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Northeast Fisheries Science Center Reference Document 12-26

Estimating Observer Sea Day Requirements in the Mid-Atlantic Region to Monitor Loggerhead Sea Turtle (Caretta caretta) Interactions

by Kimberly T. Murray
NOAA Fisheries, Northeast Fisheries Science Center, 166 Water Street, Woods Hole, MA, 02543 USA

Web version posted November 5, 2012

Citation: Murray KT. 2012. Estimating Observer Sea Day Requirements in the Mid-Atlantic Region to Monitor Loggerhead Sea Turtle (Caretta caretta) Interactions. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 12-26; 10 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543-1026, or online at

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.

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Four species of sea turtles are found on the Northeast continental shelf, all of which are protected under the U.S. Endangered Species Act (ESA). These include the loggerhead (Caretta caretta), green (Chelonia mydas), Kemp's ridley (Lepidochelys kempii), and leatherback (Dermochelys coriacea) turtle. Having responsibility to implement programs to conserve marine life listed as endangered or threatened, the National Marine Fisheries Service (NMFS) places observers on commercial fishing vessels to gain information about interactions with listed species. Information about sea turtle interactions with fisheries may be gained via monitoring under the authority of the Endangered Species Act, Marine Mammal Protection Act (MMPA), or Magnuson-Stevens Act, though in the latter two cases sampling is designed for marine mammals or fish, not sea turtles.

Information collected by fisheries observers has been used to estimate the total magnitude of loggerhead or hard-shelled turtle interactions with commercial fisheries in the Mid-Atlantic region via model-based methods (Murray 2011, Warden 2011, Murray 2009). The model-based estimates pool several years of data across multiple fishing fleets within the same gear type, and account for gear or environmental correlates with turtle interactions rates over the entire Mid-Atlantic. The total estimated interactions with each gear type are subsequently allocated across fisheries, where a "fishery" is defined as a managed fish or invertebrate species landed, to provide information requested by the Northeast Regional Office (NERO) for ESA Section 7 consultations and other management actions (Warden 2011b, Murray 2009b). While green, Kemp's ridley, and leatherback interactions have occurred, there has been insufficient information to model the rates or magnitude of these species' interactions in commercial fishing gear. In addition, incidental captures of sea turtles have generally been rare on Georges Bank and in the Gulf of Maine, so analyses of turtle interactions to date have been limited to the loggerhead species only within the Mid-Atlantic.

This document presents the Protected Species Branch's (PSB) approach to estimating the magnitude of observer coverage needed to monitor loggerhead interactions rates in the Mid-Atlantic (i.e. west of 70°W) with 30% precision, based on available analyses (Warden 2011b, Murray 2009b, Murray 2011). While a 30% precision goal has been recommended by the National Working Group on Bycatch (NMFS 2004), monitoring requirements for a range of precision goals are also reported here. Estimated sea days to monitor loggerhead interactions are subsequently integrated with sea day projections for fish, estimated annually under the Standardized Bycatch Reporting Methodology (SBRM) Omnibus Amendment to fishery management plans of the Northeast region (Wigley et al. 2012). Sea day projections for non-loggerhead species outside the Mid-Atlantic are not computed in this report, but may be possible in the future should new information become available.


Fishery observer sea days are estimated for vessels using sink gillnet, bottom otter trawl (including scallop trawl), and scallop dredge gear, the primary gear types with documented loggerhead interactions in the Mid-Atlantic. Projected amounts of observer coverage for vessels fishing gillnet or trawl gear are derived from CVs around total estimated loggerhead interactions in specific fisheries (Warden 2011b, Murray 2009b), where a fishery is defined within each gear type by the highest amount (by weight) of landed fish or invertebrate species on a trip. For dredge gear, sea day projections are derived from CVs around estimated loggerhead interactions after chain mats were required in the Mid-Atlantic (Murray 2011). CVs reported in Murray (2011) are associated with bycatch rates on trips catching sea scallops.

Because the goal of monitoring is to achieve a 30% precision around loggerhead interaction rates with all fisheries within a gear type, the lower bound on coverage is the amount required to achieve a 30% CV for the fishery with the highest observer need. For example, if there are 2 fisheries (A and B) using gillnet gear in the Mid-Atlantic, and fishery A requires 100 days and fishery B requires 80 days, we would estimate 100 days needed for both fishery A and B. The maximum amount of projected coverage in a particular fishery is considered the desired level of annual sampling for that gear type in the Mid-Atlantic (i.e. it serves as an umbrella for monitoring in all other fisheries).

Data collected from these fisheries will eventually be pooled together within each gear type to estimate the total magnitude of loggerhead interactions. Therefore, the estimated sea days will remain in place each year until new bycatch estimates are published (currently every 5 years), and will be reassessed if there are major changes in the fishery (such as a gear modification).

Estimation of Desired Sea Days

The number of observed sea days needed to achieve a 30% coefficient of variation (CV), and other levels of precision, around an estimate of total loggerhead interactions was derived from (Rossman 2007):

equation (1)

where eq2= the amount of projected effort required to achieve a given precision level (converted to sea days); equ3= the precision levels around estimated interactions levels as reported in Warden 2011 (trawl), Murray 2009 (gillnet), or Murray 2011 (dredge);eq4= the observed effort as reported in the above publications; and eq5= the projected precision level to be achieved. This yielded a desired level of sampling for trips catching each fish or invertebrate species. The maximum amount of projected coverage across all the fisheries was considered the desired level of sampling to monitor turtle interactions for that gear type. Alternate levels of sampling under different precision goals are presented for the fishery that required the maximum amount of coverage, i.e. the "driver" fishery for overall monitoring levels. Projected effort amounts were converted to sea days based on species specific catch information on observed hauls or VTR trips (Appendix).


Desired Sea Days

An estimated 4,838 sea days are needed in the Mid-Atlantic to monitor loggerhead interactions with 30% precision in bottom otter trawl fisheries, based on estimated precision levels for trips catching species managed under the small and large mesh Northeast Multispecies FMPs (i.e. NE Multispp) (Table 1). Roughly 2,170 fewer days are required to estimate loggerhead interactions with 40% precision instead of 30% (Table 2, Figure 1a). An estimated 1,440 days are needed for 30% precision in Mid-Atlantic sink gillnet fisheries, based on estimated precision levels for trips catching spot (Table 1). Roughly 600 fewer days are needed in this fishery to estimate loggerhead interactions with 40% precision (Table 2, Figure 1b). Lastly, an estimated 1,293 days are needed for 30% precision in the Mid-Atlantic scallop dredge fishery, based on loggerhead bycatch precision levels after chain mats were implemented in the fishery (Table 1). Roughly 550 fewer days are needed in this fishery to estimate loggerhead interactions with 40% precision (Table 2, Figure 1c).


Monitoring at the Level of Fisheries

Observer coverage is estimated in this document at the level of individual fisheries in order to better meet the information needs for ESA Section 7 Consultations, prepared for Fishery Management Plan actions, or for management actions implemented at the fishery level. Typically, however, analyses of turtle interactions and allocation of observer coverage are not carried out at the fishery level. Instead, the magnitude of turtle interactions are estimated by gear type using several years of data, using models which account for gear or environmental correlates that significantly affect estimated interaction rates. Days are then allocated in proportion to the previous year's commercial fishing effort, in times and areas where turtles are likely to be present. In developing these models, analysts have found that the species landed or targeted does not significantly affect estimated interaction rates (i.e. Murray 2009, Warden 2011), so the "fishery" is not taken into account when describing variation in estimated interaction rates. Annual coverage amounts are estimated here at a finer resolution to collect information about interaction rates between turtles and a multitude of managed fisheries.

Within each gear type (trawl, dredge, or gillnet), fisheries that required the largest estimated number of sea days to reach the 30% precision goal became the "drivers" for monitoring all other fisheries. For instance, while 4,838 days were estimated for trips historically catching NE multispecies, this does not mean 4,838 days will be allocated entirely to this fishery. Instead, these 4,838 days monitor all fisheries using otter trawls for fish or scallops in the Mid-Atlantic. This approach is not expected to bias future estimated rates because analyses suggest that interaction rates do not vary by fishery. On the contrary, this approach helps ensure data are collected within a gear type from a variety of fishing methods, using different gear characteristics. Choosing the maximum number of days needed across all fisheries is intended to ensure that other fisheries also meet the 30% precision goal, but variability in the distribution and magnitude of catch, as well as the level of sea days achieved, will influence ultimate precision levels in a given fishery.

Monitoring needs for other turtle species, in other regions, and for other gear types

Loggerheads are the most commonly observed species of turtle in Mid-Atlantic waters and thus have the richest level of information available for estimating interactions and coverage needs. Sea day requirements for other turtle species are not estimated here because too few have been observed to estimate the magnitude of interactions with model-based approaches similar to those done for loggerheads. Because observers document all protected species interactions on trips, monitoring of other turtles species will still occur via days intended to monitor fish or loggerheads. Interaction rates between non-loggerhead turtles and fishing gear can be analyzed across several years once sufficient levels of data become available, and subsequently, similar analyses to those described here can be used to determine monitoring requirements to meet various precision levels around estimated interaction rates.

Incidental captures of sea turtles are generally very rare on Georges Bank and in the Gulf of Maine. These regions have not been included in PSB's model-based bycatch analyses because turtle captures there are too sparse to support robust model-based analyses. For instance, in ~70,000 observed otter trawl hauls on Georges Bank and the Gulf of Maine over a 15 year period there was 1 observed loggerhead interaction (Warden 2011). Sampling of fleets in the Northeast region has increased in recent years with the rise of sectors and at-sea monitors. Once analyzed these data may provide new information on turtle capture rates outside of the Mid-Atlantic, which could subsequently lead to better estimates of monitoring needs on Georges Bank and in the Gulf of Maine.

While almost all loggerhead interactions observed by northeast fisheries observers have occurred in trawl, gillnet, or dredge gears, some have occurred in other gear types (for instance, one loggerhead was observed in beach seine gear between 2009-2011, Wigley et al. 2012). To date there has not been enough information to estimate turtle interactions in these other gear types, though monitoring is still estimated under SBRM for fish discards or as pilot coverage when there is insufficient observer coverage. Monitoring for turtle interactions in these gear types can be reassessed if sufficient information becomes available.

References Cited

Murray KT. 2011. Interactions between sea turtles and dredge gear in the U.S. sea scallop (Placopecten magellanicus) fishery, 2001-2008. Fish. Res 107:137-146.

Murray KT. 2009. Characteristics and magnitude of sea turtle bycatch in U.S. mid-Atlantic gillnet gear. Endang. Species Res. 8:211-224.

Murray KT. 2009b. Proration of estimated bycatch of loggerhead sea turtles in U.S. mid-Atlantic sink gillnet gear to vessel trip report landed catch, 2002-2006. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 09-19; 7 p.

National Marine Fisheries Service (NMFS). 2004. Evaluating bycatch: a national approach to standardized bycatch monitoring programs. U. S. Dep. Comm., NOAA Tech. Memo. NMFS-F/SPO-66, 108 p.

Rossman MC. 2007. Allocating observer sea days to bottom trawl and gillnet fisheries in the Northeast and Mid-Atlantic regions to monitor and estimate incidental bycatch of marine mammals. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 07-19; 17 p.

Warden, ML 2011. Modeling loggerhead sea turtle (Caretta caretta) interactions with US Mid-Atlantic bottom trawl gear for fish and scallops, 2005-2008. Biol. Cons. 144: 2202-2212

Warden, ML. 2011b. Proration of loggerhead sea turtle (Caretta caretta) interactions in U.S. Mid-Atlantic bottom otter trawls for fish and scallops, 2005-2008, by managed species landed. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 11-04; 8 p.

Wigley SE, Blaylock J, Rago PJ, Shield G. 2012. 2012 Discard estimation, precision, and sample size analyses for 14 federally managed species groups in the northeast region. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 12-17; 146 p.
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