A Biomass Index for Northern Shrimp (Pandalus borealis) in Davis Strait Based on Multiplicative Modelling of Commercial Catch-per-unit-effort Data (1976-97)

摘要

Four biomass indices for the northern shrimp (Pandalus borealis) stock in Davis Strait, derived from commercial catch-per-unit-effort (CPUE) data, were analysed by multiple regression to construct a single time series with the ultimate goal of tracking trends in stock biomass during the period 1976 to 1997. Indices from the offshore fisheries have been used previously in stock assessments while an index derived from logbook data for small inshore vessels operating around Disko Island has not been presented previously. The analysis used multiplicative models and linked catch rate to vessel fishing power, temporal and spatial availability and overall annual biomass density. In two of the indices, the temporal (MONTH) and spatial (AREA) effects were modelled as interaction terms. Other second order effects were treated as random error. The AREA*MONTH interaction term corresponds to an annual migration or behavioural pattern. The procedure corrects for increases in technological efficiency over the 22-year period due to fleet renewal but changes in the efficiency of individual vessels were not accounted for. The four indices were combined, weighted by the area of the total fishing grounds each index represented. The combined index was indicative of the biomass density for shrimp greater than about 17 mm in carapace length. During the 1976-88 period, the index was higher than from 1989 onwards. The combined index showed agreement with research survey biomass estimates but the contrast in the data was low during the period of comparison (1988-97). The combined CPUE index was applied to the total catches to derive estimates of total standardized effort. This effort index increased substantially in the late-1980s.