No abstract

A stratified random sample survey has been carried out since 1988 as a component of the assessment of the stock of northern shrimp Pandalus borealis in offshore West Greenland waters. In 1999, the placing of stations independently and randomly was replaced by buffered random sampling, in which stations were randomly placed but prevented from being closer together than a prescribed limit. Buffered random sampling gave a more even distribution of stations within strata, and nearest-neighbour distances were on average increased by 50%. The statistical effects were difficult to determine, but did not appear to be large, and the estimated standard errors did not change much from previous years. However, the buffered sampling method generated designs in which stations were evenly distributed over the strata, and did away with the need for subjective manual adjustment of the positions of stations which independent random sampling sometimes placed too close to another station.

No abstract

No 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 1323%, and have been only weakly correlated with catchesper-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, logtransforming 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 halfhour 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.