Report on fish by-catch during research fishing activities in Division 58.4.4b between 2008 and 2018

We report on objective 3 “Investigate population structure of bycatch species and inform bycatch mitigation measures” of the ongoing research plan on Patagonian toothfish (Dissostichus eleginoides) conducted by Japan and France in Division 58.4.4b (WG-SAM-2019/08). 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. Macrourus spp., Antimora rostrata and skates (mostly Amblyraja taaf) were the three dominant species caught as bycatch on longlines. Results from both captain declaration and observer reports showed strong effects of gear, bathymetry and locations (b_1 versus b_2) on bycatch composition and biomass. As shown in others regions, autolines were less selective than trotlines and Spanish lines. While trotline has a bycatch to catch ratio of about 15%, autolines ratio was up to 70% in some years. Hence, bycatch number and biomass increased strongly in the years autolines operated. Skate bycatch was higher in the b_2 region, particularly in the eastern part, where unusually high concentrations of Pennatulacea (Sea pens) where reported in 2018. A large number of skates were caught (up to 1,400 individuals in one year). 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 58.4.4b.