Seasonal and spatial patterns of adult Antarctic krill at the Antarctic Peninsula: insights from a 41 year data analysis

Locating the spawning grounds of Antarctic krill Euphausia superba is key to understanding their population dynamics and managing the commercial krill fishery. In the shelf waters of the Antarctic Peninsula where krill and their fishery are concentrated, a recent relaxation of fishing quotas has meant that locally successful spawning grounds could become increasingly targeted. This study revisits the existing paradigm that in summer, larger adult krill migrate away from these shelf-based fishing grounds, out towards oceanic waters to spawn off the shelf break (>1000 m). Our findings support a seasonal on-shelf division of adult krill, with smaller krill closer inshore. However, contrary to current understanding, our results suggest that 85% of the largest and most fecund >50 mm krill remain on-shelf during the summer spawning season. All adults (>30 mm) were strongly concentrated over the shelf throughout the whole spring-summer-autumn transition, with no evidence for any summer redistribution off-shelf. The Western Antarctic Peninsula (WAP) has strong on-shelf-off-shelf gradients in chlorophyll a and sea surface temperature, and based on generalised additive models, our study suggests that the high food requirement of adult krill, coupled with an oligotrophic off-shelf environment, contributes to the distribution of krill observed at the Antarctic Peninsula. The concentration of large, nutritious krill over the shelf throughout the season may be advantageous to land-based predators but raises concern for the management of fisheries that are becoming increasingly concentrated within the key adult krill habitat along the northern shelf of the WAP.