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Steller sea lions are distributed from Japan to the California coast, USA, and population demographics vary spatially, with populations in some regions increasing while others are declining. To assess changes in population size, aerial surveys are conducted annually to quantify pup production. The timing of these surveys is critical for accurate population estimates, and survey windows were determined based on historical estimates of mean pupping date. We re assessed the timing of pupping for Steller sea lions at 2 breeding islands in the central Gulf of Alaska, USA (Marmot Island) and the eastern Aleutian Islands (Ugamak Island) for evidence of temporal shift. Using land-based counts of pups, we quantified mean pupping date and the duration of the pupping season between 2003 and 2013 and compared these data to historical mean pupping dates between 1977 and 1999. The mean pupping date of 9 June on Marmot Island was not significantly different than the mean pupping date on Ugamak Island, 8 June. On Marmot Island, mean pupping date differed by 3.7 ± 0.9 d between beaches; however, mean pupping date did not differ between beaches on Ugamak Island. On Ugamak Island, mean pupping date was significantly earlier than previously reported by 2.5 d, but this may be an artifact of the limited number of years available for comparison. On Marmot Island mean pupping date was not different from historical dates. On both islands, 94.2 ± 1.6% of the pups were born prior to the planned start of aerial surveys in Alaska (23 June). Our results demonstrate that although mean pupping date was variable and may have shifted earlier relative to historical data at Ugamak Island, the current timing of the aerial survey is suitable for obtaining peak pup counts for Steller sea lions in these regions.

A re-examination of the timing of pupping for Steller sea lions Eumetopias jubatus breeding on two islands in Alaska