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 period 2014 to 2020. Fish by-catch comprised 12 species or groups of species. In 2019 and 2020, exploratory fishing occurred exclusively in research block 58.4.2_1 where by-catch represented 4.8% of the total catch in both years. 98% of the biomass was represented by two families: Macrouridae and Channichthyidae. The other 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.

In research block 58.4.2_1, the ratio of Macrourus to Dissostichus decreased in the last two years compared to 2017, while the ratio of Channichthyidae in the total catch increased. None of the bycatch thresholds set in CM 33-03/A were reached. Macrourus catch rates was relatively low in 2019 and 2020 (9.7 and 9.4 kg/1000 hooks) compared to the average value across the two divisions (23 kg/1000 hooks).

As found in others areas of the Convention, reported Macrourus catch rates was 2 times higher for autolines than Spanish lines and trotlines, and it peaked at depths between 900 and 1300m. Catch rates of other by-catch species were much lower and highly heterogeneous in space. Macrourus catch was dominated by females in all research blocks and their length frequency distribution did not reveal any temporal changes within research blocks.