Defining the relationship between Patagonian toothfish and their environment in Subarea 48.3

This paper presents initial findings from a research project evaluating climate change risks to toothfish in subareas 48.3 and 48.4. Here we focus on Patagonian toothfish (Dissostichus eleginoides) in Subarea 48.3, using groundfish survey data to develop preliminary distribution models. Data were split into size-classes before analysis and distribution models constructed, informed by relationships with environmental covariates. Based on the expectation of ontogenetic shifts in depth range and the potential effects of temperature, we used models to estimate depth ranges, and temperature values, occupied by each size-class of Patagonian toothfish. Initial findings indicate strong inter-annual variability in juvenile recruitment at Shag Rocks, with considerably weaker recruitment on the South Georgia shelf, where juvenile abundance was lower and larger fish predominated. Over the groundfish survey time-period (1986-2023) sea surface temperatures (SST) have increased at both South Georgia and Shag Rocks, most rapidly at the former. For both locations, the rate of warming is greatest during austral spring and summer. Despite a long-term increasing trend in average SST, periods of cooler summers/autumns occurred in some years and cooler winters in others. Spatial patterns of Patagonian toothfish biomass consistently showed significant relationships with depth and mean SST, with preferences for specific depths and temperature regimes that varied by size class. We note that the long-term mean SST threshold of 1.8°C, which divides areas of high and low Patagonian toothfish abundance, aligns with the geographic division between Shag Rocks and South Georgia shelf. This suggests SST relationships may also serve as a proxy for biogeographic and/or other oceanographic factors that characterize these areas. This work provides a foundation for determining species-environment relationships across different life history stages, offering insights into important determinants of distribution, as a basis for better understanding the effects of climate change. We outline the next steps for this project and welcome input and engagement from WG-FSA.