CONTENTS Abstract Introduction Methods Results Discussion References Cited
Northeast Fisheries Science Center Reference Document 10-10
Estimates of Cetacean and Pinniped Bycatch in the 2007 and 2008 Northeast Sink Gillnet and Mid-Atlantic Gillnet FisheriesChristopher D. Orphanides
Northeast Fisheries Science Center 28 Tarzwell Drive, Narragansett, RI 02882.
Web version posted September 1, 2010Citation: Orphanides CD. 2010. Estimates of Cetacean and Pinniped Bycatch in the 2007 and 2008 Northeast Sink Gillnet and Mid-Atlantic Gillnet Fisheries. US Dept Commer, Northeast Fish Sci Cent Ref Doc. 10-10; 45 p. Available from: National Marine Fisheries Service, 166 Water Street, Woods Hole, MA 02543-1026, or online at: http://nefsc.noaa.gov/nefsc/publications/
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.
Rossman and Merrick (1999) documented the methods used to estimate harbor porpoise bycatch in the NESG and Mid-Atlantic gillnet (MAG) fisheries. These methods were subsequently used to estimate the bycatch of other marine mammal species bycaught in the NESG and MAG fisheries (Blaylock et al. 1995; Waring et al. 1997; Waring et al. 2004; Belden et al. 2006; Belden 2007; Belden and Orphanides 2007).
The NESG fishery extends from Maine to Connecticut and is dominated by bottom-tending sink gillnets. Less than 1% of the fishery utilize a drift gillnet (not tending the ocean bottom). Monofilament twine is typically used with stretched mesh sizes ranging from 6 to 12 inches (Waring et al. 2004). According to data collected by the NEFOP from 1999 through 2008, string lengths ranged from 150 to over 10,000 feet, though most were about 3,000 feet. Mesh size and string lengths varies by the primary fish species targeted for catch (Waring et al. 2004).
The MAG fishery generally 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 typically ranging from 2.5 to 12 inches (Waring et al. 2004). According to data collected by the NEOP from 1999 through 2008, string lengths ranged from 100 to over 10,000 feet, though typically were between 1,000 and 1,500 feet. The mesh sizes and string lengths vary by the primary fish species targeted for catch (Waring et al. 2004).
After the 2005 bycatch estimates, the division between the Northeast and Mid-Atlantic changed from a system based on vessel home port (divided at the Connecticut-Rhode Island border) to one based on reported fishing location. For the 2006, 2007, and 2008 bycatch estimates, the NESG and MAG fisheries were defined by a division at 72º30’W longitude, extending south to the NC/SC border. This change will be further discussed in this report.
The present analysis of the 2007 and 2008 data uses the same general ratio estimator methodology that was used to calculate cetacean and seal bycatch for the 2006 NESG and MAG fisheries (Belden and Orphanides 2007). However, there have been a few minor changes in the stratification and how the total fishery effort was calculated. These changes and the resulting bycatch estimates are described in this report. As in previous years, bottlenose dolphin (Tursiops truncatus) bycatch is not estimated. Bottlenose dolphin estimates can be found in the upcoming 2010 U.S. Atlantic and Gulf of Mexico Marine Mammal Stock Assessment Reports (Waring et al. in prep.).
Three databases were used to estimate the total marine mammal incidental takes in 2007 and 2008: the NEFOP database, Allocated Commercial Landings, and Northeast Vessel Trip Reports (VTR). The NEFOP data were used to estimate the bycatch rate of marine mammals caught in the 2007 and 2008 Northeast and Mid-Atlantic gillnet fisheries. The NEFOP has two types of sampling protocols when observing gillnet fishing trips: (1) complete fish sampled trips where the observer samples the catch for fish discard information, thus the observer is not able to watch the net as it is being hauled in and so might miss an incidental take; and (2) limited fish sampled trips where the observer watches the net for incidental takes as it is being hauled in and thus should not miss any incidental takes.
In the NESG and MAG fishery, hauls observed from both trip sampling protocols were used to estimate the 2007 and 2008 bycatch rates for all species, as had been done for the 2006 MAG common dolphin (Delphinus delphis) bycatch estimates (Belden and Orphanides 2007) and all species in the 2006 NESG fisheries. Historically, only limited fish sampling trips were used in the MAG fishery to estimate the bycatch rates of most marine mammal species. However, because of increased bycatch observed on complete trips (see results section for details), the 2007 and 2008 Mid-Atlantic estimates were calculated using both complete and limited trips. Using data from both types of sampling protocols avoids discarding many observed incidental takes and increases the sample sizes to provide more robust estimates.
The Allocated Commercial Landings and Northeast Vessel Trip Reports (VTR) were used to calculate the total landings of all finfish caught in the 2007 and 2008 Northeast and Mid-Atlantic gillnet fisheries. Though this approach differs from previous years, it should provide a more accurate calculation by significantly limiting the amount of proration applied to the commercial landings data. This approach should also provide a more accurate split between the Northeast and Mid-Atlantic fisheries because in most instances the VTR locations are linked directly with the commercial landings data.
The Allocated Commercial Landings data merges the VTR logbook and Northeast Dealer Report data by trip, wherever possible (71% and 70% of VTR gillnet trips in 2007 and 2008, respectively, were matched to Northeast Dealer Report data). Thus the gear characteristic information of the VTR logbooks was linked with the near census of landings in the Dealer Report data (Wigley et al. 2008).
In the cases where VTR and Allocated trips were successfully matched one to one, the Allocated landings, locations, and other characteristics for these trips were used in this analysis. In the cases where the VTR and Allocated trips could not be matched one to one, a proration scheme was used which was based on strata defined by state, season, and year, as was done in previous years (e.g., Belden and Orphanides 2007). That is, for strata where the total Allocated landings were greater than total VTR landings, the landings of each VTR trip in that strata was multiplied by a raising factor that ensured the total VTR landings for that strata equaled the total Allocated landings for that same strata. Thus, it was assumed that the available VTR trips were spatially and temporally representative of the trips that did not provide VTR logbooks or under-reported landings in their VTR logbooks. In the cases where the VTR landings in a particular stratum were larger than landings in the corresponding stratum in the Allocated data (16% and 9% of all VTR trips in 2007 and 2008, respectively), the Allocated landings were retained unless no Allocated landings were present for those strata, in which case the VTR landings were used. This approach respects the assumption that the commercial Northeast Dealer Report landings data represents a near census of all landings in the fishery, while still allowing for a limited amount of flexibility that ensures that the spatial and temporal distribution of landings is representative of effort in the VTR. The resulting landings combining the VTR and Allocated data will be referred to as the prorated metric tons of landings.
AnalysisAn “incidental take” is defined as any observed incidentally caught marine mammal that was recorded as either alive with injuries or dead (fresh or under various stages of decomposition). Incidental takes not identified to species were not included in the bycatch estimates. This included 2 unknown porpoise/dolphin animals, 1 unknown toothed whale, and 18 unknown seals in 2007, and 6 unknown seals in 2008.
The level of sampling (observer coverage) within each stratum was calculated by dividing the observed metric tons (mtons) of landings by the prorated metric tons of landing recorded in the effort datasets. This value represented the fraction of total landings that were sampled.
The strata as defined in Rossman and Merrick (1999) were used to estimate NESG fishery incidental takes, as has been done since 1999. That is, the NESG fishery data were stratified temporally by season, spatially by port group-area and time/area closures (Figure 1), and also by the presence/absence of pingers (Table 1 and Table 2). Seasons were defined as winter (January to May), summer (June to August), and fall (September to December). The temporal/spatial/pinger strata were based on the harbor porpoise take reduction plan (NMFS 1998) and the migration patterns of the harbor porpoise. When estimating the 2007 gray seal (Halichoerus grypus) NESG bycatch, the summer Offshore port stratum was combined with the summer East of Cape Cod port stratum because the observed bycatch event in the Offshore port stratum occurred on the only observed haul in this strata and the recorded location for this haul fit well with the typical distribution of trips from the East of Cape Cod port group. A similar pooling of strata was used in the 2005 pinniped estimate calculations when incidental takes were observed on a stratum’s only observed haul (Belden 2007).
Prior to 2006, the MAG and NESG fisheries were defined for the purposes of these bycatch estimates by port landed, where Connecticut (CT) and states south and west were included in the MAG fishery, and Rhode Island (RI) and states north and east were included in the NESG fishery. For the 2006, 2007, and 2008 bycatch analyses, the division of the NESG and MAG fisheries was determined by the recorded locations of the gillnet gear. For the 2006-2008 bycatch estimates, the 72º30’W longitude line (Figure 1) was used to divide the two fisheries (Belden and Orphanides 2007). As a result, trips landing in CT, NY, and NJ which fished east of 72º30’W were included in the NESG fishery and were within the South of Cape Cod port group, while data from trips which fished west of this line were included in the MAG fishery (Tables 1, 2, 3 and 4).
The MAG bycatch estimates for 2007 harbor porpoises and harp seals (Pagophilus groenlandicus), and for 2008 harbor seals (Phoca vitulina) and harp seals, were calculated using strata defined by state and season, where the season was January-April. Some past year’s MAG marine mammal estimates (e.g., Belden et al. 2006) were calculated using state and month strata (instead of state and season strata) because spatial/temporal patterns of the fishers and marine mammals were not well known. Now, however, more information on the spatial/temporal patterns of these animals is known so it is possible to create strata that are more representative of the fisheries and the migration patterns of these marine mammals, resulting in more robust and representative bycatch estimates.
Then again, there is still relatively little known about the fine scale seasonal distribution for some rarely incidentally taken species such as Risso’s dolphin (Grampus griseus). Risso’s dolphin estimates for 2007 were calculated using November effort from Maryland, Delaware, and Virginia. Effort from three states and only one month was used because fishing effort from these three states occurred in the same general area (Figure 2), and because November appears to represent a transitional period where Risso’s dolphin habitat shifts further offshore (Waring et al. 2009). Therefore, there was more certainty in the spatial distribution of the Risso’s dolphin during November than if adjacent months were pooled. In addition, pooling the data spatially resulted in a more robust and representative estimate than just using state and month, which would have resulted in a small sample size of observed hauls.
The 2008 MAG harbor porpoise bycatch estimate approach was modified somewhat from past approaches to include stratification by mesh size categories (< 6.535”, 6.535-9.150”, and > 9.150”), along with stratification by season (Jan-Apr) and state (NJ) (Figure 3), as has been done for the 2005, 2006, and 2007 estimates (Belden 2007; Belden and Orphanides 2007). Including mesh size in the Mid-Atlantic harbor porpoise stratification was suggested by Orphanides (2009) in a thorough examination of the most appropriate means to estimate harbor porpoise bycatch in the northwestern Atlantic U.S. gillnet fisheries. Harbor porpoise bycatch rates were shown to be different in nets with different mesh sizes (Orphanides 2009; Palka et al. 2008a), as has also been shown for other marine mammals (Palka and Rossman 2001) and sea turtles (Murray 2009).
The estimated number of marine mammal incidental takes (B) is the sum of the estimated number of incidental takes within each stratum (S):
The estimated number of incidental takes within a stratum is the product of the observed bycatch rate within that stratum multiplied by the total effort within that stratum. The observed bycatch rate within a stratum is defined as the number of incidental takes observed within a stratum divided by the observed mtons of landings (effort) in that stratum.
Some gillnets in the NESG fishery are equipped with pingers, and the bycatch rate of nets with pingers differs from the rate of nets without pingers (Palka et al. 2008b). To accommodate this difference, a weighted bycatch rate (WBR) was calculated for strata that have both hauls with and without pingers. Within a stratum, two weighted bycatch rates were first calculated, one from hauls with pingers (WBRp) and one from hauls without pingers (WBRnp):
Next, within a stratum, a total weighted bycatch rate (WBR) was calculated that incorporates hauls both with and without pingers:
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 estimated CV (Bisack 2003).
Northeast sink gillnet fishery2007
The overall annual observer coverage in the NESG was 7.1%, ranging from 4.4% in the summer to 11.1% in the winter (Table 1). This level is roughly double the coverage level in 2006, which was 3.6%, ranging from 1.3% in the summer to 6.1% in the winter (Belden and Orphanides 2007). One common dolphin, 35 harbor porpoises, 2 unknown porpoise/dolphins, 80 gray seals, 6 harbor seals, 11 harp seals, 18 unknown seals, and 1 unknown toothed whale were observed incidentally taken in the 2007 NESG fishery. Unidentified animals were not included in the bycatch estimates.
The 2007 NESG estimated total incidental takes of cetaceans included 11 (CV = 94%) common dolphins (Table 5) and 395 (CV = 38%) harbor porpoises (Table 6). The 2007 estimated total incidental takes of pinnipeds in the NESG fishery included 889 (CV = 24%) gray seals (Table 7), 93 (CV = 49%) harbor seals (Table 8), and 119 (CV = 36%) harp seals (Table 9).2008
The overall annual observer coverage in the NESG was 4.6%, ranging from 3.9% in the summer to 6.1% in the winter (Table 2). Two common dolphins, 4 white-sided dolphins (Lagenorhynchus acutus), 30 harbor porpoises, 31 gray seals, 9 harbor seals, 14 harp seals, and 6 unknown seals were observed incidentally taken in the 2008 NESG fishery. Also, two humpback whale (Megaptera novaeangliae) interactions were observed, but NESG fishery bycatch estimates were not calculated because the extent of any possible injuries, or lack thereof, could not be determined. Unidentified animals were not included in the bycatch estimates.
The 2008 estimated total incidental takes of cetaceans in the NESG fishery included 34 (CV = 77%) common dolphins (Table 10), 81 (CV = 57%) white-sided dolphins (Table 11), and 666 (CV = 48%) harbor porpoises (Table 12). The 2008 estimated total incidental takes of pinnipeds in the NESG fishery included 618 (CV = 23%) gray seals (Table 13), 243 (CV = 41%) harbor seals (Table 14), and 238 (CV = 38%) harp seals (Table 15).
Mid-Atlantic gillnet fishery2007
The 2007 observer coverage for the MAG fishery using both complete and limited trips was 4.1% (Table 3). The 2007 observer coverage for Jan-Apr off of NJ was 2.6% (Table 16). The 2007 observer coverage for Jan-Apr off of VA was 1.7% (Table 16). The observer coverage off of MD, VA, and DE during November was 3.0% (Table 16)
There were 1 harbor porpoise, 1 Risso’s dolphin, and 1 harp seal observed incidentally taken in the MAG fishery in 2007. All 2007 Mid-Atlantic bycatch events occurred on complete hauls. The observed hauls for the 2007 winter harbor porpoise time-area strata included 18 complete hauls, and 73 limited hauls. The 2007 Mid-Atlantic harp seal time-area strata included 10 complete hauls and 60 limited hauls. The 2007 Mid-Atlantic Risso’s dolpin time-area strata included 15 complete hauls and 65 limited hauls.
The 2007 estimated total incidental takes for cetaceans in the MAG fishery included 59 (CV = 104%) harbor porpoises (Table 17) and 34 (CV = 73%) Risso’s dolphins (Table 18). The 2007 estimated total incidental takes for pinnipeds in the MASG fishery was 38 (CV = 90%) harp seals (Table 19).
The 2008 observer coverage for the MAG fishery using both complete and limited trips was 2.8% (Table 4). The 2008 observer coverage for Jan-Apr off of NJ (used for harp and harbor seal estimates) was 2.3%, and the 2008 observer coverage for the Jan-Apr harbor porpoise strata (NJ large mesh) was 2.6% (Table 20).
There were 9 harbor porpoises, 4 harp seals, and 2 harbor seals observed incidentally taken in the MAG fishery in 2008. Mid-Atlantic 2008 bycatch on complete hauls included 4 out of 9 observed harbor porpoise incidental takes, 1 out of 4 observed harp seal incidental takes, and 1 out of 2 harbor seal incidental takes. The observed hauls for the 2008 winter harbor porpoise time-area strata included 11 complete hauls and 38 limited hauls. The 2008 Mid-Atlantic NJ winter season (the estimating strata for harp and harbor seals) included 17 complete hauls and 44 limited hauls.
The 2008 estimated total incidental takes for cetaceans in the MAG fishery included 350 (CV= 75%) harbor porpoises (Table 21), 176 (CV= 74%) harp seals (Table 22), and 88 (CV=74%) harbor seals (Table 23).
The calculation of the 2007 and 2008 cetacean and pinniped gillnet bycatch estimates involved several small changes from past estimate approaches, while still largely using the same structure. Perhaps the most important difference is the utilization of the Allocated Commercial Landings data in this analysis. Being able to link VTR trips to Dealer Reported effort should significantly improve the accuracy of the total effort calculations for the Northeast and Mid-Atlantic fisheries.
Another change involves the modification of the Mid-Atlantic harbor porpoise bycatch stratification. Including mesh size categories as strata should result in more realistic estimates, particularly for areas with low bycatch rates (Orphanides 2009). Mesh size has been shown to influence the bycatch rate, with larger mesh sizes generally having higher bycatch rates (Palka and Rossman 2001; Orphanides 2009; Murray 2009). Because harbor porpoise bycatch appears to be driven primarily by time of year and area (Palka et al. 2008a; Orphanides 2009), using single states for the Mid-Atlantic bycatch stratification can sometimes ignore fishing in the same general time and area that should be included when calculating the bycatch rate and effort. However, for 2008 NJ effort, the spatial separation from other states was nearly complete (Figure 3), and there was enough effort in NJ alone to avoid pooling effort that would have been spatially unrepresentative and may have had a different bycatch rate. In order to avoid future confusion on whether to group states or not, modifying the location strata to replace states or port groupings with strata based directly on actual reported fishing locations and historic bycatch patterns should be considered.
Another change that should be considered for future bycatch estimates is the effect of complete and limited trips on bycatch estimates. Historically, the Mid-Atlantic observer effort consisted primarily of limited trips, and very few incidental takes were observed on the small number of complete trips. Under this situation, it made sense to use only limited trips when estimating bycatch in the Mid-Atlantic. However, in the Mid-Atlantic in recent years the number of observed incidental takes on complete trips has increased and thus both complete and limited trips have been used in the bycatch estimate. It is unknown whether this increase is the result of a random effect, or whether there is an unknown factor driving the increase on these types of trips, and this should be further examined. Despite the recent increase in observed bycatch on Mid-Atlantic complete trips, it should also be investigated whether the observed bycatch rate on complete trips is statistically lower over time than that on limited trips. This is possible because on complete trips the observer is not dedicated to watching for incidental protected species takes. If there is a statistical difference, then, it might be appropriate to employ a correction factor for complete trips. More fully investigating incidental takes on complete and limited Mid-Atlantic trips would ensure that the most appropriate methods are used for future Mid-Atlantic bycatch estimates.
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