Impacts of management measures to address skate bycatch in the Southern Ocean: Evaluating Whiteleg skate (Amblyraja taaf) abundance and fishing mortality trends in Crozet Islands

Skates are common bycatch species in the toothfish (Dissostichus sp.) fisheries operating in the Southern Ocean. While spatial density patterns and abundance trends are difficult to estimate for elasmobranch species due to a lack of data, this information is crucial to quantify fishing mortality and inform bycatch management. In this study, we developed a modelling framework consisting of fitting vector autoregressive spatio-temporal (VAST) models to catch rate data collected over the period 2007–2023 by scientific observers onboard longliners targeting toothfish in the Crozet region. We generated catch rates in number of fish per km² by calculating an area swept for each catch observation as the distance fished multiplied by a bait plume value. We employed the model predictions to map the spatial density patterns of Whiteleg skate (Amblyraja taaf) and reconstructed the annual abundance trends of the species. Using the predicted annual abundances of Whiteleg skate and recent post-release survival estimates, we quantified the annual bycatch mortality of Whiteleg skate in the Crozet region. Overall, our results indicated an enhancement in Whiteleg skate abundance over the study period, with significant increases in the early 2010s and post-2020. Bycatch mortality rates decreased over time, coinciding with the implementation of conservation and management measures in the toothfish fishery. Our modelling framework provided valuable insights into skate density patterns, abundance trends and fishing mortality, highlighting positive effects of bycatch management. This modelling framework can be adapted to other bycatch case studies where accurate catch data are available, including through observer programmes.