Northeast Fisheries Science Center Reference Document 04-06
Stock Assessment of the Gulf of Maine - Georges Bank Atlantic Herring Complex, 2003W.J. Overholtz1, L.D. Jacobson1, G.D. Melvin2, M. Cieri3, M. Power2, D. Libby3, and K. Clark2
1National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543
2Dept. of Fisheries and Oceans Canada, St. Andrews Biological Sta., 531 Brandy Cove Rd., St. Andrews, NB E5B 2L9
3Maine Dept. of Marine Resources, Boothbay Harbor Lab., P.O. Box 8, West Boothbay Harbor, ME 04575
Web version posted March 5, 2004Citation: Overholtz, W.J.; Jacobson, L.D.; Melvin, G.D.; Cieri, M.; Power, M.; Libby, D.; Clark, K. 2004. Stock assessment of the Gulf of Maine - Georges Bank Atlantic herring complex, 2003. Northeast Fish. Sci. Cent. Ref. Doc. 04-06; 290 p.
Information Quality Act Compliance: In accordance with section 515 of Public Law 106-554, the Northeast Fisheries Science Center completed both technical and policy reviews for this report. These predissemination reviews are on file at the NEFSC Editorial Office.
The Transboundary Resource Assessment Committee (TRAC) met during 10-14 February, 2003 in the Conference Center, Biological Station, St Andrews, NB, Canada, to assess the Gulf of Maine-Georges Bank Atlantic herring complex. Some data were preliminary (i.e. 2002 landings) at the time of the meeting and all analyses were completed with these data. Two assessments were presented at the meeting, a forward projection analysis (Chapter 10) and an ADAPT assessment (Chapter 14). The review committee did not reject either assessment and therefore, the results of both approaches are contained in this document. However, much progress was made on many other facets of the status of the herring complex. For example, both counties agreed that an assessment of the overall complex was warranted, that historic tagging information was still relevant, that multiple research survey time-series should be used in the analysis, and that the new acoustics results should be used.
The assessment focused on the fishery during 1959-2002, but historically landings of herring in coastal Maine have occurred over several centuries. The fishery on Atlantic herring in the region shifted from fixed gear with landings dominated by juvenile herring in the 1950s and 1960s to an intense foreign trawl fishery that occurred offshore (Georges Bank) by ICNAF countries in the mid 1960s through the late 1970s. In recent years, the fishery captures adult herring and landings are dominated by mid-water trawlers. Landings during the last 15 years have averaged slightly over 100,000 mt, and almost 123,000 mt during 1998-2002.
The herring assessment utilized research survey data from a variety of sources. Indices are available from NMFS research bottom trawl surveys (winter (1992-2002), spring (1968-2002), autumn 1963-2002), Canadian research bottom trawl surveys (winter 1986-2002), US and Canadian larval herring surveys (US 1971-1994, Canada 1987-1995), US herring acoustic surveys on Georges Bank (1998-2002) and Maine DMR inshore herring acoustic surveys (1999-2002). Trends from US and Canadian bottom trawl surveys indicate a decline in herring during the late 1960s through the 1970s, a very low period of abundance during the late 1970s through the late 1980s, and recovery during the 1990s. Both larval herring surveys indicate an increasing trend during the late 1980s and early 1990s. The US herring acoustic survey on Georges Bank indicates that a major recovery of herring has occurred on Georges Bank and a large herring biomass is present, while the acoustic survey in Maine inshore waters indicates a relatively stable biomass for the inshore component.
The forward projection analysis suggests that a major recovery of the entire herring complex occurred during the 1990s. Fishing mortality increased steadily to about F=0.8 during the late 1960s and then increased further to above F=1.0 in the mid to late 1970s and early 1980s. Fishing mortality declined in the late 1980s and 1990s and has remained low during recent years (F2002=0.06). Total stock biomass declined from a high of 1.4 million mt in 1962 to a low of 87,000 mt in 1982. Stock biomass increased gradually thereafter to 1.0 million mt in 1994 and 1.8 million mt in 2000. Trends in spawning biomass are very similar to the pattern observed in total stock biomass, reaching about 1.6 million mt in 2001. Recruitment was very poor during the late 1970s and 1980s, but steadily improved in the 1990s with two very large year-classes, the 1994 and 1998 cohorts.
Results for the ADAPT assessment also suggest that Atlantic herring from the Gulf of Maine-Georges Bank complex have also recovered from low biomass in the 1980s. Fishing mortality increased stead lily from the late 1960s through the late 1970s, reaching F=1.1 in 1980. After 1980 fishing mortality declined and averaged about F=0.3 during 1983-1997. Recent F’s have averaged about F=0.2 and F in 2002 was 0.18. Stock biomass declined from a high of about 1.2 million mt in 1967 to less than 100,000 mt in 1982. Total stock biomass recovered very slowly during 1983-1994 to about 220,000 mt and then more quickly to about 700,000 mt in 2002. Spawning biomass followed the same pattern, reaching about 600,000 mt in 2002. Recruitment was relatively low during 1972-1994 and two large year-classes occurred in 1994 and 1998.
Yield per recruit reference points were re-estimated and results were Fmax=0.40, F0.1=0.18, and F40%=0.15. Biomass dynamics based reference point estimates were obtained with a Fox (1975) model and results were Fmsy=0.25, MSY=222,000 mt, and Bmsy=896,000 mt. An Fmsy proxy (F95% Fmsy) was estimated in the ADAPT assessment from parametric and non-parametric stock-recruitment relationships and results were F95% MSY=0.20-0.22.
The prognosis from forward projection model results suggests that fishing the stock at an F of 0.1 would produce a catch of 170,000 mt in 2004 and a 2+ biomass of about 1.79 million mt in 2005. An F of 0.2 in 2004 would produce a catch of 323,000 mt in 2004 and a beginning year stock size of 1.64 million mt in 2005. Corresponding projections with the ADAPT results produce a 2004 catch of 60,000 mt (F=0.1) and a 3+ biomass of 550,000 mt. Fishing the stock at an F=0.2 would produce a 2004 catch of 100,000 mt and a 2005 biomass of 500,000 mt in 2005.
Assessment results from the two modeling approaches suggest a three fold difference in 2002 biomass (1.8 million mt vs. 600,000 mt and) and F (0.06 vs. 0.18). These results were not reconciled by the TRAC working group during the February 10-14, 2003 meeting, and future work was suggested.
1.0. Overview of Atlantic Herring in the Region
Atlantic Herring exhibit a high degree of "population richness" with a number of separate spawning areas and discrete egg and larval distributions throughout their range in the northwest Atlantic (Sinclair 1988, Sinclair and Iles 1988). The population structure of Atlantic herring has been described as a metapopulation (McQuinn 1997) and fitting the population complex models of Stephenson et al. (2001) and Smedbol and Stephenson (2001). In these models, it is recognized that herring form identifiable, relatively discrete and self-sustaining populations that persist both spatially and temporally.
In recent years there has been increasing emphasis on preserving all aspects of biodiversity, including within species diversity (Stephenson and Kenchington 2000). The biological rationale for preserving this diversity is that such variation allows adaptation to changing conditions (Smedbol and Stephenson 2001). The economic rationale is that the decrease or elimination of population richness may lead to the loss of fisheries, such as occurred during the mid 1970s when the Georges Bank herring stock collapsed.
Most fishery management units for herring are at the scale of the stock complex (Stephenson et al. 2001) rather than at the level of the individual spawning ground. The three recognized spawning groups within the Gulf of Maine-Georges Bank Atlantic herring stock complex present a unique challenge to management. Given the intermixing of these spawning groups, and the timing of the index surveys, it is currently not possible to assess each spawning group separately. At the same time, it is recognized that conspecific populations often differ in productivity and may not support equal levels of exploitation (Smedbol and Stephenson 2001). Thus, appropriate fishing levels may not be the same for the different populations within the stock complex and individual spawning components must be monitored to ensure that they are not eroded by overfishing.
1.2. Herring stocks
Western Atlantic herring (Clupea harengus) range geographically from Labrador to Cape Hatteras, with major spawning areas restricted to the northern regions of this distribution (Scott and Scott 1983; Collette and Klein-MacPhee 2002). In the Gulf of Maine/Bay of Fundy region there are three separate stock components recognized; Southwest Nova Scotia-Bay of Fundy (4WX), coastal waters of the Gulf of Maine (5Y) and Georges Bank (5Z), the latter including Nantucket Shoals (Figure 1.1). However, our the perception of stock structure has varied over time and the delineation of stock boundaries has been challenging due to the degree of inter-seasonal mixing among the components. The movement and the seasonal distribution of fish has also had a significant impact on the assessment of stock status, on how fishing effort has been assigned, on the development of a catch-at-age matrix and on management.
A fishery for adult and juvenile herring in the coastal waters of the Gulf of Maine has been conducted for several centuries, but it wasn’t until the mid to late 1960s that the fisheries in the region markedly increased (Collette and Klein-MacPhee 2002). In 1961 an international fishery for herring developed in the offshore waters of Georges Bank and southern New England. The Georges Bank fishery peaked in 1968 with a catch of 373,000t, but the fishery collapsed in 1976 due to overfishing and poor recruitment. Only 2500t of herring were harvested from Georges Bank in 1977. An excellent summary of the Gulf of Maine fisheries is provided by Anthony and Waring (1980).
Assumptions regarding the seasonal movement, intermixing, and spawning of the individual components of the Gulf of Maine/Georges Bank herring stock have changed over the years. During the 1970s when herring in the Gulf of Maine region were assessed and managed by the International Commission for the Northwest Atlantic Fisheries (ICNAF), the three components (4WX, 5Y and 5Z) were assessed independently with the basic assumption of little intermixing among them (Figure 1.1). Stock boundaries were based on ICNAF (later NAFO) statistical reporting areas, and catches assigned accordingly. Today, we recognize that catches in US waters may contain a mixture of herring, and that the distribution/mixing of fish varies from season to season. In this section of the report the information on the distribution, seasonal movement, and stock structure is summarized and reviewed.
1.3. Scientific and Management Units
Delineation of the Northwest Atlantic into statistical areas for reporting of fish landings began in US waters in the late 1800’s, but it was not until 1932 that statistical areas were first agreed (Halliday and Pinhorn 1990). Over time, and several institutional changes, reporting boundaries and management units, were redefined and renamed. Presently, the statistical reporting areas developed by the International Commission for the Northwest Atlantic Fisheries (ICNAF) are used to define stock and management units. ICNAF divisions were an attempt to define, based on the knowledge at the time, the entire stock unit while sub-divisions were used to define fisheries and fish densities (Halliday and Pinhorn 1990). Sub-areas and statistical areas within sub-divisions are used today to compile fishery data (Figure 1.1). Important modifications to the ICNAF statistical areas included the changing of the boundaries between Divisions 4X and 5Y to correspond with the US/Canada maritime boundary delineated in October 1984 and the recording and reporting of separate fisheries statistics for the Canadian and US portions of Georges Bank beginning in 1986.
1.4. Stock Structure
Perceptions of herring stock structure in the Gulf of Maine-Georges Bank region have changed over time. Mackenzie and Tibbo (1960) identified the main spawning groups in the Gulf of Maine based on distribution of <10mm larvae (Figure 1.2). Although they did not define stocks, their observations are generally consistent with later views of stock structure. In 1971 ICNAF defined the stock structure of herring in the region based on the available information (Figure 1.3). The stock structure, as revised in 1971, is today with few changes the foundation for the assessment and management of herring in the Gulf of Maine-Georges Banks region (Figure 1.4).
1.5. Spawning Locations
Herring in the Gulf of Maine and Georges Bank region are mixed during much of the year, except during spawning when they separate and return to their spawning grounds.
Documented spawning locations include:
- Inshore Coastal Areas of the Gulf of Maine: Scots Bay, Southwest shore of Grand Manan, off Eastern Maine, off Penobscot Bay, Western Gulf off Wood Island, Jeffreys Ledge, Stellwagen Bank (Figure 1.5) (Clark et al. 1999, Power et al. 2002, Reid et al. 1999, Tupper et al. 1998)
- Georges Bank (including Nantucket Shoals): Varied with time – contracted and protracted around Nantucket Shoals. Major grounds Northeast Peak (pre and post collapse), Cultivator Shoals and the Nantucket Shoals (Figure 1.5) (Melvin et al. 1996, Reid et al. 1999). Currently, spawning appears to be continuous from Massachusetts Bay into Great South Channel and along the northern fringe of Georges Bank to the Northeast Peak.
1.6. Larval Distribution
Larvae produced by the major spawning stocks in the Gulf of Maine/Georges Bank region remain discrete during the early part of the larval stage (Sinclair and Iles 1985; Tupper et al. 1998). Therefore, the distribution pattern of young larvae (<10mm) provides information on stock structure (Figure 1.6a to 1.6d).
Both Canada and the US have conducted larval surveys on Georges Bank and in the Gulf of Maine. In 1956 the Fisheries Research Board of Canada and the US Fish and Wildlife Service initiated a co-operative program to identify herring spawning grounds and nursery areas in the Gulf of Maine/Bay of Fundy region (Tibbo et al., 1958). A broad scale survey design was implemented to cover most of the offshore waters. Based on the distribution of 4-9mm larvae, the study concluded that the largest herring spawning area in the Gulf of Maine occurred on the northern edge of Georges Bank (Figure 1.7). Annual larval surveys were conducted throughout the 1960s in the Gulf of Maine (Boyar et al. 1973a, Boyar et al. 1973b; Tibbo and Legare, 1960). Again, the studies found that the largest herring spawning component occurred on the northeastern portion of Georges Bank. In 1971 ICNAF initiated an international larval survey that concentrated in NAFO sub-division 5Z (i.e., Massachusetts Bay, Nantucket Shoals, and Georges Bank) and which formed the foundation for the future US larval programs summarized by Smith and Morse (1993).
1.6.1. US Larval Survey
The ICNAF larval herring survey in the Gulf of Maine ended in 1977, but was followed during 1977 to 1987 by a comprehensive US fisheries ecosystem study known as the Marine Resource Monitoring, Assessment, and Prediction program (MARMAP). Following the completion of the MARMAP program, larval surveys of the area were continued by the Northeast Fisheries Science Center (NEFSC) under a herring/sand lance interaction study. US larval surveys ceased in January 1995.
The information collected by the ICNAF and US larval programs provide an overview of herring abundance and distribution during the pre-collapse, the collapse, the post-collapse, and the recovery stages of the Georges Bank herring stock. Smith and Morse (1993) clearly illustrate the transition from pre-collapse to the recovery. Larval distribution figures from their study are reproduced here as Figure 1.6a to 1.6d. During the early to mid 1970’s, recently hatched herring larvae (4-8mm) on Georges Bank were concentrated on the northeastern portion of the bank (consistent with previous studies) and in the Great South Channel, just south east of Cape Cod, but west of 69o longitude. Only small concentrations were observed in the Massachusetts Bay region. Later stage larvae, 2 to 5 weeks of age, were dispersed from these epicenters of spawning and were distributed over most of the bank by the time they were 5 to 8 weeks of age (Figure 1.6a).
During 1976-1984, when the Georges Bank herring stock had collapsed, the distribution and abundance of larval herring contracted to the west with the only strong signs of newly hatched larvae occurring in the vicinity of Massachusetts Bay. The large and dense aggregations of young larvae on the eastern portion of the bank had all but disappeared. Older larvae were restricted to a much narrower geographical range in the western portion of the bank, with only small and sparse occurrences of 13+mm larvae on the eastern half of Georges Bank (Figure 1.6b).
During 1985-1987, herring on Georges Bank began to show signs of a recovery. While the newly hatched larvae were still restricted to the Massachusetts Bay/ Nantucket Shoals area, older larvae were found over more of the bank (Figure 1.6c). This time frame represents the transition period back to the pre-collapse distribution By 1988-1990, young herring larvae were still concentrated in the Massachusetts Bay/Nantucket Shoals area, but occurred across the bank to almost the Hague Line (Figure 1.6d). Large aggregations of newly hatched larvae first appeared on the Canadian portion of Georges Bank in 1992 (Figure 1.7). By the time the US larval surveys ended in 1994-1995, large aggregations of newly hatched larval herring were distributed throughout most of the Bank with dense aggregations characteristic of the pre-collapse era occurring on the northeastern portion. Dense concentrations of 4-7mm larvae were also found in the Nantucket Shoals area during this same time period (SARC, 1996).
1.6.2. Canadian Larval Survey
Canadian larval surveys on Georges Bank were initiated in 1987 to monitor the distribution and abundance of herring larvae during what appeared to be the early stages of recovery. Annual larval surveys were conducted from 1987 to 1995 with expansion of the survey area occurring in 1990 and 1992 to provide better coverage of potential spawning areas. Information pertaining to the timing of each survey and the number and size range of larvae caught is presented in Table 1.1. The spatial distribution of larvae <10 mm total length (size generally considered to reflect spawning area) sampled in 1988, 1990, 1992, and is presented in Figure 1.7. Detailed annual plots of sampling stations and total larval distribution are provided in Melvin et al. (1996).
During 1992-1995, the Canadian larval survey was conducted during late October to early November. During 1992-1995, the surveys were conducted 2-3 weeks later, from mid to late November. In these latter surveys, the number and size range of larvae caught markedly increased.
Canadian survey results clearly show herring spawning during 1986-1991 spawning, as reflected by aggregations of larvae <10mm in length, was concentrated west of the Hague line in the vicinity of Georges and Cultivator shoals. By 1992, however, the distribution of larvae <10 mm expanded well into Canadian waters. This pattern continued annually through the last survey in 1995. Interestingly, in October 2001 a plankton tow conducted just east of the Hague line collected over 50,000 5-7mm larvae.
Annual distributions of herring larvae in the Canadian surveys were generally consistent with the those in US larval surveys through 1990, the last year for which detailed US data are available.
1.6.3 Summary of Information on Larval Distributions
Herring larvae produced on spawning grounds in eastern Maine and New Brunswick are transported in a westerly direction and recruit to the juvenile herring population along the Maine coast (Tupper et al 1998). Larvae from spawning grounds in the western Gulf of Maine recruit to the juvenile herring populations along the coast of central and western Maine and along the coast of New Hampshire and Massachusetts (Lazzari and Stevenson 1992, Tupper et al. 1998). Larvae produced in the Jeffreys Ledge area move inshore and disperse in all directions (Tupper et al 1998).
Georges Bank larvae may be retained in a clockwise current gyre for several months (Boyar et al. 1973a, Reid et al 1999). However, larvae from Georges Bank and Nantucket Shoals may also migrate inshore (herring younger than two years of age are not usually found on Georges Bank) (Anthony and Waring, 1980). This would most likely occur when the Georges Bank and Nantucket Shoals spawning populations are large (Tupper et al, 1998). Graham et al. (1972) report herring larvae entering the Sheepscot estuary of Western Maine in the early fall, soon after hatching. In the spring, additional larvae also entered the coastal area. The authors postulate that the spring larvae originated from Georges Bank because when the Georges Bank component declined so to did the abundance of spring larvae along the coast.
1.7. Distribution of Herring
The distribution of adult/juvenile herring on Georges Bank and in adjacent areas has changed dramatically since 1961. Figures 1.8a to 1.8d provide a chronological overview, in 5-year intervals, of the distribution of herring in the Gulf of Maine and on Georges Bank, as indicated in Canadian (1986-1995) and the US (1966-2002) fall research bottom trawl surveys. Annual plots of herring distribution (up to 1995) are presented in Melvin et al. (1996). During the early and peak years of the Georges Bank fishery, 1961-1970, adult and juvenile herring were sparsely scattered throughout the Gulf of Maine and Georges Bank, with concentrations in the vicinity of known spawning areas (i.e., northern reach of Georges Bank, Nantucket Shoals and in Massachusetts Bay) (Figure 1.8a to 1.8c). However, the survey abundance indices were relatively low during 1961-1970 compared to recent years.
Between 1971 and 1977 the abundance of herring declined sharply and the distribution of the resource contracted to a few areas on the northwestern flank of the Bank and around Nantucket Shoals (Figures 1.8c and 1.8d). By 1979 herring had all but disappeared from Georges Bank and Nantucket Shoals during the traditional spawning season (Oct/Nov) and only a single immature herring (total length 21cm) was taken in the USA 1979 autumn bottom trawl survey on Georges Bank and in the Gulf of Maine. This trend continued into 1980 where no herring were caught in 121 survey tows and into 1981 when only two mature herring (26 and 33 cm) were collected at two stations just north of Cultivator Shoals on Georges Bank.
In 1982 adult herring began to appear again in limited numbers in the survey. The distribution of herring on Georges Bank was, however, restricted to survey sampling stations in the vicinity of Little Georges and Cultivator Shoals. Trawl stations near Nantucket Shoals and in Massachusetts Bay generally showed a wider distribution and greater number of herring during 1982-85. The first herring were collected on the Canadian portion of the Bank occurred in 1985, but it wasn't until 1986 that significant amounts of adult/juvenile herring were sampled east of the Hague line.
Between 1985 and 1989, trawl survey catches of herring increased substantially on Georges Bank and the surrounding area, especially in Massachusetts Bay. Survey catches from 1988 onward exceeded those taken in the 1960s when the stock was heavily exploited. The expanded spawning distributions and increases in abundance continued through the 1990s and into the new millennium (Figures 1.8b, 1.8c), however, it wasn’t until 1992 that spawning was detected on the Canadian portion of Georges Bank. In recent years spawning herring have been consistently taken in the autumn surveys in Massachusetts Bay, throughout Nantucket Shoals and along northern flank of Georges Bank from the Great South Channel to the Northeast Peak in an almost continuous band.
Although survey coverage of the inshore waters of the Gulf of Maine is generally poor, increasing numbers of herring have been collected in the coastal areas of Maine since about 1990.
Herring from the Gulf of Maine and Georges Bank overwinter between Cape Cod and Cape Hatteras, with major aggregations occurring in coastal and shelf waters off Long Island. Distributions patterns of herring from the US Spring bottom trawl survey series, which began in 1968, illustrate the winter distribution of Atlantic herring along the US coast and depict spatial changes over time. During the late 1960’s and early 1970s, herring primarily occurred south of Cape Cod in both the inshore and offshore waters (Figure 1.9a). Limited numbers were also found east of Cape Cod and in the Gulf of Maine proper. Between 1976 and 1984 (Figure 1.9b), after the Georges Bank spawning component had collapsed, very few herring were found in the offshore waters of southern New England or the Mid-Atlantic. Herring aggregations occurred in Massachusetts Bay just north of Cape Cod, but elsewhere in the Gulf of Maine, herring were found only sporadically. This led some researchers to speculate that the herring from the inner Gulf of Maine overwinter in near-shore coastal areas, while those originating from Georges Bank overwinter offshore. During 1986 to 1990 (Figure 1.9c), the spring distribution of herring expanded; more fish occurred offshore and in Massachusetts Bay, and also became more common on Georges Bank, Nantucket Shoals and along the coast of Maine. Since 1990, herring have continued to broaden their winter distribution and increase in abundance in both coastal and offshore waters from Cape Cod to Cape Hattaras (Figures 1.9c and 1.9d).
1.8. Seasonal migration
Tagging studies and fisheries data provide the background source of information on seasonal movements of adult and juvenile herring from each of the three spawning components (4WX, 5Y and 5Z). Conclusions based on this information may only apply in a general sense because herring from this region are extremely migratory, are known to inter-mix throughout most of the year, vary their migration patterns from year to year, and the majority of the tagging programs were undertaken more than 20 years ago. Furthermore, most of the tagging was conducted when the Georges Bank component had collapsed, and so little information is available on the seasonal movement or intermixing of this group.
Larval herring move into the inshore Gulf of Maine waters from southeast New Brunswick to southern Massachusetts during fall and winter and metamorphose into juvenile (brit) herring the following spring. During the early brit stage, herring are weak swimmers and probably do not travel long distances once they reach shore. During the first year of life there is probably little mixing between the different spawning groups along the New England and New Brunswick coasts (Tupper et al. 1998). In late summer and fall, first year brit move farther offshore and overwinter close to the bottom. They return inshore the following spring at age two when they are large enough to be recruited to the sardine fishery.
The movements of juvenile herring from the Georges Bank component are not well known. Significant numbers of age 1 and 2 fish were sampled in Canadian surveys during 1988 to 1995 (Melvin et al. 1996). Davis and Morris (1976) found brit distributed widely in the open waters of the Gulf of Maine and suggested that these might have originated from Georges Bank since there was no evidence of spawning in the offshore Gulf of Maine.
Tagging studies indicate that juvenile herring migrate little during the summer (Speirs 1977; Anthony and Waring 1980; Waring 1981; Stobo 1983a), but move into deep bays or offshore areas to overwinter (Reid et al. 1999).
Prior to the collapse of the Georges Bank stock, meristic evidence indicated that the coastal Maine and New Brunswick juvenile herring populations were augmented by juveniles from Georges Bank (Anthony and Waring 1980). However, since the coastal juvenile population did not seem to be seriously affected when the Georges Bank component collapsed, the juvenile contribution may have been small. Aggregations of juvenile herring along the coast of Maine and New Brunswick are therefore likely derived from a variety of spawning grounds.
Tagging studies conducted in the late 1970s and early 1980s by the Maine Department of Marine Resources found that juvenile herring migrate westward in the late autumn and overwinter as far south as Massachusetts Bay (Tupper et al. 1998). In spring, as the waters warm, they return back east. Juveniles tagged in the Passamquoddy Bay area, however, seemed to remain in the Bay of Fundy throughout the year (Creaser et al. 1984). Tagging studies are currently underway in New Brunswick weirs that should provide further knowledge of juvenile and young adult movement and migration patterns.
After moving inshore from the deeper waters of the Gulf of Maine/Bay of Fundy in May, juvenile herring move very little during the summer feeding season. With the onset of fall, the young fish move offshore into deeper water where they remain until the next season. As the fish grow they tend to mix with adults and undergo more extensive migrations. Juvenile herring tagged along the Maine coast tended to move eastward as the feeding season progressed, while those tagged in the Passamaquoddy area over winter in the Bay of Fundy. There is no indication from any of the tagging results that the juveniles observed along the coast of Maine make a significant contribution to other spawning stocks such as those in Southwest Nova Scotia. However, Anthony and Waring (1980) found a relationship between age 3 herring recruiting to Georges Bank and catches of age 2 fish in Maine weirs.
Adult herring from all three spawning components (5Y, 5Z, and 4WX) undertake extensive summer feeding and over-wintering migrations and intermix with stocks other than their own. At spawning, each stock seems to home to its individual spawning group. Unfortunately, neither the degree of seasonal intermixing nor the integrity (fidelity) of individual stocks to spawning grounds are known. However, there is strong evidence, both historical and recent, that the stocks fluctuate independently, demonstrate different size and age structures, and undertake distinct but overlapping migrations.
Three general migratory patterns have been recognized for herring in the Gulf of Maine: (Figure 1.10)
- Most herring that spend the summer and fall in southwest Nova Scotia overwinter primarily off Chedabucto Head and in Chedabucto Bay in eastern Nova Scotia.
- Most herring in the western Gulf of Maine migrate southwest along the coast and overwinter in Massachusetts Bay and off southern New England (Tupper et al 1998).
- Georges Bank herring overwinter along the Mid Atlantic coast and spend the summer and fall on Georges Bank. Some adults move into the Gulf of Maine in the summer and return to spawning grounds on the Bank and Nantucket Shoals in the fall (Tupper et al 1998).
The annual life cycle of the herring can be divided into five seasonal phases: overwintering, spring migration, summer feeding, spawning and fall migration. Tagging of herring at each of these stages has previously been undertaken to characterize movements and identify stocks (Stobo 1983a,b, Tupper et al 1998). Gulf of Maine and Georges Bank herring components are mixed to various degrees during all phases of their annual life cycle, except during spawning.
A brief summary of information derived from various tagging studies is provided below.
1.11.1 Gulf of Maine
Herring tagged in the summer and fall along the Maine coast tend to move southwest and overwinter in Massachusetts Bay, although a few move south of Cape Cod and some move across the Bay of Fundy to Nova Scotia (Stobo 1983a; b; Tupper et al. 1998). Adult herring tagged off Cape Cod and the western Gulf of Maine move north and east from the central coast of Maine to southwest Nova Scotia during spring and summer (Grosslein 1986). Summer feeding adults and older juveniles (age 3) tagged in eastern Maine from 1976 to 1982 were recaptured on overwintering grounds in Massachusetts and Cape Cod Bays and in Southern New England (Creaser and Libby 1988).
1.11.2 Great South Channel and Jeffreys Ledge
Herring tagged in 1977 in the Great South Channel and on Jeffreys Ledge were recovered all along the northeast coast from Ipswich Bay, Massachusetts into the Bay of Fundy and along southwest Nova Scotia in the summer and autumn herring fisheries. Tagged fish were also returned during the winter fisheries in Chedabucto Bay, Cape Cod Bay and Block Island Sound (Almeida and Burns 1978, Anthony and Waring, 1980).
1.11.3 Canadian Tagging Studies
Herring tagged in the autumn in the Bay of Fundy and off Nova Scotia migrated north to Chedabucto Bay and south to Cape Cod Bay and Block Island Sound to overwinter (Stobo et al. 1975; Stobo 1976; 1982). During the feeding and pre-spawning period, the Bay of Fundy contained a large mixture of Gulf of Maine and Scotian Shelf stocks (Stobo 1982).
1.12 Other Stock Structure Studies
Studies of meristics, otolith characteristics, and genetics have also been used to investigate the distinctness of herring stocks in the Gulf of Maine. Pectoral fin ray counts were used in the past to distinguish between herring from the Maine coast, Georges Bank and Nova Scotia (Anthony and Waring 1980). However, the number of pectoral fin ray is related to water temperature and is determined at an early age. Adult herring from Georges to Cape Cod are expected to have fewer fin rays than adults from further north since they inhabit warmer waters (Reid et al. 1999). Pectoral fin ray counts from juvenile fish from the Maine coast were found to be similar to adults from Georges Bank to Cape Cod (Anthony and Waring 1980).
Libby (cited in Tupper et al.1998) examined a number of otolith size and shape characteristics from recently hatched larvae from southwest Nova Scotia, western Georges Bank and mid-coast Maine. Eighty four percent of 38 otoliths were classified to the correct spawning area.
Genetics have provided little conclusive evidence of discrete stock structure (Tupper et al. 1998). Biochemical methods for distinguishing herring populations in the Northwest Atlantic have been conducted since the 1970s. The U.S. and USSR biochemical and serological studies of the 1970s were considered flawed and thus no conclusions could be reached based on their information (Anthony and Waring 1980). Kornfield and Bogdonowicz (1987) found no evidence of genetically distinct herring populations in the Gulf of Maine based on mtDNA RFLP analysis.
More recently, McPherson (2002) found evidence for four semi-isolated groupings of herring. These groupings were herring from the Bras d’Or Lakes, Eastern Passage, Southwestern Nova Scotia and the interior Bay of Fundy/Georges Bank.
The Gulf of Maine and Georges Bank contain three major (and perhaps additional smaller) distinct but seasonally intermixing components from Georges Bank, Nantucket Shoals (Great South Channel area) and the coast of Gulf of Maine. As a result of mixing outside of the spawning season, much of the fishery takes place on mixed aggregations.
Intermixing of components in the fishery and during resource surveys precludes separate assessment and management of the components. It is therefore necessary (as in recent years) to evaluate the entire complex, with subsequent consideration of the individual components.
- Atlantic herring generally exhibits complex stock structure
- Gulf of Maine and Georges Bank support three major distinct but seasonally intermixing components
- Major components spawn on Georges Bank, Nantucket Shoals and the coastal Gulf of Maine
- Objectives related to preserving stock structure require consideration of individual components
- Intermixing of components in the fishery and in surveys preclude separate assessment and management
- The most robust strategy for evaluation and management of this resource is an assessment of the entire complex with subsequent consideration of individual components.
2.0 Management of the Stock Complex
Atlantic herring stocks in the international waters off the US coast were first managed in 1972 by ICNAF, which set quotas and country allocations during 1972-1976. With the passing of the Magnuson Fishery Conservation and Management Act in 1976 and the extension of jurisdictional waters in 1977, the New England Fishery Management Council (NEFMC) developed a management plan for Atlantic herring that was approved in December1978. During 1977 and 1978 the Atlantic herring fishery (in US waters) was regulated by a NMFS prepared preliminary fishery management plan. The 1978 management plan had two main objectives (NEFMC 1999):
- “to manage the Gulf of Maine and Georges Bank adult herring stocks so as to achieve levels of spawning biomass providing continued and relatively stable recruitment,” and
- “to manage the Gulf of Maine juvenile herring fishery resources to stabilize and rebuild the sardine industry.”
Since most of the herring fishery took place in state waters, an Interstate Sea Herring Management Plan for Maine, New Hampshire, Massachusetts, and Rhode Island was developed in 1983 by the Atlantic States Marine Fisheries Commission (ASMFC). The ASMFC plan had a main objective and two sub-objectives as follows:
- “To acquire information that will allow development and facilitate implementation of management approaches designed to minimize prospects of a collapse of herring stocks on which New England fishermen depend,”
- “to protect spawning herring,”
- “to promote complementary management of all components of sea herring fisheries throughout the range of the stocks of interest to U.S. fishermen, including relevant Canadian waters.”
During the early 1990s, the increase in the abundance of herring in the Gulf of Maine, Nantucket Shoals and Georges Bank created a situation in which the majority of catches shifted from state to federal waters. This, combined with other changes, promoted the adoption of another management plan in 1994, which defined Atlantic herring as an inter-jurisdictional resource. As the resource continued to expand, there was a need to address changing fishing patterns and the interests of new stakeholders. This eventually led to the development of the current Management Plan submitted jointly by the NEFMC and the ASMFC in 1999. The primary goals of the plan are:
- To achieve, on a continuing basis, optimum yield (OY) for the United States fishing industry and to prevent overfishing of the Atlantic sea herring resource
- To provide for the orderly development of the offshore and inshore fisheries, taking into account the variability of current participants in the fishery
- To provide controlled opportunities for fishermen and vessel in the other mid-Atlantic and New England fisheries
The FMP defined the management unit to include all the Atlantic herring within the US territorial sea and the Exclusive Economic Zone (EEZ). Three management areas were delineated to accommodate current knowledge of stock structure and existing fishing patterns (Figure 2.1), recognizing that changes might occur in the future due to new information. Area 1 includes the Gulf of Maine, Area 2 Nantucket Shoals and south, and area 3, Georges Bank east of the Great South Channel. Area 1 was further subdivided into Area 1a, the inshore waters and Area 1b, the offshore waters. In Canada, herring from the Gulf of Maine occur in two Canadian management areas: the Bay of Fundy Region of 4X, and the Canadian portion of Georges Bank (5Z).
The relative contribution of herring from each of the major spawning components (coastal Maine, Nantucket Shoals and Georges Bank) to the overall stock complex was evaluated using swept area estimates of minimum population size ( both in number and weight) derived from NEFSC autumn bottom surveys within the three management areas. Based on these estimates during the ten-year period 1988 to 1997, the coastal Maine area accounted for 27% of the total herring biomass and 26% by number, Nantucket Shoals accounted for 63% of the total (in both biomass and number) and Georges Bank accounted for 10% of the herring biomass and 11% of the total abundance. Based on the five-year period, 1993-1997, the Coastal Maine and Nantucket Shoals areas accounted for slightly less of the total than during the ten-year period and Georges Bank accounted for slightly more (Table 2.1). In the 1999 FMP, the relative contributions (portion of biomass in each area during spawning season) of herring in the Gulf of Maine, Nantucket Shoals, and Georges Bank areas were assumed to be 25%, 55%, and 20% respectively.
Autumn survey data since 1997 show an increase in the relative contribution of herring in the Gulf of Maine (Figure 2.2). The most recent 5-year survey data (1997-2001) indicate that herring in the Gulf of Maine comprise 38% of the total biomass of the complex. Furthermore, the abundance of the Georges Bank/Nantucket Shoals population appears to be declining (Figure 2.3). Further investigation may reveal whether a different a survey strata set would better reflect changes in the relative abundance of herring among the three areas.
The seasonal distribution of herring in the Gulf of Maine, as reflected by patterns in the US spring and fall surveys, has also varied substantially over time. Table 2.2 summarizes the current view of how herring the herring components are seasonally distributed among the three management areas. These percentages are used to allocate the TAC for each of the sea herring management areas. An allocation of 20,000t is provided for Canadian weir landings, but herring catches on the Canadian portion of Georges Bank are not currently addressed in the US Atlantic herring fishery management plan.
3.0. A general overview of the fishery
Atlantic herring which spawn in the Gulf of Maine (GOM) and on Georges Bank are harvested in five major fisheries: coastal Maine, New Hampshire, and Massachusetts (5Y); Nantucket Shoals/Georges Bank (5Z); Southern New England (5Zw); Mid Atlantic (SA 6); and along the New Brunswick coast (4X). An unknown portion of GOM herring are also caught in the Canadian Bay of Fundy/Southwest Nova Scotia (4WX) fishery, although the numbers are assumed to be small.
The coastal fisheries of 5Y and the New Brunswick are amongst the oldest fisheries in the western Atlantic, dating back several centuries. Landings data for these fisheries are presented from 1938 to 2002 in Figure 3.1. During 1938-1954, a marked increase occurred in landings from the Gulf of Maine coastal area while landings in the NB weir fishery ranged between 30,000 and 45,000t annually. Landings in the GOM fishery peaked at 94,200t in 1950. Since then, annual landings have averaged 52,000t (1951-2002) with lows of 25,000t occurring in the mid-1960s, mid 1970s and mid-1980s. Landings since 1989 have been about equal to or have exceeded the long term average. Conversely, NB weir landings have shown a marked decline in recent years. Since 1994, the weir landings have been below the long- term average (1951-2002) of 26,000t. and were only 11,800t in 2002. A number of factors have contributed to the decline including a reduction in the number of active weirs and changes in herring distribution.
In the early 1960s, total landings from the Gulf of Maine-Georges Bank region markedly increased with the development of a predominately foreign fleet herring fishery in the international waters of Georges Bank and Southern New England (Figure 3.2). The Georges Bank fishery began in 1961 when the former USSR landed 68,000 mt of herring. Between 1961 and 1965 the fishery was dominated by the USSR where annual catches ranged between 38,000 and 151,000 mt (Figure 3.2). The fishery expanded rapidly after 1966 when Poland and the German Democratic Republic entered the fishery. Over the next 9 years, vessels from 12 countries harvested herring from Georges Bank, including Canada and the US (Anthony and Waring, 1980). Annual catches during 1961-1977 are presented by country in Table 3.1. Fishing gear varied by country and year. Drift gillnets dominated during 1961-1963, followed by side and stern trawlers during 1963-1972, mid-water trawlers during 1971-1977 and purse seiners during 1969-1975. Fishing occurred throughout the year, but the majority of catches were taken between May and October, when large numbers of herring were on the Bank for summer feeding or spawning (September/ October).
The Georges Bank fishery dominated landings from 1962 to 1976, peaking at 373,000t in 1968. This high level of exploitation could not be maintained and by 1976 the Georges Bank spawning component had collapsed due to over-fishing and a series of poor recruitment years. No directed fishery for herring occurred on the Bank between 1979 and 1995, and it wasn’t until 1996 that any substantial landings from the area occurred. Total herring landings from Georges Bank exceeded 39,000t in 1998 and 2001, but were less than 20,000t in 2002 (Table 3.2).
The Southern New England herring fishery has also increased substantially since 1995. Traditionally, annual landings from Southern New England have been a few thousand tonnes. However, during 1996-2000, the winter fishery just south of Cape Cod exceeded 20,000t annually (Figure 3.2). Landings of herring from the mid-Atlantic region have been minimal relative to the other herring fisheries. Typically, annual landings in the mid-Atlantic region have been a few hundred tonnes and have rarely exceeded 1,000t (Table 3.2).
3.2. Recent Landings
The Maine Department of Marine Resources (MDMR) is the primary state agency in the New England region involved in Atlantic herring research, resource monitoring, and management. The two primary types of information that are collected and processed at the Department’s Fisheries Research Laboratory in Boothbay Harbor are: 1) catch and landings information from the commercial herring fishery; and 2) age, size, and other biological characteristics of the commercial catch throughout the range of the fishery. The Boothbay Harbor Laboratory has played an important role in monitoring the status of the Gulf of Maine herring resource and the US fishery for over 30 years.
Prior to 1994, US landings were collected by a combination of canning industry reports and reports by NMFS port agents. After 1994, harvesters using Vessel Trip Reports (VTR) directly reported US landings data. With implementation of the FMP in 1999, harvesters have been required to use both VTR and Interactive Voice Reports (IVR). Federally licensed dealers ware also required to submit monthly reports (NEFMC, 1999).
Harvesters report VTR data on a monthly basis. Because harvesters give location data (coordinates or Loran) on a per trip basis, this reporting system allows for summarizing catch information from specific areas. VTR data are useful for stock assessment and effort evaluation, but because they are reported on a monthly basis, the data are not useful for quota monitoring (NEFMC, 2001).
Using the IVR call in system, harvesters report catches by management area on a weekly schedule. Although trip level information and location data are not reported, this system is useful for near real time quota monitoring. IVR data are not generally useful for stock assessments, or to address management questions that require information by area or gear.
Dealer reports include detailed information on amounts landed, price paid, and utilization of landings, usually on a per trip basis. The dealer reports do not contain information on area of catch.
Both IVR and VTR data include landings to foreign vessels by domestic harvesters. Dealer data only include landings made to domestic dealers. NMFS and state observers collect data on landings to foreign processing or fishing vessels. At the end of a fishing year, all reporting systems are analyzed to detect and reconcile discrepancies.
Total landings peaked in the 1970’s (Figure 3.3), due to fishing by foreign fleets. Since 1990, total landings of herring from the stock complex have ranged between 77,000 and 150,000 mt (average; 107,000 mt).
Fixed gear was the predominate method of catching herring in the US until the early 1980s. After1981, the fishery was dominated first by purse seines, then by single mid-water trawls. Currently, most landings are taken by single and pair mid-water trawls (Figure 3.4).
Historically most of the herring landings from the coastal complex have been taken from Management Area 1A (Figure 3.5). In recent years, there has been an increase in harvests from Georges Bank and off of Rhode Island. This is in part due to the change from purse seines to mid-water gear. Purse seines tend to be less effective in deeper water.3.3. Samples
Samples of herring collected from the commercial catch are processed at the Maine Department of Marine Resources (DMR) laboratory in Boothbay Harbor. Historically samples were obtained from canning plants, some of which transported fish from other states, NMFS port agents, and fishery biologists in various states. The Canadian Department of Fisheries and Oceans would also provide samples to the State of Maine. Normally 4-8 samples are collected each month by statistical area harvested, with more extensive sampling occurring during foreign fishing or processing operations. The current sampling ratio is approximately 1 50 fish sample per 500 mt.
Usually, between 150 and 200 length samples (7,500- 10,000 fish) are processed each year (Figure 3.6). Samples of 50 fish are processed for length (mm total length), weight (grams), sex, and, where applicable, sexual maturity and gonad stage, using standard procedures and criteria. From each sample, the sagittal otoliths are removed from two fish per centimeter group and embedded in plastic trays for ageing. Periodic calibration of ageing procedure is done with NMFS scientists.
Biostat is a software program which uses catch and sample files to produce catch at age data in both numbers of fish at age and total weight of fish at age by Unit. A Unit is defined as a month, geographical area (composed of one or more statistical areas) and gear type (fixed or mobile). Currently geographical areas are defined for the purpose of catch-at-age analysis as Eastern Gulf of Maine, Western Gulf of Maine, Southern New England/ Mid-Atlantic, and Georges Bank. Gear type is defined as fixed (stop seine and weir) or mobile (purse seine, pair mid-water trawl, single mid-water trawl and bottom trawl). The sample parameters for a given Unit are weighted by the total catch from that Unit. In the event that sample data is unavailable for a particular Unit, sample data are borrowed from the next adjacent Unit, with preference given to borrowing between months as opposed to geographically. The catch-at-age matrix for each Unit are summed across all Units for total catch at age for the year for all US landings from the complex.
BIOSTAT first sums the catch (in metric tons) by a Unit. A length frequency grouped by centimeters is then developed from the sample data in that Unit. An age-length-key is then developed from the frequency of age by centimeter length group (mm as total length) from all samples in that Unit. The age frequencies are proportioned across ages for each length group. Mean weights (grams) at age are calculated from all individual weights within an age class from all samples in a particular Unit. The mean weights at age are then multiplied by the sum of numbers at age, which gives an expanded weight at age in that Unit. Catches in weight (mt) at age are derived from total weight of the catch multiplied by the weight at age proportion. Catch in numbers at age is calculated from catch in weight at age divided by the mean weight at age times 1,000,000 (convert grams to metric tons) for each Unit.
Strong 1994, 1996, and 1998 year classes are evident in the catch-at-age matrix (Table 3.4. Table 3.5). Since the early 1990s, greater numbers of older fish (7+) have occurred in the stock. This is probably due to the demise of the inshore fixed gear fishery (which tended to catch smaller fish) and an overall reduction in fishing mortality on the stock complex during the last decade.
4.0. Research Surveys
Over the years both Canada and the United States have surveyed the distribution and abundance of herring in the Gulf of Maine (Table 4.1). While both bottom trawl and larval surveys have been explored as indices of abundance for herring, only the former provide a continuous time series and both countries in the mid-1990s abandoned larval surveys in the mid 1990s. Recently, the US and Canada have each moved toward acoustic surveys to estimate herring biomass. The US now conducts annual acoustic surveys to assess the abundance and distribution of herring in the Gulf of Maine and on Georges Bank. Canada and the US also use industry based acoustic surveys to provide supplemental estimates of spawning stock biomass on specific inshore spawning grounds.
4.1. Indices of abundance
Indices of abundance, which are considered to reflect changes in the population, are critical in the evaluation of stock status Both Canada and the US have conducted fall larval surveys on Georges Bank and in the Gulf of Maine. The US larval survey, which extends from 1971-1994, was used in past Gulf of Maine/Georges Banks assessments (1991, 1993, 1996) as a tuning index along with indices from bottom trawl surveys. The US index of the number of 4-7mm larvae per 10 m2 (#/larvae/10m2) was developed as a composite of four individual annual surveys conducted under various programs (Smith and Morse, 1993). Canada conducted fall larval surveys from 1987-1995 and used the # larvae (<10mm)/m2 as an abundance index on Georges Bank (Melvin et al. 1996).
Both surveys showed that rebuilding began in the mid-to late 1980s and continued during the early 1990s (Figure 4.1).
4.2. Research Vessel Bottom Trawl Surveys
Several research vessel bottom trawl survey series have been conducted within the geographical range of the Gulf of Maine-Georges Bank herring complex. These surveys vary in temporal and spatial coverage from almost the entire range to selected portions on Georges Bank and southern New England. Trends in survey abundance indices are shown in Figure 4.2.
The Canadian spring bottom trawl survey (conducted primarily to assess the abundance of ground fish, on Georges Bank and west as far as Nantucket Shoal (in some years)), covers the northern extent of the winter/spring distribution of herring in the GOM and on Georges Bank (Table 4.2).
The US autumn bottom survey covers the entire distribution of herring off the northeast coast. The survey, which began in 1963, occurs when the majority of adult herring are aggregated to spawn (Table 4.4). During the early years of the survey, catches of herring were relatively low (Figure 4.3). Catches of herring increased during the mid 1980s until 1992, declined slightly in 1993-94, and then sharply increased in 1995. The 1995 increase is the result of large catches of age 1 herring which nearly doubled the index. Catches thereafter declined through 1998, but increased afterwards and reached a record high in 2002. Autumn survey indices for ages 2 to 8 are presented in Figure 4.4.
The US winter bottom trawl survey was initiated in 1992 and covers a large portion of the spatial distribution of over-wintering herring. The annual survey, which begins in early February (Table 4.3), extends from Cape Hattaras to Cape Cod and along the southern flank of Georges Bank. The survey indices were variable during the early 1990s, peaked in 1996, and then declined in 1997. Since 1998, the index has steadily increased (Figure 4.5). Catch-at-age indices for age groups 2 to 8 during 1992-2002 are presented in Figure 4.6.
The US Spring bottom trawl survey covers the entire US range of herring during the late winter and early spring. This survey series , which began in 1968 (Table 4.5), has used the same sampling gear except for a net change to the Yankee 41 trawl during 1973-1981 and for a door change in 1985. The survey vessel has variously been the R/V Albatross IV and the R/V Delaware II with appropriate fishing power corrections incorporated into the index (Figure 4.7).
The spring survey herring abundance index was relatively flat during 1975-1983 when herring were scarce, then gradually increased during the early 1990s, the index markedly increased and peaked at a record high in 1999. The index has since declined to about the long-term (1983-2002) mean in 2000 (Figure 4.8; Figure 4.9).
Annual growth, as represented by the mean length-at-age of herring collected during the fall spawning season on Georges Bank, has undergone some marked changes (P<0.01) since the early 1980s. Herring from the 1983-1985 year-classes grew more rapidly than those spawned during 1987-1991 (Figure 5.1). At age 2, the year-class mean lengths are distributed over a 2cm interval, however by age 4 there is almost a continuous decrease in the mean length from 1983-1991. By the time the fish reach age 5 and 6, a difference in mean length of 2 cm or more can be observed. The 1986 year-class seems to represent a transition between the two trajectories. Assuming a constant weight length relationship, a 6 year old from the early period would weight 245g compared to196g for the same size fish collected in the late stages of the recovery, a 20% difference in biomass.
6.0. Canada/US Age comparisons
Consistent and comparable aging is critical when data are being combined from two independent sources. To investigate potential difference or biases in aging, 213 otoliths, from the Gulf of Maine, were selected for aging by readers from both countries. Three independent agers, one from the NMFS Northeast Fisheries Science Center (US-1), one from the Maine Department of Marine Resources (US-2), and one from the DFO St. Andrews Biological Station (Can-1), read each otolith.
Age determinations between the two US readers were very consistent; percent agreement between the two agers was 85% with a slight bias toward under-aging (10%) vs over-aging (5%) by US-1 relative to US-2 (Table 6.1). However, significant differences (P<0.01) were detected between the age determinations of the Canadian reader and both US readers (Tables 6.2 and 6.3). Percent agreement was 76% with US-1 and 78% with US-2. In both cases, there was a tendency for the Canadian reader to under-age (16% and 18%) rather than over-age (8% and 4%) relative to US age readers. Beyond age 6, the disagreements exceeded 50%.
Differences between the Canadian and US age reading are of concern and will require some time and effort to resolve. An aging workshop amongst the readers from both countries will be convened during 2003 to address this issue. In the interim age-length keys will be applied according to the data origin. That is, data collected by the US will use US ages and Canadian data Canadian ages. While this may affect the catch-at-age matrix and age disaggregated indices of abundance, mixing of the data sources would further complicate the issue.
7.0 Acoustic Surveys and Results
This section describes various acoustics surveys, their design, and results. Only available as PDF file (7.5 KB). Tables and figures linked below.
Table 7.1 Parameters for length weight equations for Atlantic herring from NMFS autumn research vessel bottom trawl surveys in the Gulf of Maine-Georges Bank Region Table 7.2 Mean herring backscatter and SD of backscatter from acoustic surveys on Georges Bank during 1999-2002 Table 7.3 Intercepts from target strength equations from studies on herring stocks in the North Atlantic Table 7.4 Geostatistical estimates of biomass, CV, CV inverse, weighted biomass and weighted CV for Acoustic surveys on Georges Bank during 1999-2002 Table 7.5 Point estimates of mean Sa and biomass from standard statistical analysis for surveys on Georges Bank during 1999-2002 Figure 7.1 Cruise tracks for surveys on Jeffreys Ledge, Platts Bank, Cashes Ledge and Fippennies Ledge during the 1998 Atlantic Herring Hydroacoustic Survey. Figure 7.2 Cruise track for the systematic parallel survey in the Gulf of Maine during the 1998 Atlantic Herring Hydroacoustic Survey Figure 7.3 Cruise track for one zigzag survey on the northern edge of Georges Bank during the 1998 Atlantic Herring Hydroacoustic Survey. Figure 7.4 Cruise track for surveys on Jeffreys Ledge, Platts Bank, Fippennies Ledge, Cashes Ledge, Franklin Swell, and Georges Bank during the 1999 Atlantic Herring Hydroacoustic Survey. Figure 7.5 Cruise track for the systematic parallel survey circumscribing Georges Bank during the 1999 Atlantic Herring Hydroacoustic Survey Figure 7.6 Cruise track for systematic parallel surveys conducted on Jeffreys Ledge, Platts Bank, Fippennies Ledge, Cashes Ledge, and Georges Bank during the 2000 Atlantic Herring Hydroacoustic Surveys Figure 7.7 Cruise track for the systematic zigzag survey on Georges Bank during 2000 Figure 7.8 Cruise track for the stratified random survey design on Georges Bank during 2000 Figure 7.9 Complete set of potential stratified random parallel transects for survey in Atlantic herring on Georges Bank during the 2000 Atlantic Herring Hydroacoustic Survey Figure 7.10 Cruise tracks for systematic parallel surveys on Jeffreys Ledge, Platts Bank, Fippennies Ledge, and Cashes Ledge during 2001 Figure 7.11 Cruise track for the systematic parallel survey on Georges Bank during the 2000 Atlantic Herring Hydroacoustic Survey Figure 7.12 Cruise track for the random parallel survey on Georges Bank during the 2001 Atlantic Herring Hydroacoustic Survey Figure 7.13 Cruise track for systematic zigzag survey and experimental work on Georges Bank during the 2001 Atlantic Herring Hydroacoustic Survey Figure 7.14 Stratified random parallel transect design for surveying Atlantic herring on Georges Bank during 2001 Figure 7.15 Parallel design for surveying Atlantic herring on Jefferys Ledge during 2002 Figure 7.16 Parallel design for surveying Atlantic herring on Georges Bank during 2002 Figure 7.17 Herring backscatter on transects from a zigzag survey design on Georges Bank during 1998 Figure 7.18 Herring backscatter on transects from a zigzag survey design on Georges Bank during 1999 Figure 7.19 Herring backscatter on transects from a zigzag survey on Georges Bank during 1999 Figure 7.20 Herring backscatter on transects from a parallel design on Georges Bank during 1999 Figure 7.21 Herring backscatter on transects from a zigzag survey design on Georges Bank during 2000 Figure 7.22 Herring backscatter on transects from a Parallel survey design on Georges Bank during 2000 Figure 7.23 Herring backscatter on transects from a stratified random survey on Georges Bank during 2000 Figure 7.24 Atlantic herring backscatter on transects from a parallel survey design on Georges Bank during 2001 Figure 7.25 Atlantic herring backscatter on transects from a zigzag survey design on Georges Bank during 2001 Figure 7.26 Atlantic herring backscatter on transects from a stratified random design on Georges Bank during 2001 Figure 7.27 Atlantic herring backscatter on transects from a parallel survey design on Georges Bank during 2002 Figure 7.28 Variogram from the parallel survey design on Georges Bank during 2000 Figure 7.29 Biomass from bootstrap analysis for zigzag part 1, zigzag part 2, and parallel survey designs on Georges Bank during 1999 Figure 7.30 Biomass from bootstrap analysis for zigzag, parallel, and stratified random survey designs on Georges Bank during 2000 Figure 7.31 Biomass from bootstrap analysis for zigzag, parallel, and stratified random survey designs on Georges Bank during 2001 Figure 7.32 Biomass from bootstrap analysis for a parallel survey design on Georges Bank during 2002
VPA Calibration and Diagnostics
Assessments of Atlantic herring have always been complicated due to the migratory behavior and the intermixing of stocks. Over the years, a variety of assumptions have been made about stock structure and seasonal composition.
During the 1970s independent assessments of the Gulf of Maine and the Georges Bank herring stocks were undertaken by ICNAF (Anthony and Waring, 1980, Stevenson et. al., 1997). Estimates of population size were based on un-tuned VPAs or cohort analyses, had no fishery independent information to estimate F, and relied on juvenile catch information to estimate recruitment. These models estimate population biomass to be approximately 1.3 mt in 1967-1968, and 204,000t (4+) in 1976. In 1976, the population size of the western GOM was estimated to have been 159,000t, less than 100,000t in the early 1970s and only 65,000t in 1976 (ICNAF Redbook, 1976).
One approach to avoiding the problem of stock intermixing is to perform a “pooled assessment”. Anthony (1977) combined herring catches from south of Cape Cod, Georges Bank, the Gulf of Maine, and the Bay of Fundy and as far north as Chedabucto Bay. Sissenwine and Waring (1979) did the same thing when they pooled catch-at-age data for all herring fisheries between Southwest Nova Scotia and Cape Hatteras in their analysis of herring fisheries of the Northwest Atlantic.
After the decline in fishing effort by foreign fleets and the collapse of Georges Bank herring stock around 1976, assessments undertaken during the 1980s concentrated on the GOM stock. Three assessments of the GOM stock were conducted during this period using the spring bottom trawl survey indices (number/tow) to tune a VPA developed using pooled catch-at-age data. These assessment were considered flawed in that the tuning indices did not solely represent the GOM stock, but, also included fish from GB and Nantucket Shoals . Spawning stock biomass estimates for the GOM stock were relatively low in the late 1970s, 30,000t in 1982, then increased rapidly throughout late 80s to exceed 150,000t.
Confusion over the definition of Georges Bank versus Gulf of Maine fish continued in the 1980s. Anthony et al. (1981) attempted to exclude Georges Bank fish in their assessment of herring stocks of the Gulf of Maine, but they included herring from the southern New England winter fishery in their analysis. At the 1989 SAW Fogarty et al. (1989) stated that “Atlantic herring throughout the Gulf of Maine, Southern New England and mid-Atlantic regions are considered to be part of a single stock. Accordingly, we developed a single abundance index for this region”. Until 1989, it was assumed that the US fall survey might provide an index of abundance for the individual stocks. However, the fall survey data were determined to be too variable, to be a reliable indicator of abundance for either the individual stocks or the stock complex.
Uncertainties in distribution, stock inter-mixing, and assignment of catches continued to plague the assessment in 1990. At the Eleventh SAW (NEFC, 1990, p. 58) the Gulf of Maine herring stock was defined as:
“The Gulf of Maine stock was considered to include all fish found in NAFO areas 5Y and 5Zw (i.e., excluding fish from area 6, which were assumed to belong to either Georges Bank or Nantucket Shoals stocks; and excluding fish from Sub-area 4, which were assumed to belong to Atlantic Canadian stocks). However, an unknown amount of mixing occurs during winter/spring between Gulf of Maine, Georges Bank and Nantucket Shoals stocks in the Mid Atlantic and Southern New England Areas”.
Prior to 1991, the Georges Bank/Nantucket Shoals and the coastal Gulf of Maine stocks were therefore assessed separately (Anthony and Waring, 1980, Fogarty and Clark, 1983 and Fogarty et al, 1989).
In 1991, two major changes were made in the assessment of GOM/GB herring. The first was the introduction of a correction factor (approximately 50% reduction in catch rates) to account for differences in fishing power of the R/V Delaware II vs. R/V Albatross IV. Secondly, a change was made from assessing the stocks separately to treating them all as a single stock complex. Examination of the NEFSC spring survey data series revealed that no geographical grouping of strata could be used to represent either the Georges Bank or Gulf of Maine stock unit. Consequently, for the purposes of assessing abundance, herring from the coastal Gulf of Maine, Nantucket Shoals and Georges Bank were treated as a single highly migratory coastal herring population that had distinct spawning areas.
“...the SARC consensus was that both the catch at age matrix and the spring survey indices of abundance reflect not only the “coastal” stock but also intermixing of fish from New Brunswick weir catches and Georges Bank stocks. The SARC, therefore, decided that the assessment should be based on an aggregate stock complex including coastal, Georges Bank and New Brunswick weir caught fish” (NEFSC, 1992, p. 62).
Since 1991, Georges Bank/Nantucket Shoals and the coastal Gulf of Maine stocks have been considered part of a migratory coastal herring complex possessing distinct spawning components. Other important changes to the assessment have included; the inclusion of New Brunswick fixed gear (weir/shutoff) catches in the catch-at-age in the 1993 assessment; the use of a larval abundance estimate as an index of SSB in the 1991,1993 and 1996 assessments; and the introduction of the NEFSC winter survey index which in the 1998 assessment. The fall bottom survey was also reexamined in 1998, but was not used as a tuning index.
The last assessment was conducted on the coastal stock complex in 1998 and estimated SSB through 1997. The VPA was tuned using the US spring bottom trawl index from 1968-1997 (ages 2-8), and the winter survey index during 1992-1997 (ages 2-8). The spring indices were based on herring catches in survey strata 1-30, 36-40, and 61-76, while the winter survey indices were based on herring catches in survey strata 1-3, 5-7, 9-11, 13-14,16, 61-63, 65-67, 69-71, and 73-75.
Input data (catch-at-age, winter and spring survey indices, mean weights, etc.) for the VPA were updated through 2001 and a new calibration was completed using the same formulation as in the previous (1998) assessment. The VPA results indicate that spawning biomass increased greatly in the 1990s and fishing mortality was very low in 1999-2001 (Figure 9.1 and Figure 9.2). Recruitment improved dramatically in the 1990s with several large year classes (1994, 1998) and moderate year classes (1993, 1995, 1996) being produced (Figure 9.3).
Retrospective Analysis of VPA
A retrospective analysis of the VPA for the herring complex was completed using the FACT 1.05 software for the years 1995-2001. The formulation was the same as used in the 1998 assessment except that catch-at-age and research survey indices for winter and spring were added for 1998-2001. Results were similar to those obtained in the last assessment, in that severe retrospective patterns were apparent in spawning stock biomass, fishing mortality, and recruitment. Both spawning stock biomass and recruitment were overestimated in successive years during 1995-2001 (Figure 9.1 and Figure 9.3), while fishing mortality was underestimated (Figure 9.2). Recent landings of herring from the complex are low relative to stock size; resulting in fishing mortality (F) being very low relative to natural mortality (M) A succession of moderate to large year classes in the 1990s has apparently made it difficult to estimate recruitment accurately. In addition, the increase in biomass apparent from survey indices in the 1990s is not estimated very well. Examination of trends in SSB from the 1997 and 2001 VPAs, revealed that increases in biomass are very abrupt with SSB reaching high values in only the last year of each run (Figure 9.4). Because of the retrospective pattern and the inability to precisely estimate biomass in the 1990s, a forward projection modeling was used to assess stock status.
10.0. Forward Projection Approach-Application and Description
This section is only available as a PDF file. Tables and figures are linked below.
Table 10.1 Variance, number of observations, and degrees of freedom from spawner recruit models for various North Atlantic stocks of herring Table 10.2 Landings of Atlantic herring from the Gulf of Maine - Georges Bank complex during 1959-2002 Table 10.3 Research survey catch per tow (kg) for age 2 and age 3+ for US winter, spring, and fall and Canadian spring during 1963-2002 Table 10.4 Time series of survey catch for the US acoustic survey, the US larval survey, and the Canadian larval survey during 1971-1995 Table 10.5 Likelihood profile analysis for base case forward projection model Figure 10.1 Overall proportion of mature herring at different maturity stages during acoustic survey cruises during 1999-2002 Figure 10.2 Maturity stages observed during consecutive herring acoustic surveys (starting with zz01) on Georges Bank during 1999 Figure 10.3 Maturity stages observed during consecutive herring acoustic surveys (starting with pl00) on Georges Bank during 2000 Figure 10.4 Maturity stages observed during consecutive herring acoustic surveys (starting with zz01) on Georges Bank during 2001 Figure 10.5 Maturity stages observed on a herring acoustic survey on Georges Bank during 2002 Figure 10.6 Log recruit numbers plotted against spawning biomass for ten North Atlantic Herring Stocks Figure 10.7 Distribution of variance estimates for log recruitment residuals from nonparametric stock recruit models for ten North Atlantic herring stocks Figure 10.8 Surface-Bottom gradient from differencing the surface and bottom temperatures from the Gulf of Maine during 1963-2000 Figure 10.9 Sea surface temperature anomalies for the Gulf of Maine during 1963-2000 Figure 10.10 Autumn survey timing (mean Julian date) during 1963-2001 Figure 10.11 Autumn survey residuals and mean survey timing (Julian date) for 1963-2000 Figure 10.12 Spring surface-bottom gradient from differencing the surface and bottom temperatures from the Gulf of Maine during 1968 Figure 10.13 Spring survey timing (mean Julian date) during 1968-2002 Figure 10.14 Spring surveys age 2 weight/tow showing differences in residual patterns for age 2 without and with a door covariate for 1968-2002 Figure 10.15 Autumn surveys age 2 and age 3+ weight/tow showing differences in residual patterns for age 2 without and with a door covariate and age 3+ for 1963-2002 Figure 10.16 Average biomass estimates from F/V Providian for inshore and nearshore Gulf of Maine and Georges Bank during 1999 and 2000 Figure 10.17 Observed vs predicted (log scale) and residuals vs time for the spring age 2, spring age 3+, and winter age 2 US surveys Figure 10.18 Observed vs predicted, and residuals vs time for the hydroacoustic, US Larval survey, and the Canadian Larval survey Figure 10.19 Observed and predicted relative abundance and residuals vs time.for the Canadian age 2 survey, and Canadian age 3+ survey
11.0 Forward Projection Analysis Results
A full table of output and results is provided in Appendix II.11.1 Estimates of Fishing Mortality
Fishing mortality was below 0.2 during the early 1960s followed by a large increase in F to about 0.8 during the late 1960s (Figure 11.1). This coincides with a major increase in fishing effort during this period. F increased again in the mid and late 1970s to above 1.0 but declined sharply in 1984 to F=0.2 (Figure 11.1). F remained steady at about this rate during 1985-1989 and then fell further after 1990. The fishing mortality in 2002 on the coastal complex was about 0.06 (Figure 11.1).
11.2 Estimates of Biomass
Total stock biomass in the Gulf of Maine-Georges Bank herring complex was about 1.4 million mt in 1962, and steadily declined to a low of 87,000 mt in 1982 (Figure 11.2). Stock biomass increased gradually after 1983, reaching 1.0 million mt in 1994 and 1.8 million mt in 2000 (Figure 11.2).
Spawning stock biomass followed a trend nearly identical to total biomass, declining from 1.2 million mt in 1962 to a low of 42,000 mt in 1982 (Figure 11.3). SSB increased steadily afterward this to 1.0 million mt in 1996 and 1.7 million mt in 2001.11.3 Recruitment
Recruitment during the 1960’s was generally moderate with large 1968 and 1970 year-classes (Figure 11.4). All subsequent year classes through 1986 were below average or poor (Figure 11.4). Recruitment markedly improved during the 1990s with the very large 1994 and 1998 year-classes.11.4 Stock-Recruitment
A Beverton-Holt stock-recruitment relationship was estimated within the forward projection model. This relationship added some stability to the model and provided a reasonable fit to the available time-series of data (Figure 11.5). Most years fit the model well with the exception of large residuals (implied) for the 1994 and 1998 year-classes (Figure 11.5).11.5 Precision of FPA Estimates
The precision of terminal 2002 year estimates of spawning biomass and fishing mortality were estimated using bootstrap procedures. Estimates of spawning stock biomass in 2002 ranged from 0.8-2.7 million mt with a median of 1.5 million mt and 80% CI of 1.2-1.8 million mt (Figure 11.6). Estimates of F in 2002 ranged from 0.02-0.12 with a median value of F=0.066 and 80% CI of 0.054-0.084 (Figure 11.7).11.6 Retrospective Analysis of FPA
A retrospective analysis was performed using the FPA model including terminal catch years 1996-2002. No discernable patterning was evident in the estimates of fishing mortality in the retrospective runs. Estimates of fishing rates were relatively close in successive terminal years (1996-2002) (Figure 11.8).
Similarly, no retrospective patterns were detected in spawning stock biomass estimates (Figure 11.9). There is a break between 1998 and 1999 associated with the time when estimates of biomass from hydroacoustic surveys first become available (Figure 11.9). This discontinuity continues back until 1992 where it disappears.
Estimates of recruitment exhibit little retrospective patterning over the 1996-2002 period (Figure 11.10). There are some differences in the estimation of the size of the 1994 and 1998 year-classes in 1996 and 2000 respectively, but these do not occur in a sequential pattern.
11.7 Losses to Natural Mortality
12.0 Biological Reference Points
12.1 YPR and SSB/R
Yield per recruit and SSB per recruit reference points for the Gulf of Maine-Georges Bank herring complex were last estimated in the assessment conducted in 1996 (NEFSC 1996). Reference points from that analysis were F0.1=0.20, F20%=0.34, and Fmax=0.40. Yield per recruit and SSB per recruit reference points were re-estimated with more recent data (last 5 years) using the Thompson and Bell (1934) model (Table 12.1). Herring were assumed to be fully recruited at age 2 and fully mature at age 3. Estimated reference points were F0.1=0.18, F40%=0.15 and Fmax=0.40 (Table 12.2; Figure 12.1).
12.2 Surplus Production
Estimates of surplus production parameters from the 1998 assessment (NEFSC 1998) were derived using an ASPIC model that was conditioned with the B1 ratio fixed at 1.0 to produce stable estimates of parameters. This was the model accepted by the Overfishing Definition Review Panel (ODRP) in 1998 (NEFMC 1998). Estimates of biological reference points from 1998 surplus production analysis were MSY=317,000 mt, Bmsy=1.066 million mt and Fmsy=0.30.
Surplus production parameters were re-estimated using a Fox (1975) model and also a Schaefer (1954) model. The Fox model is asymmetric and was considered a better match with the Beverton-Holt stock-recruitment model. Biological reference points from the Fox model were MSY=222,000 mt, Bmsy=896,000 mt and Fmsy=0.25 (Figure 12.2). Reference points from the Schaefer model were MSY= 243,000 mt, Bmsy=1.03 million mt, and Fmsy=0.24.
During the early 1960s through the late 1970s, landings and surplus production were about equal (Figure 12.3). Starting in 1982, landings declined leading to a gradual and then large increase in the stock during the 1990s. Surplus production in the 1990s exhibited several large peaks representing the recruitment of the very large 1994 and 1998 year-classes (Figure 12.3).
Given that total stock biomass of the Gulf of Maine-Georges Bank herring complex has been above Bmsy since the mid 1990s, projections were conducted to estimate 2+ stock size in 2004 and 2005 under several assumptions of fishing mortality. The landings in 2003 were assumed to be 100,000 mt, approximately equal to that in 2002. Natural mortality was assumed to be 0.2 and two levels of F were used in the projections; F=0.2, a fishing rate approximately equal to the Fmsy reference point for the complex and F=0.1. A delay-difference projection model was constructed to simulate the dynamics of the herring complex. Bootstrap estimates of stock biomass for 2001 and 2002 were input to the model. Recruitment was modeled using the Beverton-Holt stock-recruitment relationship, parameters from the FPA final model, and a lognormal error structure. Results were summarized for the 750 bootstrap runs and median (50%) 2+ stock size and F values were produced.
An F=0.2 in 2004 would produce a catch of 323,000 mt and a reduction in stock size from 1.80 million mt in 2004 to about 1.64 million mt in 2005. An F of 0.1 in 2004 would produce a catch of 170,000m t but no change in biomass (1.79 million mt in 2005).
14.0 Gulf of Maine Herring Complex Adapt (VPA) analysis
This section of the report deals with the ADAPT formulation used to assess the status of the Gulf of Maine/Georges Bank herring stock complex. Specific input parameters such as age composition, mean weight at age, percent maturity and details on tuning indices are only discussed in general as they are discussed in depth in other sections of this report.
14.2 Analytical Approach
The assessment process was initiated with the reproduction of the 1998 assessment formulation using data through 1997. Some difficulty was initially encountered in exactly matching the F’s on the older ages and the use of the age 11 as a plus group. However, when age 11 was considered as a non-plus group the results were almost identical to the 1998 VPA results. The ratio of population numbers in the preliminary VPA was compared with population numbers from the 1998 assessment (Table 14.1). Most ratio values are close to 1 indicating little or no difference. Blank cells in the table are due to zero values in the 1998 assessment while large values (>1) are assumed to be due to precision errors from using population numbers rounded to millions.
As an initial run, the data series were updated for 1998-2002 and the VPA re-run using the 1998 assessment formulation. Indices of abundance included the NMFS spring bottom trawl survey stratified mean number per tow for strata 1-30, 36-40, and 61-76 from 1968 to 2002 and the winter NMFS bottom trawl index for strata (1-3, 5-7, 9-11, 13-14, 16, 61, 63, 65-67, 69-71, 73-75 for 1992-2002).
The results were examined in relation to how the age 11 in the catch at age was used and what effect various assumptions had on the results. Treatment of age 11 as a plus group appeared to be inconsistent with the observations from research surveys and the fishery. Specifically:
- Older ages formed a very small portion of the catch at age (less than 0.5% for ages 9 and 10 since 1973 (Table14.2).
- There was no accumulation effect in the plus group.
- A high and variable mortality after age 7, especially in most recent years, was seen in the VPA results and PR patterns.
- Very few fish older than age 8 were observed in the any of the survey series.
Several modified versions of the initial formulations were investigated. Estimates of SSB from the various treatments of age 11 are shown in Figure 14.1. All runs used the same formulations.
- age 11 as a plus group using FIRST method with F11=F10, F10 = weighted avg. 5-9. This method closely matches the 1998 assessment formulation.
- age 1-11 no plus group, oldest age using F11=F10
- age 1-10 no plus group, age 11 removed, oldest age using F10 = weighted average ages 5-9.
However, since 11+ fish do not occur in either the fishery or in the population (from trawl surveys) the treatment of these older fish should not be a significant source of error for short-term projections. The issue remaining is, will there be accumulation in the population at older ages if F is maintained at moderate levels and the implications for stock biomass, stock recruitment and reference points.
14.3 Summary of the 1998 Extended VPA
Input parameters and results for the initial VPA matching the 1998 assessment formulation and extending time series are summarized as follows.
1998 extended analysis (Appendix III):
- catch at age for 1967 to 2001 for ages 1-10 only (note that the 2002 catch at age was not available when this analysis was first done)
- no plus group assumed for reasons explained
- tune using winter survey 1992-2002 for ages 2-8 and spring survey 1968-2002 for ages 2-8; no special weighting with each value getting equal weight
- m=0.2; F on oldest age 10 calculated based on F on age 9
- Year 2002: estimate ages 5,6,7 and assign ages 1-2
- Year 2001: remaining ages 2,3,7,8,9 calculated as a weighted F of ages 4,5,6 in 2001
Details of the 1998 extended ADAPT run are presented in Appendix III. Using the above assumptions, total stock (age 1+) biomass in 2003 was calculated to be 1,580 kt; 3+ biomass, 1,400 kt; and SSB, 1,350kt (Figure 14.11). The mean squared residuals (MSR) by age were high for both surveys with many values >1.0 (Table 14.3, Figure 14.3) and the residual plots by year and age showed patterns that were either all positive or all negative by year and are large (Figure 14.2, Figure 14.3, and Figure 14.4). Age by age plots of observed vs predicted abundance show a relatively poor fit for the winter survey, but a much better pattern for the spring (Figure 14.5 and Figure 14.6) Diagnostic plots of survey q’s by age also showed time trends and were not consistent over the series (Fig. 14.7, 14.9). Overall survey q’s by age were dome shaped (Fig. 14.8, 14.10) indicating lower catchability for younger and older ages included in the formulation. There was an indication of several strong recruiting year-classes in recent years, a pattern of reduced fishing mortality since 1982, and a trend of increasing biomass since about 1997 (Figure 14.11). A severe retrospective pattern was detected beginning in 1999 but it was not as pronounced in 2000 and 2001 (Fig 14.12).
14.4 Final VPA
Several factors affecting the input parameters and the diagnostics were examined before adopting the final formulation. These included the analysis of a truncated time series and the use of the mean square residual as a selection criteria for the tuning indices.
14.5 Analysis using split survey series
In reviewing the survey abundance indices, several of the indices exhibited inconsistencies between the pre and post collapse period. A breakpoint was identified around 1984 which corresponded with a door change in the NMFS spring survey. Catch rates after 1984 were consistently higher than the previous period. Although several VPA scenarios were explored, including a truncated time series (1983-present), the final VPA run utilized all of the survey data; the spring and fall bottom trawl indices were split into two series (1968-1984; 1985-2002).
14.6 Selection of inputs based on Mean Square Residual (MSR)
An important diagnostic output of the ADAPT formulation is the Mean Square Residual (MSR) of the overall formulation as well as for each survey index by age and year. Given that tuning of the VPA uses loge (ln) transformed data, the square root of the MSR is considered approximately equal to the CV (CV=sd/mean) on the linear scale. Consequently, a MSR of 2.0 implies that the standard deviation of the survey is about 1.4 times the size of the mean. MSR values can thus be used as a selection criteria for the inclusion or exclusion of indices and age groups within a survey series. Weighting based on the MSR could also achieve the same end but exclusion was initially considered simpler given the divergence in the MSR between survey series and ages.
The observed overall MSR for various ADAPT formulations on the Gulf of Maine herring complex was quite large (typically > 1.0) indicating poor resolution of the data. An MSR of < 3.0 was therefore used for selection of ages and survey series to calibrate the VPA. This corresponds to CV’s that are up to 1.7 on a linear scale.
ADAPT has an intrinsic weighting option that uses the inverse of the mean square residual to weight the indices. This provides a mechanism to incorporate all indices of abundance with an objective weighting function. The final VPA run, reported in the SSR and as follows, utilized this approach to weight the individual indices.
14.7 Summary of final ADAPT formulation ‘Final Run’
The final ADAPT formulation had the following features:
- catch at age for ages 1-10 without a plus group for 1967 to 2002
- m=0.2; F on oldest age 10 calculated based on weighted F on ages 7,8,9
- Year 2003: estimate ages 3,4,5,6,7 and assign age 1 at 2000 in year 2002 and 2003
- Year 2002: remaining ages 7,8,9 calculated as a weighted F of ages 4,5,6 in 2002
- all available survey abundance indices were used with the ADAPT intrinsic weighting option
1. fall 1967-1984 ages 2-8; with split in series for gear change
2. fall 1985-2002 ages 2-8; with split in series for gear change
3. spring 1968-1984 ages 2-8; with split in series for gear change
4. spring 1985-2002 ages 2-8; with split in series for gear change
5. winter 1992-2002 ages 2-8
6. US larval 1971-1988 weighted mean compared to 3+ midyear biomass; with split due to data calculation issues
7. US larval 1989-1994 weighted mean compared to 3+ midyear biomass; with split due to data calculation issues
8. Canadian larval 1987-1995 mean compared to 3+ midyear biomass
9. US 1999-2002 acoustics survey abundance compared to 3+ midyear biomass
10. Canadian spring bottom trawl 1986-2002 ages 2-8 (with 1993 and 1994 surveys excluded due to lack of complete survey coverage)
Details of the ‘Complex Final Fit’ ADAPT formulation and input parameters are presented in Appendix III. Based on the above specifications and inputs, the total stock (age1+) biomass in 2003 was estimated to be 692 kt; 3+ biomass, 629 kt; and SSB 599 kt (Figure 14.31).
The MSR for the overall formulation was 1.106, while the average for each of the indices was 1.023, substantially better than the initial formulation. However, residual plots by year and age still exhibit patterns that were either all positive or all negative by year (Fig. 14.13, 14.14, 14.15). Age by age plots by survey of observed and predicted values show poor correspondence for most surveys and ages (Fig. 14.16, 14.17, 14.18, 14.19, 14.20, 14.21). Diagnostic plots of the aggregated survey indices of observed and predicted also show poor fits (Fig. 14.22). Diagnostic plots of the survey q’s by age show time trends and are not consistent over the series (Fig. 14.23, 14.25, 14.27, 14.29).Overall survey q’s by age were variable; dome shaped for the US fall and US spring showing lower catchability for younger and older ages and increasing over the ages for the US winter and Canadian spring surveys (Fig 14.24, 14.26, 14.28, 14.30).
The ‘final run’ formulation still has severe retrospective patterns starting in 1999 but slightly improved in 2001 and 2002 (Figure 14.32). The reasons for these patterns may be due to a change in the survey q’s since 1996. When annual PR patterns in the fishery were investigated a prominent dip (or saddle-back) was detected with age 2 fully recruited, ages 3-5 fully recruited and then full recruitment from age 6 onwards (Fig. 14.33, 14.34). The trends were found to be consistent over the time series (1967-present) but there was increased variability in most recent years, especially in the older age groups.
14.8 Complex Final Fit Adapt formulation projections
Deterministic projections were conducted using the bias adjusted VPA results. Two F scenarios were considered, F = 0.2, (approximating the F estimated by the VPA in recent years and corresponding roughly to an FMSY proxy), and F = 0.1.
Landings in 2003 were assumed to be 100,000t, approximately equal to those in 2002. The fishery partial recruitment was assumed to be 0.01 for age 1 and 1.0 (full recruitment) for ages 2 and older. Fishery and stock weights at age were set as the average from 1992 to 2002. Natural mortality was assumed to be 0.2.
A catch of about 100,000t in 2004 corresponds to an F = 0.2 and would generate a decrease in 3+ biomass from about 550,000t in 2004 to about 500,000t in 2005 (Table 14.4B). With an F = 0.1 in 2002, the resulting catch is about 60,000t and the 3+ biomass stays constant at about 550,000t (Table 14.4A).
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