Evaluation of sampling methods to quantify discarded fish using data collected during discards project EC 95/094 by Northern Ireland, England and Spain

Two sampling methods, equal probability and probability proportional to x ( ppx), were investigated in order to recommend an optimum sampling strategy for carrying out a discards survey in the Irish Sea. The objective of the s urvey, which took place from April 1997 to August 1998, was to estimate the quantity of fish discarded at fleet level by Northern ireland trawlers. Ad ditional data collected from English and Spanish trawlers were used to test the generality of the results. Simple random. regression, ratio and ppx es timators were evaluated. The regression, ratio and ppx estimators use addit ional information on a variable x in order to improve the precision of the estimate of the variable of interest y and offer a potential improvement ov er the simple random estimator if there is a significant correlation betwee n x and y. In order to compare the various estimators, 1000 independent dat a sets were generated using a bootstrapping technique for both equal probab ility and ppx with replacement selection. When comparing the estimators thr ee criteria were examined: (i) bias, if any, of the estimator; (ii) relativ e precision of the estimators: (iii) accuracy of the formula for the varian ce of the estimator. Linear relationships between potential x-y combination s were assessed for significance and the residuals about the fitted line ex amined. This was carried out for each country and gear type for the main co mmercial species individually and all species combined. Different estimator s proved optimal according to the x-y. combination examined. In some instan ces the improvement in precision when using the ppx sampling method was mar ginal over the equal probability method. For the data sets used in this stu dy the ppx scheme offered insufficient advantage over the simpler equal pro bability method to justify the greater complexity in implementing ppx. (C) 2001 Elsevier Science B.V. All rights reserved.