SALTONSTALL KENNEDY GRANT PROGRAM
Estimates of Effective Fishing Effort for the....
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GRANT NUMBER:  NA46FD0397           NMFS NUMBER:   93-NWR-012

REPORT TITLE:  Estimates of Effective Fishing Effort for the U.S. West Coast Groundfish Trawl Fishery

AUTHOR:  Sampson, David B.

PUBLISH DATE:  December 1996

AVAILABLE FROM: National Marine Fisheries Service, Northwest Regional Office, 7600 Sand Point Way, NE, Seattle, WA 98115.  PHONE: (206) 526-6115

ABSTRACT

In this study, logbook data from the bottom trawl fisheries of California and Washington were used to estimate effective, standardized catch-per-unit-effort (CPUE) for thirteen groundfish species: ten species off each state. The data, which included skippers' tow-by-tow estimates of retained catch, were compared with landing receipts to remove inaccurate information; trips influenced by regulatory trip limits were also excluded.  From the remaining data a subset for each state was chosen for detailed analysis to identify influential factors, to develop simplified statistical models, of catch rates for each species, and to identify boats that could be used for estimating standardized CPUE. Excluded from the standardized analyses were boats that did not operate throughout the study period, and areas in which there was limited fishing. 

The selected data were analyzed in a stepwise manner using generalized linear models for catch rates to measure the importance of the factors Year (1985-91 in CA and 1986-92 in WA), Season (bimonthly intervals) Boat (23 boats per state), Net type (generic bottoms trawl versus trawl with roller gear), Latitude (20 minute intervals), and Depth (40 fathom intervals). Because for each species there were large numbers of tows with catches that were zero, catch rates were modeled using a delta-lognormal distribution; the numbers of tows with zero catch were treated as binomial random variables and the catch rates for the non-zero tows were treated as lognormal random variables. 

The process of data verification and screening resulted in the exclusion of data from about half the fishing trips. The data subsets that were subjected to the detailed analyses consisted of tow-by-tow catch rates (lb/hr) form 63,532 tows from the California data and 53,273 tows from the Washington data. In the logistic regression analyses of the zero catch tows, essentially all factors were found to be highly significant (P<1%) for all species from both states. Depth was the first or second most influential factor in 19 of the 20 different state-species combinations, and Boat was the first or second most influential factor in 18 combinations. In the analyses with pairwise interactions, the Year-Boat interaction was the first or second most influential interaction for 14 of 20 combinations.  In the Analyses with lognormal models of the non-zero tows, essentially all factors were found to be highly significant (P<1%) and Boat was the first of second most influential factor in 19 of the 20 combinations, Depth was the first or second most influential factor in 6 combinations, and the Year-Boat interaction was the first or second most influential interaction for 15 of 20 combinations.  For each state and species estimates of annual fishing power coefficients were examined to identify boats with stable fishing power.

To estimate standardized CPUE for each species for individual areas (defined by the factors Latitude and Depth), the simplified statistical models, developed from the detailed analyses, were applied to data from the top 40 boats for each species from each state. The data were further restricted to those areas that had been fished in during the entire study period. The estimates of area-specific CPUE, which was defined as the average catch (lb per hour of towing for those boats selected as the standards, were the average to estimate the effective, standardized CPUE for each species. In the estimates from the California data there were substantial declines indicated for rex sole, moderate declines for Dover sole, and slight declines for thornyheads; and there were moderate increases indicated for petrale sole and lingcod. In the estimates from the Washington data there were substantial declines indicated for rockfish, large declines for sablefish, and slight declines for lingcod; and there were moderate increased indicated for English sole.   Estimates of effective trawl fishing effort were derived for each state and species by dividing total reported trawl landings by the estimates of effective, standardized CPUE.

 
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