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This dataset contains measurements of the diameter and area of Antarctic krill (Euphausia superba) oocytes collected from the northern Antarctic Peninsula during the austral winter from 2012 to 2016 (August each year). Female oocytes were examined using the "squash technique" developed by Cuzin-Roudy and Amsler (1991) for assessing ovarian development and sexual maturity. The squash method provides a detailed view of oocyte size, which serves as an indicator of reproductive status and energy allocation during the winter months. Data were collected and processed according to the methods described by Steinke et al. (2024). The dataset includes .csv files containing oocyte diameter and area measurements for individual female krill. These data are valuable for understanding krill reproductive cycles and their ecological responses to environmental variability. Data users should acknowledge the project and grant number, as well as credit Dr. Kim Bernard, Dr. Kirsten Steinke, and Dr. Christian Reiss. The research was conducted in collaboration with the US National Oceanic and Atmospheric Administration Antarctic Marine Living Resources group. 

The northwestern Antarctic Peninsula is an important spawning, recruitment, and overwintering ground for Antarctic krill. The region is warming rapidly, and the current impacts of climate and environmental variability on the reproductive cycle of krill remain unclear. Here, we examine the reproductive stage of female krill in the austral winter from 2012 to 2016 in relation to climate and environmental data to assess what factors influence the timing of reproductive development. We observed significant interannual variability in the degree of maturation in female krill, ranging from 48% of female krill measured at a station in 2016 to a maximum of 94% of female krill measured at a station in 2014. On average, across all five years, three-quarters of the female krill sampled were in the stage known as previtellogenesis, the point at which the onset of sexual maturity begins. The preceding spring, summer, and autumn Southern Annular Mode and the Multivariate El Niño Index explained most of the variance in the data and indicated a strong, preconditioning storm-related effect on environmental conditions leading up to winter, affecting krill maturation status at the end of the winter season. Results from our study can be used to improve krill population models that are necessary for the management of the krill fishery and for conservation at the northwestern Antarctic Peninsula.