Chiswell Island Steller Sea Lions Having Another Tough Year

In the Gulf of Alaska and farther west, Steller sea lions are listed as Endangered under the Endangered Species Act of the United States.  Most of this western distinct population segment has exhibited a significant positive trend over the previous decade and appeared to be well on the road to recovery.  However, this recovery may have encountered a setback as pup numbers have taken a turn for the worse in more recent years.

Researchers at the Alaska SeaLife Center (ASLC) have been studying Steller sea lions since 1999 using a remote-control video system at Chiswell Island in the Gulf of Alaska.  This video system, operated from a control room at the ASLC, has given us the ability to conduct detailed scientific research regarding many aspects of behavior and population dynamics in these sea lions.  Long-term recorded histories of individual animals now span more than three generations.  What we have learned is that maternal care, reproduction and survival rates had been good up through 2015 when Chiswell reached a peak of 127 pups born on the rookery.  Then the warm-water blob hit the North Pacific Ocean.  Subsequently, pup numbers dropped by 23% in 2016, with a further decrease of 16% last year when only 82 pups were born. With more than half of the pupping season over by 15 June this year, we are currently expecting only about 65 pups to be born in total, cutting potential recruitment in half within 3 years.  Furthermore, stillbirths and pup abandonments are trending higher this year.

The leading theory for why these sea lions are struggling in recent years is related to the spatial and temporal redistribution of their prey resources due to warming ocean temperatures (e.g. see Auth et al. 2018).  Prey such as cod, pollock, and herring that they feed upon during winter have likely become harder to find and more unpredictable in the timing of aggregated spawning events.  When mature Steller sea lion females are energetically compromised from reduced food intake during the winter to spring months, they may spontaneously abort a developing fetus to spare their own fitness.  This leads to lower reproductive rates and fewer pups being born during the subsequent summer.  The effects of the warm water blob appear to be lingering for some upper trophic level species such as the Steller sea lion.  How long into the future these effects will continue is still unknown.

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Written by: John Maniscalco and Pam Parker

Image is from Chiswell Island May 28, 2018.

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