Demographic and maturity patterns of Antarctic krill (Euphausia superba) in an overwintering hotspot

Overwinter environmental conditions are a major driver of larval survival, recruitment, and reproductive pre-conditioning in Antarctic krill (Euphausia superba). Few winter studies exist from which to infer impacts of changing environmental conditions on the biology of krill. Here demographic and maturity patterns of krill during five winters are examined at an overwintering hotspot in the northern Antarctic Peninsula. Two different recruitment pulses were present over the 5 years of the study. A large recruitment event was evident in winter 2012, followed by cohort growth tracked through winter 2014. A recruitment event also occurred in 2015, despite lower abundance of larvae in 2013 and 2014. The recruitment pulse in 2015 was correlated with the intrusion of high salinity water, characteristic of the Weddell Sea, during a positive El Nino-Southern Oscillation phase, suggesting recruitment was driven by advection of recruits into the northern Antarctic Peninsula. Despite the inter-annual environmental variability, most female krill were in resting reproductive stages in all years. The lack of variability in female reproductive status during winter may impose a developmental constraint to earlier spawning than previously considered; the time necessary to advance from resting stages to reproductive maturity may limit the ability of krill to adapt to trends in the timing and magnitude of the spring bloom. The observed patterns of recruitment and variation in maturity stages highlight the complexity of krill population dynamics and should be considered when projecting population responses of Antarctic krill to climate-driven changes in the Southern Ocean.