Consumption of Antarctic krill Euphausia superba by mackerel icefish, Champsocephalus gunnari off the South Orkney Islands: filling an information gap in the current ecosystem-based management approach

The Antarctic marine ecosystem is largely dominated by Euphausia superba, the most abundant krill species consumed by a wide array of predators, including whales, penguins, flying birds, seals, fish and cephalopods. The current management of the krill fishery follows an ecosystem-based approach which takes into account the distribution, abundance of E. superba and its main predators plus their interactions. Mackerel icefish, Champsocephalus gunnari, was once considered the most abundant meso-pelagic fish species and a very important consumer of E. superba until its population was collapsed by overfishing in the early 90s. Currently, C. gunnari populations are slowly recovering which may increase predation on E. superba. Therefore, in this study, we analyze the distribution and density of E. superba and C. gunnari, gain some insights about the type of foraging strategy of C. gunnari and estimate its consumption of E. superba under three different biomass scenarios of C. gunnari: 7000 (B_t), 70,000 (B_50%) and 140,000 (B₀) t. We focus particularly on the potential area of overlap between C. gunnari and Adélie penguin (Pygoscelis adeliae) off the South Orkney islands. Bayesian geostatistical models were employed to test whether C. gunnari followed an ideal or generalized ideal-free distribution. Spatially explicit consumption estimates were derived under the scenarios of 7000 (B_t), 70,000 (B_50%) and 140,000 (B₀) t of C. gunnari biomass. We found that both species exhibit a higher probability of presence and density north of the South Orkney Islands, where they present a high overlap. The probability of presence of Champsocephalus gunnari was best explained by the ideal-free distribution whereas its acoustic density was best explained by an independent spatial model, showing no relation to the distribution of E. superba. Individual consumption of E. supeba by C. gunnari was estimated to be 153.5 ± 77.9 g ind⁻¹. Thus, current biomass (B_t) would only remove ~ 0.07% of E. superba biomass in the focus area. Under a pre-exploitation biomass level (B₀), C. gunnari would remove ~ 1.4% of E. superba biomass. This study provides novel insights into the relationship between a key species like E. superba and a highly specialized consumer such as C. gunnari around the South Orkney Islands and contributes to fill in an important gap regarding fish consumption of krill in the Antarctic ecosystem.