Spatial and environmental factors associated with Patagonian toothfish (Dissostichus eleginoides) distribution at South Georgia and the South Sandwich Islands (Subareas 48.3 & 48.4)

Understanding the environmental factors that drive the distribution of harvested fish species is essential for fisheries management, particularly in the context of climate change. Here, we investigated the spatial distribution of Patagonian toothfish (Dissostichus eleginoides) around South Georgia (Subarea 48.3) and the South Sandwich Islands (Subarea 48.4) in the southwest Atlantic sector of the Southern Ocean. Using scientific demersal trawl survey data, we identified and explored environmental factors linked to spatial distribution of juvenile to sub-adult Patagonian toothfish at South Georgia, with an emphasis on temperature and ontogenetic shifts. These findings were combined with an assessment of adult distribution across South Georgia and the South Sandwich Islands using longline fishery data. Trawl survey data collected at South Georgia were used to fit distribution models, informed by environmental covariates, for six different size-classes (total length: < 26 cm to > 66 cm) of Patagonian toothfish selected to approximately represent annual age groups. These were indicative of strong relationships with depth and temperature, with larger size-classes occupying progressively deeper habitats. Temperature effects were evident across all size-classes but were strongest for the three smallest size-classes, with higher abundances predicted at locations with annual mean sea surface temperature (SST) > 1.8°C. This suggests that spatial distribution patterns are initially structured by surface or near surface temperatures for the smallest size-classes, patterns which are maintained throughout the first 3-4 years of growth. Fishery-derived data from the South Sandwich Islands, limited to toothfish available to the fishery within the depth range 700-2000 m, showed a step-like transition in catch-per-unit-effort (CPUE) in relation to seafloor temperature, with CPUE declining to near zero at or below 0.2-0.3°C, generally limiting their distribution to the northern part of the island chain. This study contributes to the work of WG-FSA in relation to understanding the drivers of spatial distribution across size-classes and life-history stages, including progressing recommendations from the CCAMLR Workshop on Climate Change. Our analyses of the complementary datasets confirm a strong influence of depth and temperature on Patagonian toothfish distribution, while providing new insight into the timing and nature of ontogenetic shifts and the environmental thresholds shaping spatial patterns. Understanding life stage-specific environmental sensitivities is important for improving forecasts of distributional shifts and informing ecosystem-based fisheries management in the context of climate change.