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 large number of predators, including whales, penguins, flying birds, seals, fish and cephalopods. 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 will increase E. superba predation. Therefore, in the present work, 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 on E. superba is consumed by C. gunnari under three different biomass scenarios: 7,000 (Bt), 70,000 (B50%) and 140,000 (B0) t. We put special focus on the potential area of overlapping between C. gunnari and Adélie penguin (Pygoscelis adeliae) off the South Orkney islands. Bayesian geostatistical models were used to test whether C. gunnari followed an ideal or generalized ideal free distribution. Spatially explicit consumption estimates were obtained under scenarios of 7,000 (Bt), 70,000 (B50%) and 140,000 (B0) t of C. gunnari biomass. We found that both species have 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 that lacked any relation with 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 (Bt) 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. The present work present novel information on 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.