Estimators of discards using fishing effort as auxiliary information with an application to Iberian hake (Merluccius merluccius) exploited by the Portuguese trawl fleets

Abstract Estimating discards is an important issue for fisheries management so that total mortality caused by fishing is considered. In 2003, a sampling programme addressing Iberian hake discards by the Portuguese fleets was implemented according to the requirements of the European Commission Data Collection Regulation (Reg. EC No 1543/2000). A preliminary analysis of the data collected showed trip and haul duration to be potential sources of bias, because their relationship to discards per unit effort (DPUE) was not linear. This paper develops estimators for discards by weight using fishing effort as auxiliary information, describes a jack-knife procedure to identify influential observations, applies the methodologies to Iberian hake as a case study and compares the results obtained by each estimator. The estimators tested are based on Cochran's estimator of the total ( Cochran, 1977 ). Four estimators are compared, each one consisting of a combination of (i) weighting or not weighting the observations by fishing effort and (ii) post-stratifying or not post-stratifying the trips by duration in days-at-sea. A jack-knife analysis of observations’ influence on the final estimate is performed by year and fleet. The results indicate that the estimator that uses both a weighted DPUE and post-stratification by the trip duration performs best and is the most robust to influential observations. The estimates of hake discards increase with time, while a stable but oscillating pattern is observed in the demersal fleet. Total hake discards show a fluctuating trend until 2009 and a large drop in 2010. The minimum discard estimates after removing influential observations was 795 t in 2006 and the maximum was 1956 t in 2009, with coefficients of variation (CV) between 22% and 74%. Estimates using the full data set were higher but had lower CVs. The observed trend in discards agrees with the recruitment estimates by the Portuguese International Bottom Trawl Survey.