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 European software T3 and those recorded on sale slips completed by the canning factories purchasing fish from 48 vessels registered with OPAGAC in the Atlantic and Indian oceans, over the period 2011-16. The analysis identified potential sources of bias in estimates of catch of tropical tunas that have been reported to the ICCAT and the IOTC during those years, or probably a longer period. The magnitude of the biases identified varied depending on the ocean, fleet, and size category, with the largest bias recorded in the Indian Ocean, where the catches of yellowfin tuna and bigeye tuna recorded by the IOTC, especially of large size, appear to be well below those obtained from sale slips. To a lesser extent, in the Atlantic Ocean the catches of yellowfin and bigeye tunas seem to be also subject to bias, although in this case underestimation of both large and small fish seem to be responsible. Even though the study is preliminary and the available datasets need to be further explored and cross-verified with actual monitoring of fish in processing plants, the results obtained, if confirmed, could have consequences on the statistics, stock assessments, management advice, and management measures adopted by ICCAT and IOTC.