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

Abstract

The North Pacific marine heatwave of 2014–2016 (PMH), one of the most geographically-extensive and severe marine heatwaves on record, resulted in widespread and persistent perturbation of the Gulf of Alaska and California Current ecosystems. Negative effects of the PMH on marine mammals have been observed, but are not yet well understood. The endangered Steller sea lion Eumetopias jubatus is an important top predator in the Gulf of Alaska that is also particularly vulnerable to sudden or severe ecosystem shifts. We examined survival of 4,178 known-aged Steller sea lions marked from 2000 to 2016 from Kodiak Island through Southeast Alaska, using mark-recapture models and 12,811 resightings collected from 2001 to 2021. Survival of adult females aged 3–15 was reduced -0.05 to -0.23 during the PMH in the areas east, but not west, of Cook Inlet. Survival of Kodiak females was unaffected by the PMH, but survival of Sugarloaf females aged 5–8 was reduced -0.13 from summer 2015 to summer 2016. Lowest survival in Southeast Alaska occurred from summer 2016 to summer 2017, but was also reduced from summer 2014 to summer 2016. Reduced survival continued post-PMH in Kenai Peninsula/Prince William Sound, but not in Southeast Alaska. Survival of adult males was insensitive to the PMH, except in Southeast Alaska where male survival was reduced -0.25 from summer 2016 to summer 2017. Prolonged or intermittent high adult female mortality may reduce population growth and initiate regional declines. Survival response of Steller sea lions to the PMH varied regionally despite similar patterns of ocean warming throughout our study area, suggesting areas east versus west of Cook Inlet were affected differently by the PMH, perhaps due to habitat and oceanographic differences.

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

“…Concerning female survival, age, region, and year effects on female survival are well documented (Hastings et al, 2023), and survival estimates in this study were consistent with past analyses of these data (Hastings et al, 2011, 2018, 2023, estimates provided in Table S3). In this paper we addressed female survival in relation to reproductive state at the end of the pupping season.…”

“…Surprisingly, a cost of reproduction on female survival was not detected in our study and causes for low adult female survival in 2014–2016 (Hastings et al, 2023) may have similarly impacted females with and without dependents. If negative changes to the prey field during the PMH (Suryan et al, 2021) contributed to adult female mortality, we expected that females with dependents would be especially impacted.…”

“…Surprisingly, a cost of reproduction on female survival was not detected in our study and causes for low adult female survival in 2014-2016 (Hastings et al, 2023) Pitcher & Calkins, 1981;Pitcher et al, 1998). Also, during that decline, juvenile mortality was high, with potential periods of high adult mortality (Pendleton et al, 2006;York, 1994;York et al, 1996).…”

“…For female survival probability, our global model was based on previous analyses of these data: ac + nr3 + y1415 + y16 (Hastings et al, 2023), where ac was the annual survival of six age‐classes (age 0, age 1, age 2, age 3–15, age 16–17, and age 18+), natal rookery group (nr3) were three categories (Forrester and Hazy pooled, White Sisters, and Graves Rocks; Figure 1), and y1415/y16 were three poor years for survival of prime‐aged females (2014–2016; Hastings et al, 2023). We fit 25 additional models including models in which survival was affected by reproductive state, by reproductive state*age or *year (the three poor years of survival 2014–2016 and 2014+, the years during and following the PMH).…”

“…Annual variation in population dynamics may also indicate sea lion response to abrupt environmental change: an abrupt decline of −0.05 to −0.23 in adult female Steller sea lion survival occurred in Southeast Alaska, in Prince William Sound and at Chiswell Island (Figure 1) during and following the severe North Pacific marine heatwave of 2014Hastings et al, 2023). The effects of the PMH on reproduction are of interest because widescale and persistent changes in the Gulf of Alaska food web during the PMH are well documented (Arimitsu et al, 2021;Suryan et al, 2021), and food intake relative to whole-body energy balance strongly determines reproductive success in female mammals (Bronson, 1985;Wade & Schneider, 1992), including Steller sea lions (Pitcher et al, 1998).…”