Temperature variation associated with interannual variability in abundance of juvenile Patagonian toothfish (Dissostichus eleginoides) at South Georgia

A detailed understanding of the life cycles of fished species in relation to their environment is important for predicting the impacts of environmental change and integrating these considerations into management to ensure long-term sustainability. Here, we present findings from a study exploring the relationship between the abundance of early life stages of Patagonian toothfish (Dissostichus eleginoides) and temperature in the waters around the sub-Antarctic island of South Georgia and nearby Shag Rocks (Subarea 48.3). Using counts and size data of Patagonian toothfish collected during demersal trawls performed on the South Georgia and Shag Rocks shelf (1986-2023), we constructed a series of models relating interannual variability in juvenile (< 26 cm and 26-37 cm size-classes) abundance to regional temperatures during spawning, egg-dispersal, larval-dispersal, and post-benthic settlement developmental stages. During those analyses, we considered temperatures at multiple depth-strata, ranging from the surface to the seafloor, informed by the known distribution of Patagonian toothfish during those stages of development. We then evaluated long-term trends in temperature to identify stages that have potentially experienced the largest changes in thermal conditions over the last 30 years. Juvenile Patagonian toothfish showed marked interannual variability overlaid onto an apparent long-term decline in abundance from 1987 to 2023. Interannual variation in abundance of juveniles was highly correlated with sub-surface temperatures during the spawning and egg-dispersal time-windows, with cooler temperatures associated with lower abundance. While regional sea surface temperatures increased from 1993 to 2023, temperatures below the surface mixed layer during the spawning period appear to have decreased, which may be contributing to the apparent decline in juvenile abundance. This study contributes to the work of WG-FSA in progressing recommendations from the CCAMLR Workshop on Climate Change, particularly in relation to drivers of distribution and inter-annual variability in abundance and recruitment patterns. Our findings underscore the importance of considering factors beyond sea surface temperature changes when assessing the impacts of climate change on fished species, as well as the role of environmental conditions across key developmental windows. Understanding these drivers provides the opportunity for future recruitment assumptions used in management to be based on an understanding of the underlying processes, rather than empirical approaches, ultimately leading to more informed and effective decision-making.