On the potential biases of scientific estimates of catches of tropical tunas of purse seiners the EU and other countries report to the ICCAT and IOTC

This document represents a first attempt to explore potential differences between the catches of tropical tunas estimated using the EU software T3 and those recorded on sale slips completed by the canning factories purchasing fish from vessels registered with OPAGAC in the Atlantic and Indian oceans, over the period 2011-16. The analysis identified potential sources of bias estimates of catch of tropical tunas may be subject to, of different magnitude depending on the ocean, fleet, and size category. The largest bias was recorded in the Indian Ocean. In the Atlantic Ocean the catches of yellowfin and bigeye tunas seem to be also underestimated, with underestimation of both large and small fish. Although the study is preliminary, the results obtained indicate that the system the EU is using to sample purse seine landings and estimate catches may be subject to bias which, if confirmed, could have consequences on the statistics, stock assessments, management advice, and management measures adopted by ICCAT and IOT