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CONTENTS
Introduction
Methods
Results
Discussion
References

Northeast Fisheries Science Center Reference Document 06-13

Estimates of Cetacean and Seal Bycatch in the 2004 Northeast Sink Gillnet and Mid-Atlantic Coastal Gillnet Fisheries

Dana L. Belden, Christopher D. Orphanides, Marjorie C. Rossman, and Debra L. Palka
National Marine Fisheries Serv., 166 Water St., Woods Hole MA 02543

Web version posted July 17, 2006

Citation: 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. U.S. Dep. Commer., Northeast Fish. Sci. Cent. Ref. Doc. 06-13; 24 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 2004 Northeast Sink Gillnet (NESG) and Mid-Atlantic Coastal Gillnet (MACG) fisheries, and documents the methodology used to produce the estimates.  The total estimated marine mammal takes in the 2004 NESG (2,292 animals, coefficient of variation [CV] = 18%) and MACG (231 animals, CV = 60%) fisheries was 2,523 animals (CV = 17%).  For the NESG fishery, the species included in this estimate are: 655 harbor porpoises (CV = 36%); 7 Atlantic white-sided dolphins (CV = 98%); 498 gray seals (CV = 35%); 786 harbor seals (CV = 34%); 303 harp seals (CV = 30%); and 43 hooded seals (CV = 95%).  For the MACG fishery, the species included in this estimate are: 135 harbor porpoises (CV = 91%); 68 gray seals (CV = 92%); and 28 harbor seals (CV = 77%).


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 US 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. 2003).  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 Northeast Sink Gillnet (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 as an effort to learn more about marine mammal interactions occurring in MA gillnet fisheries.

Rossman and Merrick (1999) documents the methods used to estimate harbor porpoise bycatch in the NESG and Mid-Atlantic Coastal Gillnet (MACG) fisheries.  The methods used to estimate harbor porpoise bycatch 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. 2003).  However, to date there has been no documentation of the methods used to estimate takes for all marine mammals incidentally bycaught in both the NESG and MACG fisheries to support the estimates that have been reported in the marine mammal SARs.

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 to 12 inches.  String lengths range from 600 to 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 to 12 inches.  String lengths range from 150 to 8400 feet.  The mesh size and string length vary by the primary fish species targeted for catch (Waring et al. 2003).

The purpose of this manuscript is to first document incidental take estimates for all marine mammals that were observed in the NESG and MACG fisheries during 2004.  Secondly, this manuscript’s purpose is to discuss the shortcomings of a 10-year-old method that was spatially and temporally designed to estimate harbor porpoise takes, but has been used to estimate takes on a variety of other marine mammal species.  Suggestions are provided for improvements to reflect current fishing patterns in gillnet fisheries and improved data collection.


METHODS

Data Sources

Three databases were used to estimate the total marine mammal takes in 2004: 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 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.  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: one unknown dolphin species; four unknown porpoise/dolphin species; and nine unknown seal species.  These animals were not included in the bycatch estimates for the strata they were caught in.

Second, the Northeast (NE) Dealer Report landings database was used to determine the total landings in 2004 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).

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 2004.  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; Figure 1).  Seasons are defined as: winter (January to May), summer (June to August), and fall (September to December).  Fishing effort observed in time/area closures was analyzed in separate time/area closure strata.  The MACG data were not stratified, but were examined by month and state for each bycaught species.  See Figure 2 for the time/area closures for the MA region. 

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. 

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

For the NESG fishery, observer coverage was 3%, 8%, and 10% during the winter, summer, and fall seasons, respectively (Table 2).  The total annual coverage for the NESG fishery was 6% (Table 2).  There were 110 observed incidental takes of marine mammals: 27 harbor porpoise, one Atlantic white-sided dolphin, 21 gray seals, 45 harbor seals, 15 harp seals, and one hooded seal (Figure 3 and Figure 4).  For the MACG fishery, observer coverage was 2%, 2%, and 3% during the winter, summer, and fall, respectively (Table 3).  The total annual coverage was 2% in the MACG fishery.  There were four observed incidental takes of marine mammals: two harbor porpoise, one gray seal, and one harbor seal (Figure 5 and Figure 6).

For the NESG fishery there were 19,083.59 tons of fish landed with the seasonal breakdown as follows: winter (53%); summer (24%); and fall (23%; Table 2).  For the MACG fishery there were 14,627.14 tons of fish landed with the seasonal breakdown, which is the same as that used in the NESG fishery, as follows: winter (50%); fall (31%); and summer (19%; Table 3).

In the NESG fishery only one of the 11 observed harbor porpoise takes that were in time/area closures requiring pingers was actually caught in a net equipped with pingers (Table 4).  Outside of the NESG time/area closures, three of the 16 observed harbor porpoise takes were caught in nets equipped with pingers (Table 4).  The only observed Atlantic white-sided dolphin take was in a NESG time/area closure and was caught in a net equipped with pingers (Table 5).  One of the four observed gray seal takes in NESG time/area closures requiring pingers was taken in a net equipped with pingers (Table 6).  Eight of the 17 observed harbor seal takes in NESG time/area closures requiring pingers were caught in nets equipped with pingers (Table 7).  All three of the observed harp seal takes in NESG time/area closures requiring pingers were caught in nets not equipped with pingers (Table 8).  The one observed hooded seal take in NESG time/area closures requiring pingers was caught in a net not equipped with pingers (Table 9).

The 2004 estimated total takes of cetaceans in the NESG and the MACG fisheries was 797 animals (CV = 33%).  The NESG fishery had 655 (CV = 36%) harbor porpoise (Table 4) and 7 (CV = 98%) Atlantic white-sided dolphin estimated takes (Table 5).  The MACG fishery had 135 (CV = 91%) harbor porpoise estimated takes (Table 10).  One incidental take of a bottlenose dolphin in the NESG Fishery was also observed, but takes of Atlantic bottlenose dolphins are analyzed with different methods that are not reported here (Palka and Rossman 2001). 

The majority (81%) of the NESG fishery harbor porpoise takes occurred in the winter, with 19% in the fall, and no takes observed in the summer (though some summer incidental takes have been observed in previous years).  During the winter, the South of Cape Cod port group-area and the South Cape closure had 46% and 20% of the entire NESG fishery estimated harbor porpoise takes, respectively (66% of the regions annual total for harbor porpoise) During the fall season 67% of harbor porpoise takes from the NESG fishery occurred in the Mid-Coast closure area (13% of the region’s annual total for harbor porpoise).  The only observed take of an Atlantic white-sided dolphin occurred in the fall in the Mid-Coast closure area.

In the MACG fishery the total estimated takes for harbor porpoise in April off the coast of Virginia was 135 animals (Table 10). 

The 2004 estimated total takes of seals in the NESG and MACG fisheries was 1,726 (CV = 20%).  For the NESG fishery, this included 498 gray seals (CV = 35%; Table 6); 786 harbor seals (CV = 34%; Table 7); 303 harp seals (CV = 30%; Table 8); and 43 hooded seals (CV = 95%; Table 9).  For the MACG fishery, the species included in this estimate are: 68 gray seals (CV = 92%) and 28 harbor seals (CV = 77%; Table 10).

The majority of the NESG gray seal takes (89%) occurred in the winter season, with the remaining takes occurring in the summer (6%) and fall (5%).  The South of Cape Cod port group-area had 45% of the estimated takes in winter, the South Cape time/area closure had 31% of the takes in winter, and the remaining 24% of the estimated takes took place in the remaining port group-areas in other seasons (Table 6). 

For harbor seals in the NESG fishery, 43% of the estimated takes occurred in the summer, 37% occurred in the winter, and the remaining 20% occurred in the fall.  The South of Cape Cod port group-area in the winter had 28% of the annual estimated takes, the New Hampshire port group-area in the summer had 28% of the annual estimated takes, the Mid-Coast time/area closure in the fall had 18% of the annual estimated takes, and the remaining 26% of estimated takes occurred in the remaining time/area closures and port group-areas in all seasons (Table 7).

All of the estimated harp seal takes in the NESG fishery took place in the winter season.  North of Boston (39%) and South of Cape Cod (38%) port group-areas had the majority of the estimated harp seal takes.  The remaining 20% of the takes occurred in the Massachusetts Bay time/area closure and 3% in the East Cape port group-area (Table 8).

All of the estimated hooded seal takes in the NESG fishery occurred in the winter season in the South Cape time/area closure (Table 9).

There were two pinniped species caught in the MACG fishery, where an estimated 68 gray seals were caught in April off Virginia and an estimated 28 harbor seals were caught in December off New Jersey (Table 10).


DISCUSSION

The current stratification method (ratio estimator) being used to estimate marine mammal bycatch in U.S.  gillnet fisheries has been in use for some time.  The current methodology was established as an acceptable method to estimate harbor porpoise takes in the NESG fishery (Bravington and Bisack, 1996).  However, since this method was developed, spatial patterns of the NESG fishery and sampling of the fishery by the NEFOP has changed, and self reported fishery dependent data collection (VTR) has improved.  In addition, there are potential problems with this approach when applied to other species.  Each issue is discussed below.

For all marine mammal take estimates, one of the potential problems with the current methodology involves a bias associated with using observed hauls from complete fish sampling trips as well as marine mammal sampling trips to estimate takes.  Observers on a complete fish sampling trip are engaged in sampling the catch for fish discards during the haul back and are not watching the net as it is being hauled in.  Consequently, they may not see marine mammals that fall out of the net before it reaches the deck (NEFOP Observer Manual, http://www.nefsc.noaa.gov/femad/fishsamp/fsb/).  In contrast, observed trips dedicated to sampling for marine mammal interactions have observers watching the gear at all times.  See Table 11 for the complete breakdown of trips, hauls, and incidental takes by trip types.

In 2002 the NEFOP was charged with increasing the sampling of fish discards for the NESG fishery.  As a result the number of complete fish sampling trips in the NESG fishery increased from ~10% in 2002 to ~60% and ~91% in 2003 and 2004, respectively.  Based on this change, it was decided that both complete fish sampling and marine mammal sampling trips would need to be used to compute the marine mammal bycatch rates for the NESG fishery, starting with the take estimates calculated from the 2003 data, which were reported in the 2005 SAR.  These take estimates have not been corrected to account for the potential bias associated with trips dedicated to fish sampling (Table 11).  We anticipate updating the work of Bravington and Bisack (1996) utilizing new data from recent years to develop revised correction factors.  Prior to 2003 only limited fish sampling trips were used to estimate marine mammal bycatch in the NESG fishery.  The increase in trips sampled for fish discards did not impact the MACG fishery as bycatch rates from the MACG fishery were only derived from limited sampling trips. 

For both cetacean and pinniped bycatch estimates, the following methodology issues require further attention: (1) the stratification and proration scheme used for estimating takes in NE fisheries may need to be revised to account for improved VTR data reporting and the spatio-temporal patterns of animals other than harbor porpoise; (2) the observer data are likely over-stratified, which means that the data may not reflect the changes that have occurred in fishing patterns which also influences the accuracy and precision of estimated bycatch rates; and (3) the port-based stratification scheme may no longer reflect current fishing patterns.  The current stratification method was specifically designed to estimate takes of harbor porpoise in the NESG fishery.  Therefore, this method may not be appropriate for other cetacean or pinniped species due to their different spatial/temporal patterns within the habitat of the Northeast region.  In addition, the effectiveness of pingers was tested by experimental design and proven to successfully deter harbor porpoise away from gillnets (Kraus et al. 1997).  The weighting of bycatch rates for other species incidentally bycaught in gillnets implies that pingers are also effective in deterring these other species.  However, reports on the effectiveness of pingers to deter or attract other small cetaceans and pinnipeds are varied and some were inconclusive (Dawson 1998, IWC 1999, Jefferson and Curry 1996).  Therefore, the use of weighted bycatch rates for species other than harbor porpoise bycaught in nets with and/or without pingers should be considered on a case-by-case basis, given the information that is available in the literature for each individual species.

In conclusion, we are in the process of evaluating our take estimation methodology to determine whether the current method can be improved given the changes in the data.  For cetaceans, we are looking to combine the NESG and MACG data into a predictive model that will improve stratification as well as the accuracy and precision of our take estimates.  For pinnipeds, we hope to design a new stratification method that utilizes pinniped spatio/temporal patterns in relation to the fishery.


REFERENCES

Bisack KD.  1993.  Estimates of total US harbor porpoise bycatch in the Gulf of Maine sink gillnet fishery.  Northeast Fish. Sci. Cent. Ref. Doc. 93-11; 23 p.

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.  Northeast Fish. Sci. Cent. Ref. Doc. 03-18; 18 p.

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.

Dawson SM, Read A, Slooten E.  1998.  Pingers, porpoises and power: uncertainties with using pingers to reduce bycatch of small cetaceans.  Biol. Conservation 84:141-146.

International Whaling Commission (IWC).  1999.  Report of the Scientific Committee.  IWC/51/4, p. 62-65.

Jefferson TA, Curry BE.  1996.  Acoustic methods of reducing or eliminating marine mammal-fishery interactions: do they work?  Ocean & Coastal Management 31(1): 41-70.

Kraus SD, Read A, Anderson E, Baldwin K, Solow A, Spradlin T, Williamson J.  1997.  Acoustic alarms reduce porpoise mortality.  Nature 388:525.

National Marine Fisheries Service [NMFS].  1998.  Taking of marine mammals incidental to commercial fishing operations; harbor porpoise Take Reduction Plan regulations.  Federal Register 63(231):66464-66490, December 2, 1998.

Palka DL, Rossman MC.  2001.  Bycatch estimates of coastal bottlenose dolphin (Tursiops truncatus) in US mid-Atlantic gillnet fisheries for 1996 to 2000.  Northeast Fish. Sci. Cent. Ref. Doc. 01-15; 77 p.

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.  Northeast Fish. Sci. Cent. Ref. Doc. 99-17; 36 p.

Smith T, Palka D, Bisack K.  1993.  Biological significance of bycatch of harbor porpoise in the Gulf of Maine demersal gillnet fishery.  Northeast Fish. Sci. Cent. Ref. Doc. 93-23; 15 p.

Waring GT, Palka, DL, Mullin K, Hain J, Hansen L, Bisack K.  1997.  US Atlantic and Gulf of Mexico marine mammal stock assessments – 1996.  NOAA Tech. Mem. NMFS-NE-114; 250 p.

Waring GT, Pace R, Quintal J, Fairfield C, Maze-Foley K.  2003.  US Atlantic and Gulf of Mexico marine mammal stock assessments – 2003.  NOAA Tech. Memo. NMFS-NE-182; 287 p.

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