New Sampling and Data Processing Strategy for Estimating the Composition of Catches by Species and Sizes in the European Purse Seine Tropical Tuna Fisheries

Abstract

The difficulties associated with tropical tuna sampling strategies, particularly those resulting from the large-scale adoption of the method of fishing on logs, have caused the Indian Ocean tuna scientists concern since the beginning of the purse seine fishery, as well as the ICCAT SCRS (Standing Committee for Research and Statistics of the International Commission for the Conservation of Atlantic Tunas) in recent years. Upon the recommendation of this Committee, in 1996 Spain and France embarked on a European Union project aiming to study the consequences of this new fishing tactic on the estimated catch by species and by size, and to implement a new sampling strategy so as to improve the accuracy of the statistics. Many specific analyses have been necessary because of the complexity of the problem. In this regard, close attention has been given to the influence of different factors (place, time, fishing associations, flags, vessel class, etc.) on the breakdown of catches by species and sizes, the sampling coverage rate, the sample size, the unit of sampling, etc. In this document the results of the different analyses conducted are described and discussed, and a new sampling system for both east Atlantic and Indian Ocean tropical tuna purse seine catch sampling is proposed. This document is mainly focussed on Indian Ocean, while another document (SCRS/97/28) - presented at the last year’s ICCAT meeting – was aimed to the Atlantic Ocean.