Abstract
Steller sea lions experienced a dramatic population collapse of more than 80% in the late 1970s through the 1990s across their western range in Alaska. One of several competing hypotheses about the cause holds that reduced female reproductive rates (natality) substantively 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 (n = 151) 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 give birth, indicating some females are more fit than others. Natality, estimated at 69%, was similar to natality for Steller sea lions in the Gulf of Alaska prior to their decline (67%) and much greater than the published estimate for the 2000s (43%) which was hypothesized from an inferential population dynamic model. 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 in this endangered species which has heretofore been lacking. Without support for depressed natality as the explanation for a lack of recovery of Steller sea lions in the Gulf of Alaska, alternative hypotheses must be more seriously considered.
Highlights
Between the late 1970s and 2000, the western distinct population segment (WDPS) of Steller sea lions (Eumetopias jubatus) declined by more than 80% in the Aleutian Islands and Gulf of Alaska (GOA) [1] and was listed as endangered in 1997
The impetus derived in major part from two related factors: 1) the importance of walleye pollock (Theragra chalcogramma) to the nutritional status of the animals—pollock is widely consumed by Steller sea lions, and 2) pollock is the target of the largest single-species commercial fishery in the world, with an exvessel value in the order of half a billion dollars U.S Yet despite the massive monetary expenditures and scientific effort, no consensus of opinion has emerged about the cause of the decline
Inferential population dynamic models based on census counts of Steller sea lions indicate that the recent small increases are related to improved juvenile and adult survival, but that natality continued to deteriorate during the 1990s and 2000s [10,13]
Summary
Between the late 1970s and 2000, the western distinct population segment (WDPS) of Steller sea lions (Eumetopias jubatus) declined by more than 80% in the Aleutian Islands and Gulf of Alaska (GOA) [1] and was listed as endangered in 1997. In the context of bottom-up control of population dynamics, reduced natality of Steller sea lions [8] and low juvenile and adult survival [9,10] due to poor nutrition are believed by some to have been the causes of the population collapse. Inferential population dynamic models based on census counts of Steller sea lions indicate that the recent small increases are related to improved juvenile and adult survival, but that natality continued to deteriorate during the 1990s and 2000s [10,13]. Natality in the Central GOA was estimated to be 67% during the 1970s [14], 55% in the 1980s [8], and just 43% in the 2000s [13] (Figure 1)
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