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.
