Report on fish by-catch during Dissostichus mawsoni exploratory fishing in Divisions 58.4.1 and 58.4.2 (2016–2022)

In this paper, we report on fish bycatch during exploratory fishing activities undertaken in Divisions 58.4.1 and 58.4.2 during the period 2016 to 2022. Fish bycatch was composed of at least 14 species or species groups. In 2021 and 2022, exploratory fishing occurred in Division 58.4.2 only, and none of the bycatch limits set in CM 33-03/A were reached. Proportion of bycatch over total catch was higher in block 2 than block 1 of Division 58.4.2. Bycatch biomass was dominated (98%) by two families: Macrouridae and Channichthyidae. The percentage of Macrourus spp. remained stable in research block 58.4.2_1 at <10% of total catch, while the percentage of Channichthyidae remained low but showed an increasing trend since 2018. The other most common by-catch species or families were Muraenolepis spp., Antimora rostrata and Artedidraconidae. Bathyraja and Amblyraja were rarely caught. Species composition varied between research blocks except for Macrourus spp. which dominated bycatch composition in all fishing locations. At a larger scale, the proportion of bycatch in total catch was higher in the eastern part of Division 58.4.1 and western part of Division 58.4.2. As found in others areas of the Convention Area, reported Macrourus catch rates were higher for autoline than Spanish lines and trotlines, and lowest at depths between 1200 and 1600m. Exploratory fishing under a research plan with high numbers of fixed stations outside this depth range have an increased risk of high Macrourus bycatch. Catch rates of other bycatch species were much lower and highly variable between locations. Macrourus catch was dominated by females in all research blocks without changes in length frequency distribution patterns over time.