Temporal variability in mortality and recruitment jointly influence the periodic fluctuations in Antarctic krill populations.

Antarctic krill (Euphausia superba) is a key part of the food web in the Southern Ocean ecosystem. Significant inter-annual fluctuations in population dynamics make stock assessment and management of its population a significant challenge. To better understand the population dynamics and fluctuation of krill, a survey-based age-structured catch-at-length model (ACL) is used to estimate the periodic fluctuations, based on length data collected from scientific surveys under the US Antarctic Marine Living Resources (AMLR) Program between 1992 and 2011. Spectral analysis of the model estimates revealed periodic fluctuations of 5-6 years in the recruitment, total abundance, and total biomass of krill in the Antarctic Peninsula, while spawning stock biomass exhibited periodic fluctuations of both 5-6 years and 2-3 years. The variations in krill total abundance and total biomass were mainly driven by recruitment, but the variation in spawning stock biomass was likely driven by both recruitment and time-varying, age-specific mortality. Our study contributes to a better understanding of the patterns and drivers of the periodic dynamics of Antarctic krill, which may help lead to improved assessments and fishery management for this important stock.