High Natality Rates of Endangered Steller Sea Lions in Kenai Fjords, Alaska and Perceptions of Population Status in the Gulf of Alaska (2010)

Steller sea lions experienced a dramatic population collapse of more than 80 percent in the late 1970s through the 1990s across their western range in Alaska. One of several hypotheses about the cause is that reduced female reproductive rates (natality) contributed to the decline and continue to limit recovery in the Gulf of Alaska despite the fact that there have been very few attempts to directly measure natality in this species. We conducted a longitudinal study of natality among individual Steller sea lions at a rookery and nearby haulouts in Kenai Fjords, Gulf of Alaska during 2003–2009. Multi-state models were built and tested in Program MARK to estimate survival, resighting, and state transition probabilities dependent on whether or not a female gave birth in the previous year. The models that most closely fit the data suggested that females which gave birth had a higher probability of surviving and giving birth in the following year compared to females that did not. This indicates that some females are more fit than others. Natality was estimated at 69 percent. This is similar to natality for Steller sea lions in the Gulf of Alaska prior to their decline and much greater than the published estimate for the 2000s. Reasons for the disparity are discussed, and could be resolved by additional longitudinal estimates of natality at this and other rookeries over changing ocean climate regimes. Such estimates would provide an appropriate assessment of a key parameter of population dynamics  which has been lacking for this species. Without support for depressed natality as the explanation for a lack of recovery of Steller sea lions in the Gulf of Alaska, we must seriously consider alternative hypotheses.

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John M. Maniscalco, Alan M. Springer, and Pamela Parker. Professional paper from National Marine Fisheries Service (NMFS), 2010.