Plasticity and seasonality of the vertical migration behaviour of Antarctic krill using acoustic data from fishing vessels

Understanding the vertical migration behaviour of Antarctic krill is important for understanding spatial distribution, ecophysiology, trophic interactions and carbon fluxes of this Southern Ocean key species. In this study, we analysed an eight-month continuous dataset recorded with an ES80 echosounder on board a commercial krill fishing vessel in the southwest Atlantic sector of the Southern Ocean. Our analysis supports the existing hypothesis that krill swarms migrate into deeper waters during winter but also reveals a high degree of variability in vertical migration behaviour within seasons, even at small spatial scales. During summer, we found that behaviour associated with prolonged surface presence primarily occurred at low surface chlorophyll a concentrations whereas multiple ascent-descent cycles per day occurred when surface chlorophyll a concentrations were elevated. The high plasticity, with some krill swarms behaving differently in the same location at the same time, suggests that krill behaviour is not a purely environmentally driven process. Differences in life stage, physiology and type of predator are likely other important drivers. Finally, our study demonstrates new ways of using data from krill fishing vessels, and with the routine collection of additional information in potential future projects, they have great potential to significantly advance our understanding of krill ecology.

Automated summary

Understanding the vertical migration behavior of Antarctic krill is crucial for comprehending their spatial distribution, ecophysiology, trophic interactions, and carbon fluxes. This study analyzes an eight-month continuous dataset collected from a commercial krill fishing vessel in the southwest Atlantic sector of the Southern Ocean. The findings reveal variability in the krill's vertical migration behavior within seasons, suggesting that factors such as environmental conditions, life stage, and predators contribute to this behavior. Furthermore, the study showcases the potential of utilizing data from krill fishing vessels to advance our understanding of krill ecology.