Comparing electronic monitoring system with observer data for estimating non-target species and discards on French tropical tuna purse seine vessels

Electronic monitoring system (EMS) was recently implemented on French tropical tuna purse seiners to complement the current Orthongel’s observer program and to increase observer coverage both in the Indian and Atlantic Oceans. The main objective was to test the efficiency and the potential of the EM system compared to regular observer programs. In this perspective, ‘mixed’ trips involving both EMS and on-board observers were conducted on the Torre Giulia and Gevred CFTO vessels over 2015-2016. In this study, we analyzed non-target species and discard data from six mixed trips and compared EMS to observer estimations using generalized linear models (GLMs). Good matches between both methods were observed for tuna discards, including for skipjack and for the bigeye/yellowfin discard group. However divergences between estimations and methods were noted for non-target catch and the difference appeared to depend on the species. For species with high occurrence such as triggerfish and mackerel scad which are systematically discarded, EMS provided similar estimates as on-board observation. EMS could actually be more efficient than observers to describe the discarded volume of these species as it allows exhaustive counts on the discard belt. However, for larger species such as sharks and billfishes or for high commercial value species such as dolphinfish, EMS systematically underestimated occurrence and discards volume compared to observers. Indeed these species can be handled at different places on deck (individuals release directly at sea) or below deck (individuals kept for cooking or for wells) and EMS usually failed to document their retrieval due to camera distance or dead angles. We conclude that with some improvement on camera installation as well as with minimal crew collaboration, EMS on French tropical tuna purse seiners appears to be a promising tool for monitoring discards and non-target catch at an acceptable species identification resolution to supplement regional observer program.