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Methods
Results
References

Northeast Fisheries Science Center Reference Document 07-08

Estimates of Cetacean and Pinniped Bycatch in the 2005 Northeast Sink Gillnet and Mid-Atlantic Coastal Gillnet Fisheries

Dana Belden
National Marine Fisheries Serv, Woods Hole Lab, 166 Water St, Woods Hole MA 02543-1026

Web version posted May 11, 2007

Citation: Belden D. 2007. Estimates of cetacean and pinniped bycatch in the 2005 northeast sink gillnet and mid-Atlantic coastal gillnet fisheries. U.S. Dep. Commer., Northeast Fish. Sci. Cent. Ref. Doc. 07-08; 16 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.

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ABSTRACT: This report provides incidental take estimates for six marine mammal species taken in the 2005 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 26 common dolphins (CV = 80%), 630 harbor porpoises (CV =23 %), 15 Risso’s dolphins (CV = 93%), 59 Atlantic white-sided dolphins (CV = 49%), 574 gray seals (CV = 44%), 719 harbor seals (CV = 20%), and 35 harp seals (CV = 68%). For the MACG fishery, the estimated 2005 takes included 470 harbor porpoises (CV = 51%) and 63 harbor seals (CV = 67%).
INTRODUCTION

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 by 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) document 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).

The NESG fishery extends from Maine to Rhode Island and is dominated by bottom-tending sink gillnets.  Less than 1% of the fishery utilizes a drift gillnet, not tending the ocean bottom.  Monofilament twine is typically used with stretched mesh sizes ranging from 6–12 inches. String lengths range from 600–10,500 feet.  Mesh size and string length vary by the primary fish species targeted for catch.  The MACG fishery ranges from Connecticut to North Carolina and utilizes both drift and sink gillnets.  These nets are most frequently attached to the bottom, although unanchored drift or sink nets are also utilized to target specific species.  Monofilament twine is again the dominant material and is used with stretched mesh sizes ranging from 2.5–12 inches. String lengths range from 150–8400 feet. The mesh size and string length vary by the primary fish species targeted for catch (Waring et al. 2004).

To calculate cetacean and seal bycatch for the 2005 NESG and MACG fisheries, the same ratio estimator methodology was used as was documented in Belden et al. (2006).  However, there were a few changes in the stratification; these changes and the resulting bycatch estimates are described in this report.


METHODOLOGY

Data Sources

Three databases were used to estimate the total marine mammal takes in 2005: the NEFOP database, Northeast Dealer Reports, and Northeast Vessel Trip Reports.

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 in which 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 in which 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.

Second, the Northeast (NE) Dealer Report landings database was used to determine the total landings in 2005 of all finfish caught in the Northeast gillnet fishery.

Lastly, the NE Vessel Trip Report (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 manuscript, 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 whenever possible by the fishery observer.  There were several incidental takes that were not identified to species: 2 unknown dolphins; 3 unknown porpoise/dolphins; and 14 unknown seal species.  These animals were not included in the bycatch estimates for the strata they were caught in.

The level of sampling (observer coverage) for each stratum was calculated by dividing the observed tons of fish caught by the prorated tons of fish recorded in the dealer database.  This value represented the fraction of total landings that were sampled.

Analysis

The strata defined in Rossman and Merrick (1999) was used to estimate takes in 2005. The NESG data was 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–May), summer (June–August), and fall (September–December).  However, there was one change to the NESG analysis.  The Harbor Porpoise Take Reduction Plan’s Offshore Closure area requires pinger use during November–May for vessels within the NESG fishery.  In previous analyses (e.g. Belden et al. 2006), effort for this period was split into two seasonal strata: fall (September–December) and winter (January–May).  In 2005, however, there were only six observed trips in the Offshore Closure area: five in November and December (fall) and one in May (winter).  Using one observed trip in a bycatch rate estimate for the winter Offshore Closure area would result in an unrepresentative bycatch rate for this stratum.  A more representative rate is available from pooling the one May trip with offshore port group data from the adjacent months to create a new summer season, defined as May–August (Table 1).  This change affected the estimated bycatch for harbor and gray seals. Fishing effort observed in time/area closures was analyzed in separate time/area closure strata.

As indicated in Belden et al. (2006), MACG bycatch estimates have been calculated by month for each state.  In 2005, observer and VTR trip locations indicate New Jersey MACG trips during January–April fished in a similar area (Figure 1), so a winter season (January–April) was used for the 2005 analyses (Table 2) of harbor porpoise and harbor seal takes in Jew Jersey waters.

Connecticut (CT) gillnet fishing effort has historically been included in the mid-Atlantic region bycatch analyses. Spatial analysis of 2005 VTR and observer data indicated that CT vessels fish in the same time and area as vessels from the Northeast region fishing in the South of Cape Cod port group (Figure 2); therefore, CT trips were included in the 2005 Northeast South of Cape Cod port group bycatch estimates (Table 1).  This change affected the estimated bycatch of harbor porpoise, white-sided dolphin, gray seal, harbor seal, and harp seal.

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 bycatch rate for each stratum is defined as the number of observed takes divided by the observed tons (effort) of fish landed.

equation

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.  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 – within a stratum. 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 resampling unit used was an entire trip rather than individual hauls, to ensure that any within-trip dependence was carried over into the bycatch estimates (Bisack 2003).


RESULTS

The overall observer coverage in the NESG was 7.3%, ranging from 4.2% in the fall to 11.48% in the winter (Table 1).  This level is slightly higher then in previous years. Four common dolphins, 51 harbor porpoises, 1 Risso’s dolphin, 5 white-sided dolphins, 2 unknown dolphins, 3 unknown porpoise/dolphins, 33 gray seals, 70 harbor seals, 3 harp seals, and 14 unknown seals taken were observed taken in the 2005 NESG fishery.  Unidentified animals were not included in this analysis.

The 2005 estimated total takes of cetaceans in the NESG fishery was 26 (CV = 80%) common dolphins (Table 3), 630 (CV = 23%) harbor porpoise (Table 4), 15 (CV = 93%) Risso’s dolphin (Table 5), and 59 (CV = 49%) white-sided dolphin (Table 6).  The 2005 estimated total takes of pinnipeds in the NESG fishery was 574 (CV = 44%) gray seal (Table 7), 719 (CV = 20%) harbor seal (>Table 8), and 35 (CV = 68%) harp seal (Table 9).

The 2005 observer coverage for the MASG fishery was 2% (Table 2).  The 2005 observer coverage for the winter off of New Jersey was 3.2% (Table 10).  There were 15 harbor porpoise, 2 harbor seals, 2 bottlenose dolphins, 2 unknown dolphins, and 3 unknown seals observed taken in the MASG fishery in 2005.  The unidentified animals are not included in this analysis.

The 2005 estimated total takes for cetaceans in the MASG fishery was 470 (CV = 51%) harbor porpoise (Table 11).  Bottlenose dolphin bycatch is estimated by a different method (Palka and Rossman, 2001) and therefore is not reported here.  The 2005 estimated total takes for pinnipeds in the fishery was 63 (CV = 67%) harbor seal (Table 12).


REFERENCES

Belden DL, Orphanides CD, Rossman MC, Palka DL.  2006.  Estimates of cetacean and seal bycatch in the 2004 Northeast sink gillnet and mid-Atlantic coastal gillnet fisheries.  US Dep Commer, Northeast Fish Sci Cent Ref Doc. 06-13; 24 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole MA 02543-1026 or http://nefsc.noaa.gov/publications/crd/crd0613/crd0613.pdf

Bisack KD.  1993.  Estimates of total US harbor porpoise bycatch in the Gulf of Maine sink gillnet fishery.  US Dep Commer, Northeast Fish Sci Cent Ref Doc. 93-11; 23 p.  Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole MA 02543-1026.

Bisack KD.  1997.  Harbor porpoise bycatch estimates in the New England multispecies sink gillnet fishery: 1994 and 1995.  Rep  int  Whal  Commn. 47: 705-714.

Bisack KD.  2003.  Estimates of marine mammal bycatch in the Northeast ( New England) multispecies sink gillnet fishery in 1996.  US Dep Commer, Northeast Fish Sci Cent Ref Doc. 03-18; 18 p.  Available from: Natl Mar Fish Serv, 166 Water Str, Woods Hole MA 02543-1026 or http://nefsc.noaa.gov/publications/crd/crd0318/

Blaylock RA, Hain JW, Hansen LJ, Palka DL, Waring GT.  1995.  US Atlantic and Gulf of Mexico Marine Mammal Stock Assessments.  NOAA Tech Mem. NMFS-SEFSC-363; 211 p.

Bravington MV, Bisack KD.  1996.  Estimates of harbor porpoise bycatch in the Gulf of Maine sink gillnet fishery, 1990-1993. Rep  int  Whal  Commn.  46: 567-574.

National Marine Fisheries Service [NMFS].  1998.  Taking of marine mammals incidental to commercial fishing operations; harbor porpoise Take Reduction Plan regulations.  Fed Regis. 63(231):66464-66490.

Palka D, Rossman M.  2001.  Bycatch estimates of coastal bottlenose dolphin (Tursiops truncatus) in U.S. Mid-Atlantic gillnet fisheries for 1996 to 2000.  Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole MA 02543-1026 or http://nefsc.noaa.gov/publications/crd/crd0115/0115.htm

Rossman MC, Merrick RL.  1999.  Harbor porpoise bycatch in the Northeast multispecies sink gillnet fishery and the mid-Atlantic coastal gillnet fishery in 1998 and during January-May 1999.  US Dep Commer, Northeast Fish Sci Cent Ref Doc. 99-17; 36 p.  Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole MA 02543-1026 or http://nefsc.noaa.gov/publications/crd/crd9917.pdf

Smith TD, Palka DL, Bisack KD.  1993.  Biological significance of bycatch of harbor porpoise in the Gulf of Maine demersal gillnet fishery.  US Dep Commer, Northeast Fish Sci Cent Ref Doc. 93-23; 15 p.  Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole MA 02543-1026.

Waring GT, Palka, DL, Mullin KD, Hain JHW, Hansen LJ, Bisack KD, Editors.  1997.  US Atlantic and Gulf of Mexico marine mammal stock assessments – 1996.  NOAA Tech Mem NMFS-NE-114; 250 p.  Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole MA 02543-1026.

Waring GT, Pace RM, Quintal JM, Fairfield CP, Maze-Foley K, Editors.  2004.  US Atlantic and Gulf of Mexico marine mammal stock assessments – 2003.  NOAA Tech Memo NMFS-NE-182; 287 p.  Available from: Natl Mar Fish Serv, 166 Water St, Woods Hole MA 02543-1026 or http://nefsc.noaa.gov/publications/tm/tm182/

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