Report on fish by-catch during exploratory fishing activities in Division 58.4.3a (Elan Bank) between 2008 and 2018

We report on objective 2 “inform estimations of the distribution, relative abundance, and life history of the main bycatch species” of the suspended research plan on Patagonian toothfish (Dissostichus eleginoides) last submitted in 2018 by Japan and France in Division 58.4.3a (WG-FSA-18/61). We analyzed bycatch composition in number and biomass during the period 2008-2018 to investigate the inter-annual variations in bycatch patterns, spatial distribution and biological parameters of the main bycatch species. Skates (mostly Amblyraja taaf) was by far the dominant bycatch species on longlines, followed by Macrourus spp. and Antimora rostrata. Results from both captain declaration and observer reports showed strong effects of gear and bathymetry on bycatch composition and biomass. As shown in others regions, autolines were less selective than trotline. While trotlines had a bycatch-to-catch ratio of 10 to 20%, autolines ratio was up to 70% in some years. Hence, bycatch number and biomass increased strongly the years autolines operated. Skate bycatch was higher in shallower waters (900m-1200m) and the frequency of hauls with high skate CPUE led to a very high cumulated number of skates caught in 58.4.3a. While most skates were released in good or average condition and potentially survived, without more information on the post-released survival rate, we cannot rule out the fact that the use of longlines, and in particular integrated weight autolines, may have an adverse impact on skate populations in Division 58.4.3a.