Spatial distribution and krill consumption estimates of mackerel icefish (Champsocephalus gunnari) around South Georgia

Demersal fish are a critical component of Antarctic marine ecosystems and may consume relatively large proportions of Antarctic krill (Euphausia superba). Along with other krill-dependent predators, their reliance on krill as a primary prey resource places them at risk from competition with commercial krill fisheries. However, providing estimates of krill consumption to inform spatial management requires an understanding of the spatial distribution of fish biomass, and spatial variation in dietary consumption of krill. In this study we use a long-term (1986-2023) dataset of groundfish surveys to examine the spatial distribution of mackerel icefish (Champsocephalus gunnari) around South Georgia in the Southern Ocean. We found that mackerel icefish are distributed across the South Georgia and Shag Rocks shelves to 400 m depth, with density declining to zero beyond this depth. Despite their almost continuous distribution, several localized biomass hotspots were evident, related to seafloor topography (slope and curvature) and sea surface temperature. While krill predominated in mackerel icefish diet samples, constituting 60% by mass on average across all years sampled, diet composition varied among locations, primarily between the South Georgia shelf, where krill predominates, and Shag Rocks where diets are more piscivorous. However, finer-scale differences in diet composition were also evident, with higher relative krill consumption at locations coincident with hotspots of fish biomass and krill fishing. Combining distribution maps of fish biomass, daily consumption rates and a spatially constant diet consisting of 60% krill resulted in a minimum consumption estimate by mackerel icefish of 278×10³ tonnes of krill per year. Incorporating spatial variation in diet composition resulted in lower overall krill consumption (265 ×10³ tonnes per annum), but higher consumption in locations targeted by the krill fishery. However, we stress that these estimates likely represent a minimum given the unknown, and potentially considerable, proportion of mackerel icefish dispersed throughout the water column that are not caught by the ground-trawl methodology used in our analyses. Our results highlight the need to consider broad to fine-scale spatial variation in diet composition, as well as predator biomass, when examining the potential for competition between marine predators and commercial fishing operations.