CONTENTS Introduction Methods Results References Acronyms
Northeast Fisheries Science Center Reference Document 07-20
Dana Belden1,2 and Christopher D. Orphanides1
Estimates of Cetacean and Pinniped Bycatch in the 2006 Northeast Sink Gillnet and Mid-Atlantic Coastal Gillnet Fisheries
1 NOAA Fisheries, Northeast Fisheries Science Center, 166 Water Street, Woods Hole MA 02543-1026
2 Current contact information: Office of Naval Research, Marine Mammals & Biological Oceanography Program, One Liberty Center, Code 322 – Rm 1072C, 875 N. Randolph St., Arlington, VA 22203-1995. email@example.com
Web version posted January 9, 2008Citation: Belden D, Orphanides CD. 2007. Estimates of Cetacean and Pinniped Bycatch in the 2006 Northeast Sink Gillnet and Mid-Atlantic Coastal Gillnet Fisheries. US Dep Commer, Northeast Fish Sci Cent Ref Doc. 07-20; 18 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.
This report provides incidental take estimates for six marine mammal species taken in the 2006 Northeast Sink Gillnet (NESG) and Mid-Atlantic Coastal Gillnet (MACG) fisheries and documents the methodology used to produce the estimates. For the NESG fishery, the estimated take was 20 common dolphins (CV = 105%), 514 harbor porpoises (CV = 31 %), 41 Atlantic white-sided dolphins (CV = 71%), 248 gray seals (CV = 47%), 87 harbor seals (CV = 58%), and 65 harp seals (CV = 66%). For the MACG fishery, the estimated 2006 takes was 512 harbor porpoises (CV = 32%), 11 common dolphins (CV = 103%), and 26 harbor seals (CV = 98%).
Pursuant to the 1994 amendments of the Marine Mammal Protection Act (MMPA), Section 117 states that estimates of annual human-caused mortality and serious injury to marine mammal stocks must be reported in annual stock assessment reports (SAR) for each stock of marine mammal that occurs in waters under U.S jurisdiction.
The Northeast Fisheries Science Center (NEFSC) Sea Sampling Observer Program (SSOP), presently known as the Northeast Fisheries Observer Program (NEFOP), was initiated in 1989 to document the bycatch of marine mammals taken incidentally to commercial fishing operations (Waring et al. 2004). Since the initiation of the observer program, the estimation of total takes for harbor porpoise (Phocoena phocoena) has been the focus of much attention due to frequent observations of incidental takes occurring in the NESG fishery (NMFS 1998). This attention led to the development of a stratification method designed to estimate the total annual takes of harbor porpoise (Bisack 1993; Smith et al. 1993; Bravington and Bisack 1996; Bisack 1997; Rossman and Merrick 1999; Bisack 2003). The regional scope of the SSOP was expanded into the Mid-Atlantic (MA) region in 1995 in an effort to learn more about marine mammal interactions occurring in MA gillnet fisheries.
Rossman and Merrick (1999) documented the methods used to estimate harbor porpoise bycatch in the NESG and MACG fisheries. These methods have also been used by the NEFSC to estimate the bycatch of other marine mammals observed bycaught in the NESG and MACG fisheries (Blaylock et al. 1995; Waring et al. 1997; Waring et al. 2004; Belden et al. 2006; Belden 2007).
Historically, the NESG fishery extended from Maine to Connecticut and was dominated by bottom-tending sink gillnets. Less than 1% of the fishery utilized a drift gillnet (not tending the ocean bottom). Monofilament twine was typically used with stretched mesh sizes ranging from 6 to 12 inches. String lengths ranged from 600 to 10,500 feet. Mesh size and string lengths varied by the primary fish species targeted for catch.
The MACG fishery ranged from Connecticut to North Carolina and utilized both drift and sink gillnets. These nets were most frequently attached to the bottom, although unanchored drift or sink nets were also utilized to target specific species. Monofilament twine was again the dominant material and was used with stretched mesh sizes ranging from 2.5 to 12 inches. String lengths ranged from 150 to 8400 feet. The mesh size and string lengths varied by the primary fish species targeted for catch (Waring et al. 2004). Due to recent fishing patterns, the division between the Northeast and Mid-Atlantic has changed from Connecticut to Rhode Island. This will be discussed further in this report.
The same ratio estimator methodology was used to calculate cetacean and seal bycatch for the 2006 NESG and MACG fisheries as was used in Belden et al. (2006) and Belden (2007). However, there were a few changes in the stratification; these changes and the resulting bycatch estimates are described in this report. As in previous years, bottlenose dolphin bycatch was not estimated in this document. Bottlenose dolphin estimates can be found in the upcoming 2008 U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessment Report (Waring et al. in prep.), and in an upcoming paper by Rossman (in prep.).
Three databases were used to estimate the total marine mammal takes in 2006: the NEFOP database, Northeast (NE) Dealer Reports, and Northeast Vessel Trip Reports (VTR). First, the NEFOP database provided data on the observed bycatch of marine mammals. The NEFOP has two types of sampling protocols when observing fishing trips: (1) complete fish sampled trips where the observer samples the catch for fish discard information (the observer is not able to watch the net as it is being hauled), and (2) limited fish sampled trips where the observer watches the net for incidental takes as it is being hauled. In the NESG fishery only, hauls observed from both trip sampling protocols were used to estimate the bycatch rates from observed incidental takes. Only limited fish sampling trips were used in the MACG fishery to estimate the bycatch rates of most species.
Second, the NE Dealer Report landings database was used to determine the total landings in 2006 of all finfish caught in the Northeast and Mid-Atlantic gillnet fishery.
Lastly, the NE VTR database was used to assign (prorate) the NE Dealer Report landings from the NESG fishery to spatial and temporal strata historically used to estimate takes of harbor porpoise in the NESG fishery (Rossman and Merrick 1999; Bisack 2003).
For purposes of this report, a “take” is defined as any observed incidental take where the animal’s condition was recorded as either alive with injuries or dead (fresh or under various stages of decomposition). All incidental takes are identified to species by the fishery observer whenever possible. There were several incidental takes that were not identified to species: two unknown porpoise/dolphin species and eight unknown seal species. These animals were not included in the bycatch estimates for the strata in which they were caught..
The level of sampling (observer coverage) for each stratum was calculated by dividing the observed metric tons (mtons) of fish caught by the prorated metric tons of fish recorded in the dealer database. This value represented the fraction of total landings that were sampled.
The strata defined in Rossman and Merrick (1999) were used to estimate takes in 2006. The NESG data were stratified temporally by season, spatially by port group-area and time/area closures, and by bycatch avoidance techniques via the use of pingers (Table 1). Seasons are defined as winter (January to May), summer (June to August), and fall (September to December).
Connecticut (CT) gillnet fishing effort has historically been included in the Mid-Atlantic region bycatch analyses. Spatial analysis of 2005 and 2006 VTR and observer data indicated that CT vessels are currently fishing in the same time and area as vessels from the Northeast region fishing in the South of Cape Cod port group (Figure 1). Therefore, CT trips were included in the 2006 Northeast South of Cape Cod port group bycatch estimates (Table 1). This change affected the estimated bycatch of harbor porpoise, common dolphin, gray seal, harbor seal, and harp seal.
As indicated in Belden et al. (2006), until 2004 MACG bycatch estimates have been calculated by month for each state. In 2005 and 2006, observer and VTR trip locations indicate New Jersey (NJ) MACG trips during January to April fished in a similar area (Figure 2). So, a winter season (January to April) was used for the 2006 analyses (Table 2) of harbor porpoise and harbor seal takes in New Jersey waters.
In January 2006, a common dolphin take was observed off the coast of NJ. Rather than creating a winter category as we had done for harbor porpoise and harbor seal, we calculated a bycatch rate for only the month of January, due to uncertainty about the distribution of common dolphins during that time of year. The take was observed on a complete sampling trip. No common dolphin bycatch was observed on limited sampling trips. Historically, MA cetacean bycatch estimates have been calculated using only limited trips (Rossman and Merrick 1999; Belden et al. 2006; Belden 2007), but since the observed bycatch occurred on a complete trip, we used both complete and limited trips to estimate bycatch for common dolphins. Use of both types of trips is likely more representative of bycatch than complete trips alone, since the observer on complete trips is not able to watch the net as it is being hauled. Using both trip types also increased the sample size of observed hauls from 38 to 106, providing a more robust estimate.
New York (NY) gillnet fishing effort has historically been included in the MA region bycatch analyses. Spatial analysis of 2006 VTR and observer data indicated that some NY vessels, including a vessel on which takes were observed, fished in the same time and area as vessels from the Northeast region fishing in the South of Cape Cod port group or time/area closure (Figure 3). Therefore, the NY observed trips that were east of the 72º30’W line were included in the 2006 Northeast South of Cape Cod port group or time/area closure bycatch estimates (Table 1). This fits in well with the List of Fisheries definition for NESG fisheries, which states that the MA area is from 72º30’W south to 36º33’03”N. This change affected the estimated bycatch of harbor porpoise, common dolphin, gray seal, harbor seal, and harp seal.
The VTR effort for NY was divided between the NE and MA regions using the 72º30’W line, as was the observer data. Next, the proportion of NY VTR tons assigned to the South of Cape Cod port group or area was calculated for each month. These proportions were then applied to the NY dealer data to get the prorated dealer tons to be used in the bycatch estimates for the appropriate port group or area in the NE region. Thus, the NY data from the observer and VTR databases were divided into the NE region (Table 1 and Table 2) and MA region (Table 3).
The number of marine mammal takes (B) is the product of the observed bycatch rate multiplied by the total effort in each stratum (S). The observed bycatch rate for each stratum is defined as the number of observed takes divided by the observed mtons (effort) of fish landed.
There is a possibility that strings could be either equipped or not equipped with pingers in the NESG fishery. Therefore, a weighted bycatch rate was calculated for strata where there were hauls with and without pingers. Within a stratum, the weighted bycatch rate was calculated as the sum of two weighted bycatch rates, one from hauls with pingers and one from hauls without pingers. Each bycatch rate was weighted by the proportion of hauls sampled with or without pingers within its respective stratum.
Standard bootstrapping techniques were used to derive the confidence intervals and coefficients of variation (CV) for the bycatch estimates for each stratum. The re-sampling unit used was an entire trip rather than individual hauls to ensure that any within trip dependence was carried over into the bycatch estimate’s CV (Bisack 2003).
The overall observer coverage in the NESG was 3.6%, ranging from 1.3% in the summer to 6.0% in the winter (Table 1). This level is lower than in 2005, which was 7.3%, ranging from 4.2% in the fall to 11.5% in the winter (Belden 2007). One common dolphin, 26 harbor porpoises, 2 white-sided dolphins, 2 unknown porpoise/dolphins, 9 gray seals, 3 harbor seals, 3 harp seals, and 8 unknown seals were observed taken in the 2006 NESG fishery. Unidentified animals were not included in this analysis.
The 2006 estimated total takes of cetaceans in the NESG fishery included 20 (CV = 105%) common dolphins (Table 4), 514 (CV = 31%) harbor porpoises (Table 5), and 41 (CV = 71%) white-sided dolphins (Table 6). The 2006 estimated total takes of pinnipeds in the NESG fishery included 248 (CV = 47%) gray seals (Table 7), 87 (CV = 58%) harbor seals (Table 8), and 65 (CV = 66%) harp seals (Table 9).
The 2006 observer coverage for the MACG fishery using only limited trips was 3.7% (Table 2) and using both complete and limited trips was 4.3% (Table 3). The 2006 observer coverage for the winter off of NJ was 3.9% (Table 10). The 2006 observer coverage for January off of NJ was 9.0% (Table 11). There were 20 harbor porpoises, 1 common dolphin, and 1 harbor seal observed taken in the MACG fishery in 2006.
The 2006 estimated total takes for cetaceans in the MACG fishery included 512 (CV = 32%) harbor porpoises (Table 12) and 11 (CV = 103%) common dolphins (Table 13). The 2006 estimated total takes for pinnipeds in the MASG fishery was 26 (CV = 98%) harbor seals (Table 14).
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Acronyms CV = coefficient of variation MA= Mid-Atlantic MACG = Mid-Atlantic Coastal Gillnet MMPA = Marine Mammal Protection Act mtons = metric tons NE = Northeast NEFOP = Northeast Fisheries Observer Program NEFSC = Northeast Fisheries Science Center NESG = Northeast sink gillnet NMFS = National Marine Fisheries Service NOAA = National Oceanic and Atmospheric Administration SAR = stock assessment report SSOP = Sea Sampling Observer Program VTR = Vessel Trip Report