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

Abstract

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 survey, which took place from April 1997 to August 1998, was to estimate the quantity of fish discarded at fleet level by Northern Ireland trawlers. Additional data collected from English and Spanish trawlers were used to test the generality of the results. Simple random, regression, ratio and ppx estimators were evaluated. The regression, ratio and ppx estimators use additional 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 over the simple random estimator if there is a significant correlation between x and y. In order to compare the various estimators, 1000 independent data sets were generated using a bootstrapping technique for both equal probability and ppx with replacement selection. When comparing the estimators three criteria were examined: (i) bias, if any, of the estimator; (ii) relative precision of the estimators; (iii) accuracy of the formula for the variance of the estimator. Linear relationships between potential x– y combinations were assessed for significance and the residuals about the fitted line examined. This was carried out for each country and gear type for the main commercial species individually and all species combined. Different estimators proved optimal according to the x– y combination examined. In some instances the improvement in precision when using the ppx sampling method was marginal over the equal probability method. For the data sets used in this study the ppx scheme offered insufficient advantage over the simpler equal probability method to justify the greater complexity in implementing ppx.