Environmental drivers of demography and potential factors limiting the recovery of an endangered marine top predator

Abstract

Understanding what drives changes in wildlife demography is fundamental to the conservation and management of depleted or declining populations, though making inference about the intrinsic and extrinsic factors that influence survival and reproduction remains challenging. Here we use mark-resight data from 2000 to 2018 to examine the effects of environmental variability on age-specific survival and natality for the endangered western distinct population segment (wDPS) of Steller sea lions (Eumetopias jubatus) in Alaska, USA. Though this population has been studied extensively over the last four decades, the causes of divergent abundance trends that have been observed across the wDPS range remain unknown. We developed a Bayesian multievent mark-resight model that accounts for female reproductive state uncertainty. Annual survival probabilities for male pups (0.44; 0.36-0.53), female yearlings (0.63; 0.49-0.73), and male yearlings (0.62; 0.51-0.71) born in the western portion of the wDPS range, estimated here for the first time, were lower than those in the eastern portion of the wDPS range, estimated as: male pups (0.69; 0.65-0.74), female yearlings (0.76; 0.71-0.81), and male yearlings (0.71; 0.65-0.78). There was a higher proportion of young female breeders in the western portion of the range, but overall natality was lower (0.69; 0.47-0.96) than in the eastern portion of the range (0.80; 0.74-0.84). Additionally, pup mass had a positive effect on pup survival in the eastern portion of the range and a negative effect in the western portion of the range, potentially due to earlier weaning of heavier pups. Local-and basin-scale oceanographic features such as the Aleutian Low, the Arctic Oscillation Index, the North Pacific Gyre Oscillation, chlorophyll concentration, upwelling, and wind in certain seasons were correlated with vital rates. However, drawing strong inferences from these correlations is challenging given that relationships between ocean conditions and an adaptive top predator in a dynamic ecosystem are exceedingly complex. This study provides the first demographic rate estimates for the western portion of the range where abundance estimates continue to decline. These results will advance efforts to identify factors driving

Reduced survival of Steller sea lions in the Gulf of Alaska following marine heatwave

“…For this dataset, we also could examine PMH effect on survival by more specific age ranges within the prime-aged category for females (young-prime = 5-8, mid-prime = 8-12, old-prime = 11-15). We chose a start age of 5 for young-prime because few females produced pups < age 5 (Warlick et al 2022) and therefore only potentially breeding females were included. For Marmot and Sugarloaf, only the 2014 cohort could have experienced the PMH as juveniles (the next youngest cohort was age 4 in 2014).…”

Age‐specific reproduction in female Steller sea lions in Southeast Alaska

“…If a similar pattern occurs with age for Chiswell Island females, birth probabilities at peak ages would be higher at that rookery than in Southeast Alaska. A recent estimate for Steller sea lion females in the northern Gulf of Alaska also suggests higher peak reproductive output in this area (0.80, Warlick et al, 2022 ). Birth probability estimates for females in the Gulf of Alaska based on cross‐sectional data were 0.63 during 1975‐1978 (pre‐decline) and were 0.55 in 1985 and 1986 during a period of dramatic decline, due to high rates of late‐term abortions for younger, lactating females (Pitcher & Calkins, 1981 ; Pitcher et al, 1998 ).…”

“…Endangered Species Act. Recent estimates of age‐specific survival probabilities (Altukhov et al, 2015 ; Fritz et al, 2014 ; Hastings et al, 2011 ; Maniscalco, 2014 ; Warlick et al, 2022 ; Wright et al, 2017 ) are useful for population viability models, but age‐specific information on reproduction is sparse. Models have relied on reproductive rate estimates from the 1970s and 1980s which were based on pregnancy rates of cross‐sectional samples (Pitcher & Calkins, 1981 ; Pitcher et al, 1998 ) that assumed no reproductive senescence (York, 1994 ).…”

“…If a similar pattern occurs with age for Chiswell Island females, birth probabilities at peak ages would be higher at that rookery than in Southeast Alaska. A recent estimate for Steller sea lion females in the northern Gulf of Alaska also suggests higher peak reproductive output in this area (0.80,Warlick et al, 2022). Birth probability estimates for females in the Gulf of Alaska based on cross-sectional data were 0.63 during[1975][1976][1977][1978]…”