Behavioral and life history responses of an endangered sea lion (Otariidae) following a severe marine heatwave

John M. Maniscalco, Pamela Parker and Kara Bolander

Abstract:

Introduction: The northeast Pacific marine heatwave (PMH) of 2014-2016 had strong impacts on many species across a vast region. Impacts on upper trophic species, such as reduced survival and reproduction, were a clear consequence of reduced food availability.

Methods: We conducted a long-term individual-based study of behavior and vital rates in endangered Steller sea lions (Eumetopias jubatus) in the Gulf of Alaska. Vital rates were estimated using robust design, multi-state mark-recapture statistical methods.

Results: After parturition, the duration of maternal attendance declined significantly during 2015 – 2018 and again in 2021 compared to earlier years of this study. Prior to the PMH, adult female survival rates for sea lions were approximately 93%, irrespective of parturition status. Beginning in 2016, and the years following the PMH, survival dropped to 77% for females giving birth and 85% for females that were barren, demonstrating an important cost of reproduction in terms of future survival. Females born in 2016 delayed their age at first reproduction by 1 year compared to females born in earlier years, while adult female reproductive rates declined from > 70% prior to 2016, to around 60% during the years 2016 – 2018 as greater proportion of mature females skipped a year of giving birth. Following the PMH years, there was also an increased lactation interval as a greater proportion of juveniles suckled beyond one year of age. Extended lactation buffered against a reduction in juvenile survival, but with a negative impact on adult female survival.

Conclusions: Changes in animal behavior and vital rates can be expected during periods of reduced resource availability, but adult survival should be favored over juvenile survival and reproduction in slow-living K-selected species such as the Steller sea lion.  We found that adult females will bear a cost of reproduction in terms of future survival and reproductive potential during these periods. Furthermore, a preference to nurse offspring for additional years represented a type selfless behavior that also reduced maternal survival probabilities and may ultimately compromise population stability.