Appendix
A. Introduction This appendix documents the method used to estimate the spatial distribution of total landings in the 1996 Northeast sink gillnet fishery. The method presented here was new as a result of a changing data collection system. Specifically, the Northeast Fisheries Science Center (NEFSC) switched from a port agent interview system to a mandatory self-reporting logbook system. The earlier data collection system had port agents interview fishers for trip locations and the port agents then merged the trip location with dealer data. This study presents preliminary results because a trip-based allocation for the entire (all gear types) commercial data base is in progress. Two databases were used to estimate the spatial distribution of fish landings: the dealer data (WO) and the self-reporting vessel trip report (VTR) logbook data. The dealer data include the total trip landings, port, vessel, and date of landing. The logbook data include these data elements plus the location of the fishing trip. To date, the dealer data have been considered a census of total landings (Bisack, 1997 ; Bisack and Dinardo, 1992; Bravington and Bisack, 1996) and the VTR data is considered a sample of the census. Estimates of total landings by area and time were used to expand the observed marine mammal bycatch rates to derive total marine mammal bycatches. The logbook data were stratified temporally and spatially to account for marine mammal bycatch and fishery landings patterns. Historically the temporal stratification has been by seasons: winter (January - May), summer (June - August), and fall (September - December). The spatial stratification has been by port, where port is a surrogate for area: Northern Maine, Southern Maine, New Hampshire, Northern Boston and Southern Boston. (5) These strata were not sufficient in 1996 due to the implementation of time and area closures to reduce harbor porpoise bycatch. As such, in 1996, a new stratification was developed and it was the first year VTR data was used in marine mammal bycatch analysis. Data and Methods The data used in the bycatch analysis were the 1996 dealer (WO) data and 1996 vessel trip report (VTR) logbook data for the New England multispecies sink gillnet fishery. VTR and WO data from ports north of and including the south of Cape Cod port were included. Assumptions 1. Dealer data (WO) are a census of total landings. 2. VTR data are a representative sample of the census data. Stratification For details, see this section in main part of the paper. Proration VTR landings were used to prorate WO landings within a season and port by area. The steps below describe the overall proration scheme. 1. The VTR landings data and WO landings data were summed over all species landed within each season and port group. The VTR sample was then examined for biases. 2. Within a season and port group, VTR landings were spatially stratified into the following areas: 1) closure areas; 2) offshore (SA 515,522 and 464); and 3) non-closure and non-offshore area. The percentage of landings by area were then calculated within each season and port group combination. For example, in northern Maine in the winter season, no landings occurred in the closed areas, 97% of landings occurred offshore, and 3% were in the non-closure areas. 3. WO landings were prorated. Within each season port, WO season port landings are multiplied by the percentage of VTR landings in an area (Step 2). For example, if 100 lbs of fish were landed in Maine in the summer season and the VTR data revealed 90% of the landings were offshore, then 90 lbs would be prorated to the offshore area in Maine during the summer. 4. Total offshore landings were the sum of all port group offshore landings. Similarly, all landings in the closure areas were summed since several port groups may have been fishing within a closure. Data Problems Within the 1996 VTR data set, some trip records have missing information on port codes and fishing location. Of the 13,307 records in the 1996 VTR database, 3,776 records (28%) contain limited location data (only statistical areas) and 900 records (7%) have missing port codes. To utilize as many VTR logbooks as possible, all trips were sorted by vessel permit number and date of the trip. Each record with a missing port code was matched with the closest record in time for that vessel with an identified port code. It was then assumed that the vessel landed in the same port on two consecutive trips. Trips without latitude and longitude entries were eliminated. Results The landings distributions, by port and season, for the VTR logbook and dealer WO were generally similar (Table A1). WO landings were consistently higher compared to the VTR, except in the Northern Maine stratum. The percent of landings by port group in the VTR logbook and the WO differ by a maximum of 6.4% in the winter (North Boston), 6.1% (North Boston) and 6.0% (South Cape Cod) in the summer, and 3.1% in the fall (New Hampshire). VTR landings indicate sink gillnet vessels have a larger percentage of landings offshore in winter than during summer and fall; however, landings are lowest in the winter season (Table A2). Sink gillnet landings in the marine mammal closure areas were highest in the fall compared to the winter and summer seasons. ------------------------ Table A1. 1996 Northeast sink gillnet fishery VTR logbook and WO (L)andings in tons of fish and percentage (%) by port group, Winter (January - May), Summer (June - August), and Fall (September - December)
Table A2. Distribution of 1996 Northeast sink gillnet VTR landings by season and area
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