Monitoring by-catch species in the Ross Sea region toothfish fishery

This paper provides a summary of trends in performance indicators, including catches, fishing effort, catch rates, fish size, sex ratios and fish body condition, for the main bycatch species/species groups in the longline fishery targeting Antarctic toothfish (Dissostichus mawsoni) in the Ross Sea region (RSR) (Subareas 88.1 and Small-Scale Research Units (SSRUs) 88.2A–B), up to and including the 2021/22 fishing season. Key biological parameters for the main bycatch species, including length-length conversion factors, length-weight relationships and length-at-maturity schedules, are also provided as a baseline reference for related work. As documented previously for the RSR toothfish fishery, five species groups (macrourids, skates and rays, icefish, eel cods and morid cods) dominate the bycatch of the fishery, comprising 99.5% of the total reported bycatch by weight. Bycatch species composition varied between management areas; however landings of most species groups were generally highest in SSRUs 88.1H and 88.1I in the S70 (South of 70°S) management area. Based on reported catch and effort information, nominal catch rates have generally been stable for most species groups, although icefish catch rates have increased in the last three fishing seasons, particularly for autoline vessels. For macrourids, skates and rays, icefish and eel cods, a large number of records provided by scientific observers continue to be collected at coarse taxonomic levels (e.g., genus, family or order). Unscaled length frequencies and body condition of the key bycatch species (i.e., the macrourids Macrourus caml and M. whitsoni, the skates and rays Amblyraja georgiana and Bathyraja eatonii, the icefish Chionobathyscus dewitti, eel cods (Family Muraenolepididae) and the morid cod Antimora rostrata) showed little variation over time, with no evidence of progressive truncation. Unscaled sex ratios of most species across the RSR have been similarly stable over time and dominated by females for most species.

To support ongoing monitoring of bycatch species in the RSR toothfish fishery, we recommend:

1. Continuing data collection for bycatch species as proposed in WG-FSA-22/XX.
2. Requesting the CCAMLR Secretariat to investigate approaches to increase the number of scientific observer records that are identified to the species level for the main bycatch groups (particularly macrourids, skates and rays, icefish and eel cods), including collaborating with scientific observer coordinators.
3. Collaborating on targeted analyses of bycatch ratios, to understand why there are differences in catch rates of bycatch among gears and among vessels.
4. Collaborating to monitor bycatch performance indicators at regular intervals (every two years suggested), for submission to WG-FSA in bycatch-focused years.
5. Requesting the CCAMLR Secretariat periodically summarise data available for bycatch species from toothfish fisheries and incorporate this information into fishery reports.