Assessment of accuracy in processing purse seine tropical tuna catches with the T3 methodology

The multi-species nature of tropical tuna surface fisheries gives rise to a series of difficulties when estimating the catch by species and catch by size statistics. The T3 processing was built about 30 years ago in order to correct biases of the logbook on species composition and to provide more accurate estimates of catch by species for the European purse seine fleet. However, the evolution of fishing practices and fishing stock according to climate change have challenged the T3 methodology on some part of it processing. The aim of this paper is so to give key elements to understand the potential biases that could occurs in the catch assessments of tropical tunas of purse seiners and second explore some ways in order to increase accuracy of T3 processing for future. By comparing catch weight obtain from T3 processing output and from sale slips weight produced by cannery factories, we found a potential overestimation of catch of less dominant species, which lead to an underestimation of dominant species. This bias could be a consequence of the evolution of length-weight relationship used in T3 processing, for a minor part, but should mainly due to the too large spatio-temporal stratification used to predict species catch. We also discussed of the limit on the T3 processing in relation to the data quality and the reliability of the sale slips.