Report on fish by-catch on exploratory fishing in Divisions 58.4.1 and 58.4.2

In this paper, we report on fish by-catch during exploratory fishing activities undertaken in Divisions 58.4.1 and 58.4.2 during the 2012 to 2018 seasons. Fish by-catch comprised 14 species or groups of species. In 2018, by-catch represented 12% of the total catch (43 tonnes over 307 tonnes of Dissostichus spp.) in Divisions 58.4.1 and 58.4.2. 98% of the biomass was represented by 2 families: Macrouridae and Channichthyidae. The others most common by-catch species or families were Muraenolepis spp., Antimora rostrata and Artedidraconidae. Raja and Bathyraja were rarely caught. Species composition varied between research blocks except for Macrourus spp. which dominated by-catch composition everywhere. The ratio by-catch to target catch was higher in the eastern part of Division 58.4.1. The ratio Macrourus to Dissostichus increased in every research blocks during the study period, particularly in the eastern blocks where Macrourus CPUE also increased in the last 3 years. Macrourus CPUE was 23kg/1000 hooks in average with relatively little spatial variations except in the east were it could reach 180kg/1000 hooks locally. As found in others areas of the Convention, Macrourus CPUE was 2 times higher for autolines than Spanish lines and trotlines, and it peaked at depths between 900 and 1300m. CPUE distribution of the others by-catch species were much lower and highly heterogeneous in space. Macrourus catch was dominated by females in all research blocks and length frequency distribution did not reveal any temporal changes within research blocks. Length at 50% maturity was estimated for the two most common species of Macrourus (Macrourus whitoni and Macrourus caml) and for Chionobathyscus dewitti. 95% of the Muraenolepis sp. and Antimora rostrata were females in every research blocks with spatial variations in total length frequency distribution.