Comparing electronic monitoring and logbook estimates of set-level catches in tropical tuna purse seine fisheries

Abstract

Electronic Monitoring System (EMS) has been increasingly recognized as a promising tool to complement observer programs and improve the management of tuna fisheries. Since 2013, EMS were deployed on board French purse seiners as part of the voluntary Common Permanent Unique Observer program to supplement onboard observer coverage in the Indian Ocean and provide bycatch and discards estimates at sea. In this study, we assess the potential of EMS observation to evaluate the tonnage of set-level catches of tropical tuna purse seiners. Using video analyses from five vessels, we compared EMS-derived total catch estimates with those reported in fishing logbooks. Results revealed a strong linear correlation (∼0.98) between the two sources, although EMS tend to estimate slightly higher catches for sets below ∼32 mt and lower catches for larger sets compared with logbook declarations. Linear mixed models were then applied to investigate the influence of vessel, trip, catch size, and percentage of large individuals (as a proxy of tuna size catch composition) on discrepancies between EMS and logbook estimates. Differences varied across fishing trips and increased with catch size. After discussing the methodological limitations and possible drivers of these trends, we conclude that EMS represents a valid source of information, comparable to logbooks, for estimating total catch at the set level in the French tropical tuna purse seine fishery.