Improving the West Greenland trawl survey for shrimp (Pandalus borealis)

Abstract

A stratified random research trawl survey has provided estimates of biomass and biological parameters for the West Greenland stock of northern shrimp, Pandalus borealis since 1988. About 200 stations distributed over the fishing grounds at depths between 150 m and 600 m were allocated in proportion to stratum area. The survey was extended into southern fishing grounds as the fishery developed there. Gear changes included replacing a 44 mm stretch mesh liner with 20 mm since 1993, and using a trawleye to time the tow start since 1997. Total biomass estimates have had coefficients of variance in the range of 13-23%, and have been only weakly correlated with catches-per-unit effort. The survey design was reviewed in 1997, and the changes suggested included abandoning 2-stage sampling, shortening the tows and increasing their number, allocating more stations to the strata with high yields and variances, and fixing the position of some stations. Suggested changes to the analysis included pooling strata into larger depth-based groups, log-transforming the data to reduce its skewness, and smoothing the survey results. To implement some of these suggestions, a progressive shift toward shorter tows was initiated, and some half-hour tows were experimentally carried out as pairs of 15-min tows. Using shorter tows did not appear to compromise the precision of biomass estimates. The proportion of stations allocated to strata within which catches were highly variable from place to place was increased. Buffered sampling was used to control station placement so as to prevent stations from clustering within strata. Fixing the positions of a random selection from among the trawl stations from one year to the next resulted in more precise biomass estimates.