Sea ice influences Antarctic krill population structure through juvenile habitat associations in the Western Indian Ocean sector of the Southern Ocean

Abstract

Antarctic krill (Euphausia superba) is a keystone species in the Southern Ocean, yet its population dynamics and environmental responses remain poorly quantified, particularly in the Southwest Indian Ocean sector. Here, we present a 5-year survey (2019–2024) results on krill demography and its environmental drivers. Krill density and population structure exhibited pronounced interannual variability, with contrasting patterns between the Cosmonaut and Cooperation seas. Age composition and recruitment indices also varied considerably among years and regions, indicating substantial fluctuations in cohort dynamics and recruitment success. Spatial analyses revealed consistent size-dependent segregation, with young krill (1+ and 2+) predominantly found in regions of late-spring (December) sea-ice concentration. A generalized additive model showed a significant positive relationship between the proportion of young krill and sea ice (Deviance Explained = 63.6%, P < 0.001). Sea ice was significantly associated with young krill habitat occupancy, but not consistently associated with recruitment indices. We did not detect a clear long-term trend when comparing our data with historical estimates (1980s and 2006), though the limited temporal coverage of historical data warrants caution. Our findings suggest that sea-ice variability may affect the demographic structure of Antarctic krill populations by shaping juvenile habitat occupancy and subsequent cohort composition, thereby highlighting the importance of sea-ice-facilitated juvenile habitats for population dynamics under changing Southern Ocean conditions.