New Publication: Changes in Steller Sea Lion Winter Diets Following the Pacific Marine Heatwave

Extreme climate events are being experienced worldwide, and Alaska ecosystems, unfortunately, have a front-row seat. Heatwaves, droughts, floods, and hurricanes, are having profound environmental impacts, and are expected to continue for decades to come.

One of these extreme climate events, called the Pacific marine heatwave, occurred between 2014 and 2016. During this time, water temperatures rose far above normal conditions and had expansive ecological impacts throughout the Gulf of Alaska and northeast Pacific Ocean.

Luckily, the Chiswell Steller Sea Lion Remote Video Monitoring team (the ASLC Chiswell team) has been monitoring the Endangered population of Steller sea lions for nearly 25 years — long before the Pacific marine heatwave occurred. This western population of Steller sea lions was  listed as Endangered under the U.S. Endangered Species Act in 1997 following a major population decline. The ASLC Chiswell team has monitored this population since 1998 and has seen a general increase in the population over the years until 2016.

The ASLC Chiswell team conducts multiple population surveys each year in the area around Resurrection Bay and monitors the Steller sea lion rookery called Chiswell Island through remote video cameras controlled at the Center. 

The population data collected through the Pacific marine heatwave showed a continued increase in the population through 2015.  The peak impact of the heatwave occurred during the winter of 2015 and into 2016, and the Chiswell team began to see significant declines in pups and adults in the summer of 2016. Changes in food availability likely played some role in the decline. 

Dr. John Maniscalco, Ph. D. — a lead scientist at the ASLC — recently published a peer-reviewed article in Global Ecology and Conservation that looks at how the Pacific marine heatwave could have affected the winter diets of the Endangered population of Steller sea lions in Alaska over this time frame. 

Winter is considered a critical time for Steller sea lions as it is likely the most energetically challenging time of the year. In addition to regulating their body temperatures in freezing Alaskan waters, adult female sea lions can be both lactating and pregnant with new pups, which they will birth in the coming summer. Limited food availability during the winter months could have dire outcomes for adult females, leading them to abort growing fetuses, wean their pups too early, or put their own survival at risk.

Winter is also a high-risk season for scientists, as harsh and unpredictable weather at sea makes regular surveys logistically challenging, if not impossible. So how do scientists find out what sea lions are eating during this critical time period?

The answer lies not in the sea lions, but in what they leave behind: scat samples.

Two ASLC staff members collect scat samples on a Steller sea lion haul out in March of 2022.

The ASLC Chiswell team collected scat samples at three different Steller sea lion haulout sites near Resurrection Bay and Chiswell Island when weather conditions allowed. These sites were visited between January 27 and March 29 during the years 2014, 2015, 2017, and 2018 with the ASLC’s small landing craft — fittingly named the Jubatus.

Since no other pinniped species were seen hauled out at these sites when scat was collected, the team can reasonably assume the scat was from Steller sea lions alone. 

ASLC scientists scooped individual deposits into Ziploc bags, being careful that the scooper was cleaned between samples. Once back at the Center, the samples were frozen at − 20 ◦C until ready for processing.

To determine the diet of the Steller sea lion populations in question, scientists analyzed 150 samples from the years 2014-2015 and another 150 from the years 2017-2018. To find out what kind of prey was in each sample, they looked closely for hard part remains (like bones).

The samples showed that before the Pacific marine heatwave, Steller sea lions mostly fed on fish found closer to the water’s surface (the epipelagic and mesopelagic zones) including capelin, walleye pollock, and Pacific cod. Capelin, specifically, is high in calories and is an impactful prey item for the sea lions.

Following the Pacific marine heatwave, the amount of capelin consumed by the sea lions was strongly reduced, and their diet diversity increased by 12%. A greater presence of prey found closer to the bottom of the ocean (the demersal and benthic zones) like skates, lumpsuckers, snailfish, and polychaetes was detected in collected scat samples.

Dr. Maniscalco suggests that the sea lion’s preferred prey may not have been readily available in the later years, forcing them deeper into the ocean and more broadly for prey that could have provided less energy and calories. 

This change in diet corresponds with the ASLC Chiswell team’s population survey data that saw fewer sea lions and fewer pups being born after the Pacific marine heatwave years, showing that the prey availability over the winter months may have a substantial impact on population health for Steller sea lions.  

If warming in the northern Pacific Ocean continues long-term, we may very well see negative impacts on Steller sea lion populations and many more negative outcomes for the wide-ranging ecosystem.

The ASLC Chiswell team will continue to monitor this Endangered population and hopes to do so for many years to come. Without multi-year monitoring projects like this one, we would not be able to see the long-term changes in populations and the effects on this incredible ecosystem as a whole.  

Click here to read Dr. John Maniscalco’s recent publication in Global Ecology and Conservation entitled Changes in the overwintering diet of Steller sea lions (Eumetopias jubatus) in relation to the 2014 – 2016 northeast Pacific marine heatwave. 

Blog and photographs by Kaiti Grant

All activities photographed permitted by NMFS Permit No. 22293