CONTENTS Introduction The Fishery Stock Abundance and Biomass Indices Mortality Estimation of Fishing Mortality Rates and Stock Size Biological Reference Points Conclusions Acknowledgments Literature Cited
Northeast Fisheries Science Center Reference Document 06-02
by Ralph K. Mayo and Laurel A. Col
The 2005 Assessment of the Gulf of Maine Atlantic Cod Stock
National Marine Fisheries Serv., Woods Hole Lab., 166 Water St., Woods Hole, MA 02543
Web version posted March 14, 2006Citation: Mayo RK, Col LA. 2006. The 2005 assessment of the Gulf of Maine Atlantic cod stock. US Dep Commer, Northeast Fish Sci Cent Ref Doc. 06-02; 109 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.
ABSTRACT: The status of the Gulf of Maine Atlantic cod (Gadus morhua) stock is reviewed, and terminal year VPA estimates of 2004 fishing mortality, spawning stock biomass and the survivors in 2005 are presented. Precision estimates of the 2004 fishing mortality and spawning stock biomass estimates for Gulf of Maine cod are also provided.
The 2005 assessment is based on several sources of information including: the age composition of USA commercial and recreational landings, commercial fishing vessel trip reports (VTR), Northeast Fisheries Science Center (NEFSC) sea sample data, MRFSS estimates of recreational harvest, NEFSC and Massachusetts Division of Marine Fisheries (DMF) spring and autumn research vessel survey data, and standardized USA commercial fishing effort data. This assessment updates the analyses presented in the 2001 assessment of the Gulf of Maine cod stock reviewed at SAW 33 (NEFSC 2001a, b, Mayo et al. 2002) and those reviewed in 2002 at the Groundfish Assessment Review Meeting (GARM) (NEFSC 2002b). The analyses presented herein were recently reviewed at the 2005 GARM (NEFSC 2005).
Total landings of Gulf of Maine cod decreased from 4,156 metric tons (mt) in 1998 to 1,636 mt in 1999, increased to 3,730 mt in 2000 and have since remained between 3,800 and 4,400 mt. The sharp decline in landings between 1998 and 1999 and the subsequent increase in 2000 likely reflect the imposition of very low trip limits during 1999 and the subsequent relaxation of these limits in early 2000. It is probable that the extent of discarding increased sharply in 1999 in response to the reduced trip limits.
Commercial landings per unit of standardized effort declined steadily between 1982 and 1987, increased during 1988-1990, but declined sharply in 1992 and remained low in 1993. CPUE estimates have not been included in the assessment model since 1994 because of uncertainty in the effort units between the interview-based estimates and the VTR-based estimates as well as recent management initiatives including trip limits and closed areas imposed to control fishing mortality. Fishery-independent spring and autumn bottom trawl surveys conducted by the NEFSC have documented a steady decline in total stock biomass since the 1960s; the largest decreases occurred during the 1980s. Although the most recent indices suggest a modest increase since the early 1990s, the Gulf of Maine cod stock biomass remains low compared to the 1960s and 1970s. Except for the 1998 year class, recruitment during the 1990s has been well below the long-term mean. The 1999 and 2000 year classes are weak but there are signs that the 2003 year class may be well above average.
Total stock biomass (ages 1+) declined from a peak of 41,966 mt in 1990 to 15,867 mt in 1997, but increased to 29,000 mt in 2001 and has remained at about that level through 2004. Spawning stock biomass (SSB) declined from over 24,200 mt in 1990 to a low of 11,128 mt in 1997; SSB increased to 25,369 mt in 2002 but declined to 20,549 mt in 2004 due to very low abundance of the poor 1999 and 2000 year classes. Fully recruited instantaneous fishing mortality (F, ages 4-5) remained close to or above 1.0 between 1983 and 1997, but declined to 0.35 by 2002 and has since increased to 0.58 in 2004. SSBmsy is now estimated to be 82,830 mt with a corresponding Fmsy of 0.23, (fully recruited, ages 4+) (NEFSC 2002a). With respect to the age-structured MSY-based reference points, 2004 spawning stock biomass is well below ½ SSBmsy, and 2004 F is 2.5 times Fmsy.INTRODUCTION
Atlantic cod (Gadus morhua) in the Gulf of Maine region have been commercially exploited since the 17th century, and reliable landings statistics are available since 1893. Historically, the Gulf of Maine fishery can be separated into four periods (Figure 1): (1) an early era from 1893-1915 in which record-high landings (> 17,000 mt) in 1895 and 1906 were followed by about 10 years of sharply-reduced catches; (2) a later period from 1916-1940 in which annual landings were relatively stable, fluctuating between 5,000 and 11,500 mt, and averaging 8,300 mt per year; (3) a period from 1941-1963 when landings sharply increased (1945: 14,500 mt) and then rapidly decreased, reaching a record-low of 2,600 mt in 1957; and (4) the most recent period from 1964 onward during which Gulf of Maine landings have generally increased but have declined steadily since the early 1990s. Total commercial landings doubled between 1964 and 1968, doubled again between 1968 and 1977, and averaged 12,200 mt per year during 1976-1985 (Table 1). Gulf of Maine cod landings subsequently increased, reaching 17,800 mt in 1991, the highest level since the early 1900s. Total landings declined sharply in 1992 to 10,891 mt, and have since decreased steadily to 1,636 mt in 1999 before increasing to 3,730 mt in 2000. Total commercial landings have since fluctuated between 3,800 and 4,400 mt. Landed cod from the recreational sector have represented between 6 and 39 percent of the combined commercial and recreational harvest.
This report presents an updated and revised analytical assessment of the Gulf of Maine cod stock (NAFO Division 5Y) for the period 1982-2004 based on analyses of commercial and recreational data through 2004 and research vessel survey data through spring 2005. From the early 1960s through 1993, information on the catch quantity by market category was derived from reports of landings transactions submitted voluntarily by processors and dealers. More detailed data on fishing effort and location of fishing activity were obtained for a subset of trips from personal interviews of fishing captains conducted by port agents in the major ports of the Northeast. Information acquired during the course of these interviews was used to augment the total catch information obtained from the dealer. Procedures for collecting and processing commercial fishery data in the Northeast were revised after 1993.
Beginning in 1994, data on number of hauls, average haul time, and catch locale were obtained from logbooks submitted to the National Marine Fisheries Service (NMFS) by operators fishing for groundfish in the Northeast under a mandatory reporting program. Estimates of total catch by species and market category were derived from mandatory dealer reports submitted on a trip basis to NMFS. Catches (landed and discarded portions) by market category were allocated to stock based on a matched subset of trips between the dealer and logbook databases. Data in both databases were stratified by calendar quarter, port group, and gear group to form a pool of observations from which proportions of catch by stock could be allocated to market category within the matched subset. The cross-products of the market category by stock proportions derived from the matched subset were employed to compute the total catch by stock, market category, calendar quarter, port group, and gear group in the full dealer database. A full description of the proration methodology and an evaluation of the 1994-1996 logbook data is given in Wigley et al. (1998) and DeLong et al. (MS 1997).
An initial analytical assessment of this stock (Serchuk and Wigley 1986) was presented at the Seventh NEFC Stock Assessment Workshop in November 1988 (NEFC 1989) and subsequent assessments were reviewed at the 12th, 15th, 19th, 24th and 27th Northeast Regional Stock Assessment Workshops in June 1991, December 1992, December 1994, June 1997 and June 1998 (NEFSC 1991, 1993, 1995, 1997, 1998; Mayo 1995, 1998; Mayo et al. 1993, 1998). Interim assessments were reviewed by the Northern Demersal Working Group in July 1999 (NEFSC 2000) and August 2000 (NEFSC 2001a). The most recent peer review of this assessment (Mayo et al. 2002) occurred at the 33rd Northeast Regional Stock Assessment Workshop in June 2001 (NEFSC 2001b) and an updated assessment through 2001 was reviewed at the Groundfish Assessment Review Meeting in October 2002 (NEFSC 2002b, Mayo and Col 2002). The present assessment was reviewed at the second Groundfish Assessment Review Meeting in August 2005 (NEFSC 2005, Mayo and Col 2005).THE FISHERY
Fishing for Gulf of Maine cod had been managed under international treaty prior to 1977 and by domestic management authority since 1977 (Appendix 1). Annual Total Allowable Catches (TACs) were first established under the International Commission for the Northwest Atlantic Fisheries (ICNAF) for Division 5Y (i.e., the Gulf of Maine) cod in 1973 (Serchuk et al. 1994). The TAC remained at 10,000 mt from 1973-1975; the 1976 TAC was reduced to 8,000 mt and the TAC proposed for 1977 was reduced further to 5,000 mt.
Following implementation of the Magnuson Fishery Conservation and Management Act (FCMA) in 1977, management of this stock fell under the auspices of the New England Fishery Management Council. TACs were carried forward for the first few years under the Fishery Management Plan for Atlantic Groundfish, and were distributed among vessel tonnage classes and quarters of the years until 1982 when the “Interim” Plan for Atlantic groundfish was implemented. This plan eliminated all direct catch controls (quotas) and established mesh size and minimum landing size regulations as the primary regulatory measures for cod, haddock and yellowtail flounder.
Management of the Gulf of Maine cod fishery has been carried out since 1985 under the Northeast Multispecies Fishery Management Plan (FMP). This plan and its Amendments 1 through 4 essentially carried forward the regulatory measures originally implemented in 1982 under the “Interim” Plan (Appendix 1). Beginning in 1994 with the implementation of Amendment 5, the primary goal of the FMP became a reduction in fishing mortality for 5 key monitoring stocks. This was to be achieved through a combination of reductions in days at sea (DAS) usage and, under Amendment 7, an additional series of seasonal and year-round area closures oriented primarily towards Gulf of Maine stocks. Amendment 13, implemented in May 2004, added additional restrictions on Days at Sea usage and further defined the use of A DAS and B DAS to allow fishing on stocks in relatively good condition while still restricting effort on stocks of concern (including Gulf of Maine cod).
Commercial Fishery Landings
Annual commercial landings data for Gulf of Maine cod in years prior to 1994 were obtained from trip-level detailed landings records maintained by the Northeast Fisheries Science Center, Woods Hole, Massachusetts (1963-1993) and from summary reports of the Bureau of Commercial Fisheries and its predecessor the U.S. Fish Commission (1895-1962). Beginning in 1994, landings estimates were derived from dealer reports prorated to stock based on the distribution of landed catch reported in fishing vessel logbooks as described above.
Total commercial landings in 2004 were 3,798 mt, slightly below those from 2001-2003 but approximately 132% greater than in 1999 (Table 1, Figure 1). Since 1977, the USA fishery has accounted for all of the commercial catch. Canadian landings reported as Gulf of Maine catch after 1977 are believed by Canadian scientists to have been misreported catches from the Scotian Shelf stock (Campana and Simon 1985; Campana and Hamel 1990) and have thus been excluded. Although otter trawl catches account for most of the landings (averaging between 50 and 59% between 1993 and 2003), the otter trawl percentage has declined considerably compared to the period prior to 1993. Most of this change can be attributed to an increase in the percentage of cod taken by sink gillnets since 1993, although the percentage from combined handline and line trawls also increased substantially during the 1990s (Table 2). The percentage landed by otter trawls declined further in 2004 to 44%.
Commercial Fishery Discards
Discard rates have been routinely calculated for Gulf of Maine cod by quarter and gear from NEFSC sea sampling data collected since 1989 (Table 3). Discard and kept components of the catch were summed for all observed tows within each gear type occurring in Division 5Y, and the ratio of the discarded- to-kept quantity was applied to landings for the corresponding quarter and gear type within each year. Data were available for otter trawls, shrimp trawls (through 1993 only), and sink gillnets.
For otter trawl gear, discard-to-kept ratios (D/K) and absolute quantities of discarded cod declined from relatively high values in 1989 and 1990 to relatively low levels from 1991 through1998 as D/K ratios generally fluctuated between 0.002 and 0.155. In the shrimp trawl fishery, D/K ratios remained high during 1989-1991, but declined substantially in 1992 and remained negligible in 1993. Shrimp trawl sampling data for 1994-2004 were minimal; therefore, landings by this gear component were not distinguished from all other otter trawls in the proration scheme employed to derive the landings by stock for the present assessment. Consequently, discard estimates from both otter trawl and shrimp trawl gear were combined for the 1994-2004 period. D/K ratios from the sink gill net fishery remained relatively low between 1989 and 1998, generally in the range of 0.05 or so. In 1999, discard ratios increased sharply for otter trawl and sink gill nets during the second and third quarters, declined from these peak levels in the fourth quarter, but continued to remain relatively high through 2000-early 2004 compared to pre-1999 ratios. Ratios declined in the second quarter of 2004 after trip limits were relaxed.
Discards of Gulf of Maine cod ranged from 154 mt in 1998 to 3,598 mt in 1990 (Table 3). Discards exceeded 1,000 mt in each year between 1989 and 1991 before declining steadily since 1992. The relatively high discard rates calculated for otter trawl and shrimp trawl gear during 1989-1991 coincide with recruitment of the strong 1987 year class to the small mesh shrimp trawl gear and then the large mesh general otter trawl gear. Available length composition data for these gear types suggest that most of the discarded cod were about 30-50 cm with a mode around 40 cm. Discards emanating from these two gears are the likely result of minimum size regulations. In contrast, the relatively low, but persistent, discards of cod in the gillnet fishery comprised fish of all lengths, up to 125 cm. The larger size range reflects discarding resulting from minimum size regulations as well as poor fish quality (in the case of the larger, marketable cod). Discards in 1999 were estimated to be 2,630 mt, one of the highest in the data series, due to the imposition of low trip limits. Estimated discards declined to 1,170 mt in 2000 as trip limits were relaxed to 400 lbs/day in early 2000, and fluctuated between 1,500 and 2,000 mt between 2001 and 2003 before declining to about 575 mt in 2004 when the trip limit was increased to 800 lbs/day in the second quarter (Table 3).
To further evaluate discarding during 1999 - 2004 when low trip limits were imposed, all available vessel trip report (VTR) records were examined from trips reporting some catch of cod in the Gulf of Maine. All trips from vessels which never reported any discard were excluded from the discard analyses. The VTR data were treated in the same manner as the sea sample data. Comparisons between discard estimates based on Sea Sample and VTR data are provided in Tables 4 and 5.
The discard estimates of Gulf of Maine cod derived from the two data sets have been reasonably close to each other, with annual differences of 3-18 percent on the estimates of total commercial catch. Each method and data set has advantages and limitations. The sea sample data are less subjective since they are based on consistent interpretation by a small group of individuals. But these data have been sparse in some years, leading to considerable imprecision. The VTR data provide considerably more observations, which may increase precision, but these data may have been influenced by possible reporting bias in response to severe management actions beginning in 1999. Note that the discards estimated from the VTR data exceed the estimates derived from the Sea Sample data in 1999 and 2000, while the opposite is true from 2001-2004 (Tables 4 and 5).
There is no objective basis to select the results obtained from either data set. Thus, the SAW 33 SARC Panel concluded that both estimates could be used to derive annual estimates to the nearest 500 mt increment. This approach has continued in the present assessment and the results are given in Table 4 for the 1999-2004 period. While it is acknowledged that this approach is subjective, it has been accepted by the SAW33 SARC Panel and the 2 meetings of the GARM in 2002 (NEFSC 2002b) and 2005 (NEFSC 2005).
Commercial Fishery Sampling Intensity
A summary of USA length frequency and age sampling of Gulf of Maine cod landings during 1982-2000 is presented in Table 6. USA length frequency sampling averaged one sample per 155-200 mt landed during 1983-1987 but the sampling intensity was reduced in 1990 (1 sample per 387 mt) and 1993 (1 sample per 360 mt), and the absolute level of sampling was extremely low in 1993. Overall, sampling improved slightly in 1994 and 1995, but the seasonal distribution was uneven and poorly matched to the landings. Sampling improved substantially in 1996 and remained equally high in 1997, reaching all-time highs in terms of both absolute number of samples and samples per ton landed in both years.
Most of the USA samples have been taken from otter trawl landings, but sampling and the estimation of length composition is stratified by market category (scrod, market, and large). Although the length composition of cod differs among gear types (primarily between otter trawl and gillnet), the length composition of cod landings within each market category is virtually identical among gear types.
Beginning in 1998, the quality of commercial port sampling for Gulf of Maine cod declined considerably. The total number of samples taken declined sharply in 1998 and again in 1999, a possible outcome of the very low trip limits imposed in 1999. Although the number of samples collected increased in 2000, the distribution by market category was out of phase with actual landings. In particular, the number of ‘Large’ market category cod samples has diminished to the point that the representation of the older age groups may have been somewhat compromised. Sampling improved considerably in 2001, especially in the case of large market category cod as a result of augmented sampling effort from the Massachusetts Division of Marine Fisheries (DMF) samplers, and has remained high (less than 100 mt per sample).
Of the 201 samples collected in 2004, 46 were scrod samples (23%), 65 were market (32%), and 90 were large (45%). Compared with the 2004 market category landings distribution by weight (scrod: 3%; market: 41%; large: 53%) (Table 7), sampling in 2004 over-represented the scrod category and well represented the market and large categories.
As well, the seasonal distribution of samples became skewed for several years such that, although there appears to have been sufficient numbers of samples taken, there has been insufficient sampling in some quarters and half-years, requiring pooling of samples on an annual basis.
Commercial Landings Age Composition
The age composition of landings during 1982-1993 was estimated, by market category, from monthly length frequency and age samples, pooled by calendar quarter. Quarterly mean weights, by market category, were obtained by applying the NEFSC research vessel survey length-weight equation for cod:
ln Weight (kg, live) = -11.7231 + 3.0521 ln Length (cm)
to the quarterly market category sample length frequencies. Computed mean weights were then divided into quarterly market category landed weight to derive estimated numbers landed by quarter, by market category. Quarterly age/length keys were applied to the quarterly market category numbers at length distributions to provide numbers at age. These results were summed over market categories and quarters to derive the annual landings-at-age matrix (Table 8a).
Age composition of landings from 1994 through 2004 was estimated in a manner similar to that employed for the 1982-1993 estimates except that samples and landings were, at times, pooled to semi-annual or annual resolution because of the uneven distribution of length and age samples by quarter (Table 6). Semi-annual pooling was required for the 1st and 2nd quarters of 1994 because of incomplete sampling coverage of scrod and large cod landings; in 1995, samples were pooled in both semi-annual periods due to the absence of large cod samples and the sparse coverage of market cod in quarters 1 and 3. Quarterly allocation of samples to landings was achieved for all market categories in 1996 and 1997, but semi-annual and annual pooling was required in 1998 and annual pooling was required in 1999 and 2000. Quarterly stratification resumed in 2001 and continued through 2004.
Gulf of Maine cod landings have been generally dominated by age 3 and 4 fish in numbers and by ages 3, 4, and 5 in weight. Representation of age 2 cod was relatively high in the early 1980s but, in response to a series of minimum mesh size increases during the 1990s, age 2 fish have gradually all but disappeared from the landings. Cod from the strong 1987 year class predominated from 1990 through 1992 but, by 1993, fish from the 1990 year class accounted for the greatest proportion of the total number landed. In terms of weight, the 1993 landings were equally distributed between the 1987 and 1990 year classes. In 1993 these two year classes accounted for approximately 70% of the total number and weight landed. From 1994 through 1996, landings were dominated by age 4 cod in both number and weight. In 1997 age 5 fish were dominant in terms of both number and weight, reflecting the higher abundance of the 1992 year class. Although traditionally low in terms of their contribution to the total landings, age 10 and 11+ fish were absent for several years during the 1990s, and numbers of age 8 and 9 fish have also been unusually low (Table 8a). Although this pattern may be partly a result of the poor sampling of 'Large' category cod, especially in recent years, a trend towards fewer older fish in the landings began in 1991.
More recently, the 1998 year class has dominated the landings at ages 3 through 6 in 2001 through 2004, respectively. As well, the proportion of cod older than age 7 has begun to increase. In 2004 ages 8 and older represented 15% of the landed weight, more than the 7-13% contribution during 1982-1984 and the very low 1% contribution in 2000. Although the fraction of age 8 and older fish has begun to increase, the period of low representation during the 1990s precludes the use of these older fish in the assessment model. Therefore, the age 7+ group was continued in the present assessment model (Table 9a and 9b).
Adjustment of the 1999 - 2004 Commercial Landings at Age
The fishery for Gulf of Maine cod was affected by management actions that began in 1999 and have continued into 2004. The implementation of extremely low trip limits in 1999 likely precipitated a substantial increase in the amount of cod discarded compared to previous years, as noted above. While these trip limits were relaxed to some extent in subsequent years, a substantial portion of the total catch continues to be discarded. Consequently, the 1999-2004 estimated commercial landings at age presented in Tables 9 and 10 do not reflect the full extent of removals from the stock by the fishery. Therefore, prior to inclusion in the VPA, the 1999-2004 landings estimates had to be adjusted upwards at each age by the ratio of total estimated catch biomass (landings + discard) to the landed catch biomass.
This approach assumes that the age composition of the discarded component of the catch is the same as the landed component. In most cases where discards occur because the mesh selectivity in the fishery is not consistent with minimum landing size regulations, it is necessary to estimate the size and age composition of the discarded component separate from the landed component. In general, the discards comprise the smaller, younger fish compared to those that are landed. However, where regulatory discards are generated as a result of extremely low trip limits (as occurred during 1999-2004), it is presumed that cod of all sizes and ages are discarded without prejudice. Examination of the 1998, 1999 and 2000 kept and discarded length composition samples from the NEFSC Sea Sample database support this assumption. The sizes of discarded cod in 1998, when trip limits were considerably higher, were primarily below the 48 cm minimum landing size and the sizes of retained cod were approximately the same as those observed in the commercial port samples. In 1999 and 2000, however, the sizes of discarded and retained cod were generally the same, well above the minimum landing size and similar to those observed in the 1999 commercial port samples. Therefore, the 1999 -2004 commercial landings at age estimates in Table 9 were multiplied by the discard adjustment factors in Table 4 before inclusion in the VPA catch at age matrix (see pages 3 and 4 for discard estimation procedures).
Commercial Landings Mean Weights at Age
Mean weights at age in the landings during 1982-2004 are given in Table 8b for ages 1-11+ and in Table 9b and Figure 2 for ages 2-7+. Based on landings patterns these are considered mid-year values. Mean weights of age 2 and 3 cod have increased since about 1992 and mean weights of age 4 cod have increased since 2000, reflecting reduced partial recruitment of younger fish to the fishery, while the average weights for age 5 and 6 cod have fluctuated without trend. Mean weights for ages 9 and older fluctuate considerably and are particularly sensitive to sampling variability. However, when the older ages are aggregated into a single age 7+ group, a marked decline is evident in mean weights during the 1990s, followed by an increase beginning in 2001. This likely reflects the decline of older fish in the landings during the 1990s and recent increases in the proportion of the stock ages 7 and older.
The generally higher mean weights of age 2 and 3 cod since the mid 1990s may be related to an increase in minimum codend mesh size from 140 mm (5.5 in.) to 152 mm (6 in.) in 1994, while the increase in mean weights of age 4 cod occurred after an increase in the minimum codend square mesh to 165 mm (6.5 in.) in May, 1999.
Recreational Fishery Catches
Estimates of the recreational cod catch were derived from the Marine Recreational Fishery Statistics Survey (MRFSS), which has been conducted annually since 1979. Gulf of Maine cod catches were estimated assuming that catches of cod recorded in the intercept survey were removed from the ocean in statistical areas adjacent to the reported state or county of landing. The MRFSS database has been revised, resulting in adjusted catch estimates for the years 1981 through 1997. In addition, the Gulf of Maine cod catch from 1994 to 2004 were re-estimated using a revised port stratification scheme to better reflect sampling allocation in New England ports. Information on the catch prior to 1981, which has not been revised, is included in Table 10 to provide a longer-term perspective. Further information on the details of the allocation scheme and sampling intensity are given in NEFSC (1992). Estimates of the total Gulf of Maine cod recreational catch as well as the retained portion of the catch (i.e., excluding those caught and released) are provided in Table 10.
The quantity of cod retained generally exceeded 80% of the total recreational catch during 1981 through 1991, but has averaged less than 40% since 1992. The estimated total cod catch (including those caught and released) declined from over 5,000 mt in 1980 and 1981 to less than 2,000 mt between 1983 and 1986, increased to over 3,500 mt in 1990 and 1991, and fluctuated between 1,200 and 3,300 mt between 1992 and 1999. The total catch increased sharply beginning in 2000, reaching 7,700 mt in 2003 before declining to 3,500 mt in 2004. Trends in the weight of cod retained were similar to the total catch, but the magnitude of the removals is considerably less, especially since 1992.
The fraction of the total landings (commercial and recreational) taken by the recreational sector (retained cod) generally ranged from 6 to 20% between 1981 and 1998. The recreational component increased to 35 percent in 1999 and has remained between 25 and 39 percent through 2004.
Recreational Fishery Sampling Intensity
Information on the length frequency sampling levels of Gulf of Maine cod taken in the recreational fishery is also provided in Table 10. Overall, sampling of cod taken by recreational gear is poor, averaging less than 1 sample per 1,000 mt removed. Sampling of the recreational fishery improved during the 1990s, but has been relatively low in recent years. The age composition of the 1982-1996 recreational landings was derived for the 1997 assessment (Mayo 1998) but, given the highly variable sampling, these data were not formally included in the VPA conducted in 1997 (NEFSC 1997; Mayo 1998). However, given the recent increase in the proportion of the total landings accounted by the retained recreational catch, the age composition of the recreational landings in 1997-2004 was estimated for the current assessment and the 1982-2004 recreational landings at age estimates were incorporated into the total catch at age.
Recreational Fishery Landings Age Composition
Given the limited sampling coverage in this sector of the fishery, the estimation of the number of Gulf of Maine cod caught by length and age required that samples be pooled on an annual basis. The low inter-seasonal variability displayed by the sample length composition data supported this approach. Differences between the party/charter and private/rental fishing modes are also minimal. Therefore, estimates of the age composition of cod retained by the recreational sector were derived from the length composition data applied to the retained numbers of cod based on pooled annual length frequency samples from Gulf of Maine trips. Only the retained numbers of cod were included because the intercept sampling may not accurately reflect the size composition of the released cod. Age-length keys obtained from sampling the commercial landings, augmented by age samples from NEFSC bottom trawl surveys for cod less than 40 cm, were applied to the numbers retained at length on an annual basis to derive the numbers retained at age (Table 11a).
During the 1980s, Gulf of Maine cod recreational landings in numbers were dominated by age 3 fish with age 2 fish next in importance. Following the increases in minimum retention size in 1989 and again in 1996, the proportion of age 2 cod declined, and the age composition of the recreational landings now resembles that from the commercial fishery with ages 3, 4 and 5 predominant (Tables 9a and 11a). The strong 1987 year class dominated the recreational catch in 1990, 1991 and 1992. The 1992 year class can also be tracked in the estimated catch at age between 1995 and 1999 and the 1998 year class predominates after 2000. Ages 3 and 4 cod generally predominate in terms of weight caught, although the 1987, 1992, and 1998 year classes predominated at age 5 in 1992, 1997 and 2003 respectively.
Recreational Landings Mean Weights at Age
Mean weights at age were obtained by applying the NEFSC research vessel survey length-weight equation for cod to the numbers retained at age on an annual basis:
ln Weight (kg, live) = -11.7231 + 3.0521 ln Length(cm)
Mean lengths and weights at age of cod landed by the recreational sector (Table 11b) are consistently lower than those taken in the commercial fishery. This pattern persists through age 5, but for ages 6 and older, mean weights are highly variable due to the relatively poor sampling of fish at the larger sizes combined with the lack of market category stratification. Despite this variability, patterns present in the commercial landings mean weights are also evident in the recreational landings, i.e., an increase in the mean weight of age 2 and 3 cod beginning in the mid-to-late 1990s, apparent stability of mean weights of age 4, 5 and 6 cod, and an indication of a similar decline and recent increase in the mean weight of the 7+ group fish.
Total Landings Age Composition
Estimates of the age composition of total cod landings (Table 12a) were derived by combining the separate age composition estimates obtained for the commercial (Table 9a) and recreational fisheries (Table 11a). Given the general similarities between the age compositions estimated for the commercial and recreational sectors, the total age composition reflects the same dominant year classes and age structure over time. In general, ages 3, 4 and 5 have predominated; the 1987 year class dominated the total landings in 1990, 1991 and 1992, the 1992 year class can also be tracked between 1995 and 1999, and the 1998 year class dominates the period after 2000.
Total Landings Mean Weights at Age
Mean lengths and weights at age of cod landed by the combined commercial and recreational sectors (Table 12b) are intermediate to those obtained from the individual sectors. Mean weights at age are highly variable for the older ages due to the relatively poor sampling of fish at the larger sizes. Mean weights at age for calculating stock biomass at the beginning of the year are provided in Tables 13 and 14 for ages 1-11+ and 1-7+, respectively. The age 11+ values were derived from the commercial landings mean weight at age data (Table 9b) and the age 7+ values were derived from the total (commercial and recreational) landings mean weight at age data (Table 12b) using procedures described by Rivard (1980, 1982).STOCK ABUNDANCE and BIOMASS INDICES
Commercial Catch Rates
Trends in commercial landings per unit effort (LPUE) and fishing effort for the period 1965-1993 and 1994-1996 were reported by Mayo (1998). Given the uncertainty in reported fishing effort since 1994, the 1994-1997 LPUE data were not formally included in the VPA conducted in 1998 (NEFSC 1998; Mayo et al.1998). Recent management actions, including imposition of trip limits and rolling closures also make interpretation of 1997-2004 LPUE inconsistent with previous years. Until effort units are resolved in the commercial fishery database, no further treatment of the LPUE series after 1993 is meaningful.
The 1982-1993 age composition of the landings corresponding to the effort sub-fleet as presented by Mayo et al. (1994) was used with the updated standardized effort estimates to calculate a revised LPUE-at-age index. Numbers landed at age were estimated by applying quarterly commercial age-length keys to quarterly commercial numbers landed at length by market category. The LPUE-at-age indices were derived by dividing the estimated numbers landed at age by corresponding 1982 through 1993 standardized fishing effort. Further details regarding data selection, preparation and estimation procedures are provided in Mayo et al. (1994).
Research Vessel Survey Indices
Indices of cod abundance (stratified mean catch per tow in numbers) and biomass (stratified mean weight per tow in kilograms) developed from NEFSC and Commonwealth of Massachusetts Division of Marine Fisheries (MADMF) research vessel bottom trawl survey data, have been used to monitor changes and assess trends in population size and recruitment of cod populations off New England. Offshore (> 27 m) stratified random NEFSC surveys have been conducted annually in the Gulf of Maine in autumn since 1963 and in spring since 1968. Inshore areas of the Gulf of Maine (< 27 m) have been sampled during spring and autumn NEFSC and MADMF inshore bottom trawl surveys since 1978. For the NEFSC surveys, a "36 Yankee" trawl has been the standard sampling gear except during spring 1973-1981 when a modified "41 Yankee" trawl was used.
Prior to 1985, BMV oval doors (550 kg) were used in all NEFSC surveys; since 1985, Portuguese polyvalent doors (450 kg) have been used. Details on NEFSC survey sampling design and procedures are provided in Azarovitz (1981) and Clark (1981). The MADMF inshore bottom trawl sampling program is described in Howe et al. (1981). No adjustments in the survey catch-per-tow data for cod have been made for any of the trawl differences, but vessel and door coefficients have been applied to adjust the stratified means (number and weight per tow) as described in Table 15. Standardized mean catch-per-tow-at-age (number) indices are listed in Appendix 2: Tables 1 and 2, and catch-per-tow-at-age indices from DMF spring and autumn surveys are listed in Appendix 2: Table 3. The entire series of spring and autumn abundance and biomass indices was re-estimated for this assessment to better account for vessel effects between RV Albatross IV and RV Delaware II. Although the only major difference during the 1982-2004 assessment period occurred in 1987, many minor changes to the indices occurred in most years. Therefore, the indices listed in Table 15 and Appendix 2: Tables 1 and 2 may differ slightly from those provided in previous assessments.
NEFSC spring and autumn offshore stratified mean catch per tow indices for Gulf of Maine cod have generally exhibited similar trends throughout the survey time series (Table 15, Figure 3). Number-per-tow indices declined during the mid- and late 1960s, but between 1972 and 1985 have fluctuated as a result of a series of recruitment pulses. Sharp increases in the autumn number per tow indices reflect above-average recruitment of the 1971, 1973, 1977-1980, and 1985-1987 year classes at ages 1 and 2 (Appendix 2: Table 2; Figure 4). The sequential dominance of these cohorts at older ages is evident from number-per-tow-at-age values in both spring and autumn NEFSC surveys (Appendix 2: Table 2). Increases in the autumn 1994-1995 and spring 1996-1997 biomass indices may be attributed to somatic growth of fish from the 1992 year class which was the largest within a series of poor year classes.
More recently, autumn biomass indices have shown a modest increase, but the large value in 2002 is the result of a single very large tow that unduly influenced the calculation of the mean. The 1998 year class is equivalent to the 1992 year class, and the 2003 year class appears to be the strongest since the 1987 year class. Biomass indices in 2003 and 2004 suggest the population biomass remains above the low level of the early 1990s.
Spring NEFSC number-per-tow indices have remained relatively low since 1985, below the 1981-1984 average (Table 15); spring weight-per-tow indices have also remained relatively low through 1991, but the index increased substantially in 1992, and remained relatively high in 1993, due to a large contribution from the 1987 year class (Appendix 2: Table 1). The index declined markedly in 1994, remained low in 1995, increased moderately in 1996 and remained essentially unchanged in 1997. Spring weight-per-tow indices increased in 2001 and 2002 and remained high in 2003; however, the 2004 and 2005 indices suggest that current biomass remains relatively low (Figure 3).
Overall, the 1987 year class appears to have been one of the strongest ever produced; catch-per-tow indices for this cohort at ages 1-3 in the NEFSC autumn surveys and at ages 0 and 1 in the MADMF autumn inshore surveys were nearly all record-high values (Appendix 2: Tables 1 and 3). Based on MADMF and NEFSC survey catch per tow indices, the 1992, 1998 and 2003 year classes appear to be of moderate strength; the intervening year classes of Gulf of Maine cod, particularly the 1993, 1994, 1995, 1996 and 2000 year classes have been well below average (Figures 4 and 5).MORTALITY
Total Mortality Estimates
Estimates of instantaneous total mortality (Z) were calculated on an annual basis from 1964 through 2004. Total mortality was calculated from NEFSC survey catch per tow at age data (Appendix 2: Tables 1 and 2) for fully recruited age groups by the loge ratio of the pooled age 3+/age 4+ indices in the autumn surveys, and the pooled age 4+/age 5+ indices in the spring surveys. For example, the 1983 estimates were derived from:
Spring: ln ( age 4+ for 1983/ age 5+ for 1984)
Autumn: ln ( age 3+ for 1982/ age 4+ for 1983)
Different age groups were used in the spring and autumn analyses so that Z could be evaluated over the same year class within each year.
Values of Z derived from the spring surveys are generally comparable to those calculated from the autumn data (Figure 6). These values of Z exhibit considerable inter-annual variability due primarily to year effects in the surveys. When smoothed, however, the annual estimates suggest a pattern of increasing mortality during the1980s, with mortality remaining close to 1.0 through the mid 1990s. Total mortality declined during the late 1990s, but the most recent estimates suggest an increase.
Instantaneous natural mortality (M) for Gulf of Maine cod is assumed to be 0.20, the conventional value of M used for all Northwest Atlantic cod stocks (Paloheimo and Koehler 1968, Pinhorn 1975, Minet 1978).ESTIMATION OF FISHING MORTALITY RATES AND STOCK SIZE
Virtual Population Analysis Calibration
The ADAPT calibration method (Parrack 1986, Gavaris 1988, Conser and Powers 1990) was used to derive estimates of terminal fishing mortality (F) in 2004. As in previous assessments, age-disaggregated analyses were performed. Several comparative ADAPT calibrations were performed, each using the same NEFSC spring and autumn (ages 2-6) and MADMF spring (ages 2-4) and autumn (age 2) survey series. Due to uncertainty in the interpretation of effort units in the 1994-1997 VTR data, USA commercial LPUE abundance indices for ages 2-6 were included only through 1993. This change effectively removed the influence of the LPUE indices on the terminal year outcome of the calibration, while preserving the historic relationship employed in the previous assessment. As in the previous assessments (see Mayo et al. 1998), the USA commercial LPUE indices from 1982 through 1993 were derived from the catch at age corresponding to the effort sub-fleet used in the estimation of standardized fishing effort as described by Mayo et al. (1994).
The NEFSC and MADMF autumn indices were lagged forward by one age and one year whereby age 1-6 indices were related to age 2-7 stock sizes in the subsequent year for corresponding cohorts. All NEFSC and MADMF indices were related to January 1 stock sizes, and USA commercial LPUE indices were related to mid-year stock sizes. The 1982-2004 commercial landings at age as provided in Table 8a include true ages 2-10 as well as the 11+ group. In recent years, however, fish beyond age 7 have been poorly represented. As reported by Mayo (1995), a calibration run employing an extended age complement (true ages 2-9) produced high coefficients of variation (CV) on the terminal year stock size estimates and variable estimates of F on ages 7-9 in most years prior to the terminal year. Therefore, as in previous assessments of this stock (Mayo et al. 1993; Mayo 1995, Mayo 1998, Mayo et al. 1998, NEFSC 2000, NEFSC 2001, Mayo et al. 2001, Mayo and Col, 2002, 2005), all VPA formulations employed a reduced age range (ages 2-6 and 7+) (Table 9a).
Comparative VPA Calibrations
Recent changes to the VPA software and some revisions to the commercial and MRFSS data components were made in the intervening years between the current assessment and the previous assessment reviewed at the 2002 Groundfish Assessment Review Meeting (Mayo and Col 2002). These include:
- A change in the software from the FACT package to the NFT package. The major change is in the estimation of F on the oldest age.
- The discovery of additional commercial age samples for the years 2000 and 2001
- Revision to the MRFSS catch estimates for the period back to 1994
- Re-calculation of the NEFSC bottom trawl survey indices to better account for vessel effects between RV Albatross IV and RV Delaware II. Only minor differences occurred during the period of the VPA assessment.
To provide a bridge to the previous (2002) assessment, key results from six calibration runs with 2001 as the terminal year are compared in Table 16. The first set of results was taken directly from the 2002 assessment (Mayo and Col 2002) based on the FACT software package. The second set of results is based on running the NFT software on the 2002 assessment input file. The next 3 sets of results are also based on the NFT VPA software reflecting the following data changes: 1) re-estimated commercial catch at age for 2000 and 2001, 2) re-estimated recreational landings at age for 1994-2001, and 3) re-estimated NEFSC bottom trawl indices. The final set of results, also based on the NFT VPA software, incorporates all three data changes noted above.
In general, differences across all runs are minor (Table 16). The change in software did not produce a substantial change in the aggregate results (RSS, F and SSB), but there was a small difference in the estimated F at the fully recruited ages (4 and 5). Results from the remaining calibration runs incorporating the various data changes were very similar to the initial calibration run using the NFT software. Had these changes in software and data been in place in 2002, the results would have been very similar to what was obtained from the 2002 assessment. Based on these conclusions, the NFT software and the data revisions were carried forward with the updated catch and survey data for 2002-2004.
Final VPA Formulation
The ADAPT formulation employed in the final VPA calibration was the same as that used in the 2002 assessment except for the revisions to the software and data as noted above. This analysis provided direct stock size estimates for ages 2 through 6 in 2005 and corresponding estimates of F on ages 1 through 5 in 2004. Since the age at full recruitment was defined as 4 years in the input partial recruitment vector, the terminal year F on age 6 was estimated as the mean of the age 4 and 5 Fs; age 6 is also the oldest true age in the terminal year. In all years prior to the terminal year, F on the oldest true age (age 6) was determined from weighted estimates (by age group abundance, in numbers) of Z for ages 4 through 6. In all years, the age 6 F was applied to the 7+ group. Spawning stock biomass (SSB) was calculated at spawning time (March 1) by applying a series of period-specific maturity ogives. The present analysis used a maturity schedule which reflected earlier maturation beginning in 1994.
Residuals of the observed and predicted indices derived from the final VPA formulation (Figure 7) do not indicate any consistent trends over the period of the VPA, except for the MADMF age 2 autumn index.
Virtual Population Analysis Results
A complete listing of the final ADAPT VPA calibration is given in Appendix 3, and summary results, including age-specific estimates of instantaneous fishing mortality (F), stock size, mean biomass and spawning stock biomass, are presented in Table 17. All parameter estimates were significant (Appendix 3). Coefficients of variation on the stock size estimates at the beginning of 2005 ranged from 0.23 (age 4) to 0.36 (age 5), while CVs on the estimates of survey qs were between 0.03 and 0.17. Slopes of the abundance index-stock size relationships increased with age through age 6 for the NEFSC spring and autumn surveys and the USA commercial LPUE indices. The MADMF spring indices exhibited a decreasing trend in q between ages 2 and 4 (Appendix 3).
Average (ages 4-5, unweighted) fishing mortality fluctuated around 1.0 during the 1980s and early 1990s, but steadily declined thereafter to less than 0.4 in 2002 (Table 17, Figure 8). Fishing mortality in 2004 was estimated to be 0.58, an increase from 2002 and 2003. The increase in F in 2004 is notable because the two ages upon which the fully recruited average F is based (ages 4 and 5) represent the below-average 1999 and the very weak 2000 year classes. The spawning stock biomass of age 1 and older cod declined from 23,987 mt in 1982 to 15,302 mt in 1987. Following the recruitment and maturation of the strong 1987 year class, SSB increased to 24,261 mt in 1990 but declined to 10,797 mt in 1993, a 3-year reduction of 55% (Table 17, Figure 9). SSB increased to 14,996 mt in 1995 due to the growth and maturation of the 1992 year class, but declined again in 1996 and reached a record-low of 11,128 mt in 1997. SSB increased again beginning in 1998, reaching 25,369 mt in 2002, based primarily on growth and maturation of the 1998 year class (Table 17). Spawning stock biomass has since declined to 20,549 mt in 2004. Total stock size (ages 1+) has also declined sharply in recent years from 44.6 million fish in 1988 to an average of 14.0 million fish during 1996-1998 (Table 17), a decrease of 72%, but is estimated to have increased to about 32.2 million fish in 2004 due in large part to an initial estimate of the strength of the 2003 year class.
Since 1982, recruitment at age 1 has ranged from less than 4 million fish (1993, 1994, 1995, 2000 and 2002 year classes) to 25.2 million fish (1987 year class). Over the 1982-2004 period, geometric mean recruitment for the 1981-2003 year classes was 6.3 million fish. The 1987 year class is the highest in the 1982-2004 series and about twice the size of the next strongest year class. The 1992, 1998 and 2001 year classes were of moderate strength. (Table 17, Figure 9), while the 1993-1995, 1999-2000 and 2002 year classes were weak. The initial estimate of the strength of the 2003 year class (22 million) is very close to that of the 1987 year class.
Precision of F and SSB
A bootstrap procedure (Efron 1982) was used to evaluate the precision of terminal year estimates, by generating 600 estimates of the 2004 fully recruited fishing mortality rate and spawning stock biomass. Summary statistics for the bootstrap analyses are provided in Appendix 4, and the distributions of the bootstrap estimates and the corresponding cumulative probability curves are shown in Figure 10. The cumulative probability expresses the likelihood that the fishing mortality rate was greater than a given level (Figure 10a) or the likelihood that spawning stock biomass was less than a given level (Figure 10b), when measurement error is considered.
Coefficients of variation for the 2005 stock size (numbers) estimates ranged from 0.24 (age 4) to 0.49 (age 2), and CVs for qs among all indices ranged from 0.03 to 0.31 (Appendix 4). The fully-recruited fishing mortality in 2004 for ages 4+ was reasonably well estimated (CV = 0.20). The mean bootstrap estimate of F (0.60) was slightly higher than the point estimate (0.58) from the VPA. The 80% probability interval ranges from 0.46 to 0.76 (Figure 10a).
Although the abundance estimates for individual ages in 2005 had wide variances (CV = 0.24 to 0.49, the estimate of 2004 spawning stock biomass was robust (CV = 0.14). The bootstrap mean was 2.9% higher than the VPA point estimates (Appendix 4). The 80% probability interval for SSB ranges from 17,800 mt to 24,800 mt (Figure 10b). Despite this variability, current spawning stock biomass is estimated to have increased substantially from earlier record lows. In general, estimates of stock size and fishing mortality in the present assessment are estimated with about the same precision as in the previous assessment of this stock (Mayo and Col 2002).
The previous retrospective analysis for this stock was reported by Mayo et al. (2002). Although the formulation used in the present assessment is the same as in the previous assessment, changes in management measures for this stock during 2002-2004 may have imposed additional uncertainty in the interpretation of current stock status. Therefore, the retrospective analyses were conducted again, and the tabular results are given in Appendix 3.
Retrospective patterns with respect to terminal F are evident for Gulf of Maine cod in the most recent years (Figure 11), but the patterns are not consistent through time. Mean F (ages 4-5, unweighted) in the terminal year was under-estimated in 1995 and 1996 by the ADAPT calibration. The retrospective analysis presented by Mayo et al. (1998) indicated the same pattern, but was able to detect the opposite pattern (slight over-estimate of F) prior to 1994. The retrospective pattern in F reversed between 1997 and 1998 revealing a tendency to over-estimate F in 2000 and 2001. Most recently, there appears to be a slight tendency to again under-estimate F in 2002 and 2003. Convergence of estimates is generally evident within 3 years, and often within 2 years, prior to any given terminal year. The retrospective analysis provides additional evidence that current fishing mortality on this stock, although somewhat lower than in the 1980s and 1990s, is showing a tendency to increase again. The retrospective patterns for SSB and age 1 recruits show the opposite patterns, with an indication of over-estimation in 2002 and 2003.
Spawning Stock and Recruitment
The relationship between spawning stock biomass and recruitment for Gulf of Maine cod was examined from two perspectives. First, a traditional spawning stock-recruitment scatterplot (Figure 12a) was constructed over the period covering the 1982-2003 year classes. In addition, a survival ratio, expressed as recruits per unit of SSB (R/SSB) was also calculated for each year class (Figure 12b). The stock-recruitment trajectory indicates the position of the most recent levels of SSB and recruitment in the upper right corner of the plot illustrating the emergence of the 2003 year class from a relatively high spawning stock.
Survival ratios of pre-recruits up to age 1 are highest for the 1987, 1992, 1998 and 2003 year classes, the first two emerging from about average SSB, the 1998 year class from low SSB and the 2003 year class from relatively high SSB. Survival ratios were generally higher during the early-to-mid 1980s prior to the emergence of the large 1987 year class. Survival declined after the 1992 year class appeared, but increased in 1996 and 1998, declined thereafter and increased again in 2003.BIOLOGICAL REFERENCE POINTS
Yield and Spawning Stock Biomass per Recruit Reference Points
Yield and spawning stock biomass biological reference points were not revised during the preparation of the current assessment. The following reference points were taken from Mayo et al. (2002) and are the same as those applied by the Working Group on Biological Reference Points (NEFSC 2002a) and in the 2002 and 2005 GARM assessments (NEFSC 2002b, 2005; Mayo and Col 2002, 2005). These are:
F0.1 = 0.15
Fmax = 0.27
F20% = 0.36
The yield per recruit reference points (F0.1 and Fmax), and the SSB per recruit reference point (F20%) are slightly lower than those reported in the 1998 assessment (Mayo et al.1998).
Age-Structured Production Model Reference Points
MSY-based reference points were not revised during the preparation of the current assessment. An age-structured production model (Sissenwine and Shepherd 1987) was developed and applied by the Working Group on Biological Reference Points (NEFSC 2002a) using data and results from the 2001 assessment (Mayo et al. 2002). A complete description of the approach is given in Mayo et al. (2002) and NEFSC (2002a). The MSY-based reference points calculated using this method are:CONCLUSIONS
Gulf of Maine cod spawning stock biomass has increased since the late 1990s from 15,900 mt in 1997 to 20,500 mt in 2004, but the stock remains low relative to SSBMSY (82,830 mt). Fully recruited fishing mortality declined to about 0.35 in 2000 and 2001, but has since increased to 0.58 in 2004, indicating that F continues to remain very high relative to fully recruited F reference points (F0.1 = 0.15; Fmsy = 0.23; Fmax = 0.27). Thus, the stock remains in an overfished condition and overfishing continues to occur.
The 1987 year class is the strongest in the VPA assessment period (1982-2004). Subsequent year classes subsequent have been poor-to-average, except for the 1992, 1998, 2001 and 2003 cohorts. The 1993-1995, 1999-2000 and 2002 year classes are among the poorest in the VPA time series. Survival ratios (R/SSB) declined after 1998 but survival increased substantially with the appearance of the strong 2003 year class.
A retrospective pattern has existed in the VPA results for this stock, but the pattern has reversed several times over the past decade. At present, it appears that there is a slight tendency to under estimate fully recruited F and over estimate biomass in the terminal year.ACKNOWLEDGMENTSLITERATURE CITED
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