Sardine shortage cause of sea lion deaths in La Jolla

By Ashley Mackin

The National Oceanic and Atmospheric Administration (NOAA) released preliminary findings as to the cause of the unusual sea lion pup mortality event that began in January 2013 — a scarcity of sardines.

During the event, a higher than usual number of California sea lion pups were found emaciated, dehydrated and/or underweight for their age and in need of rescue and rehabilitation. The event is still under investigation.

Marine Mammal Health and Stranding Response Coordinator Sarah Wilkin reported: “A likely contributor to the large number of stranded, malnourished pups was a change in the availability of sea lion prey or forage — especially sardines, a high value food source for nursing mothers.”

Sardines, higher in fat than other fish that sea lions eat (including anchovies, juvenile hake and squid), provide sufficient nutrition for nursing sea lion mothers.

Sam McClatchie, a supervisory oceanographer at La Jolla’s Southwest Fisheries Science Center, said sardine- spawning grounds have shifted progressively offshore over the last 15 years. He said spawning sites shift further and closer to shore from year to year, but the overall trend is that they are moving offshore.

These further distances are particularly problematic for nursing sea lion mothers that don’t want to be away from their pups for long periods of time and newly weaned pups foraging on their own, both of which need the fat.

Additionally, NOAA regularly tracks the number of sardines along the coast of Southern California and found there are fewer spawning fish than in previous years. “Our spring survey showed very few sardines. We don’t know if they have temporarily disappeared or moved out of the survey area. We may find they are further north. They may have spawned earlier this year,” McClatchie said of the results of the biannual survey spanning from San Diego to San Francisco, offshore to approximately 550 kilometers (341 miles).

He added there is a summer survey that tracks San Diego to North Vancouver Island that would give further insight into spawning patterns for the sardines.

During the unusual mortality event investigation, scientists looked at viruses or disease that might have been the cause, and tested a subset of pups for radiation stemming from the Fukushima nuclear disaster in 2011. The pups tested negative for radiation and widespread disease was not determined to be a major cause.

One type of virus, known as “astrovirus,” was identified in a high percentage of sampled pups, but Wilkins noted astrovirus is found in healthy animals in the wild and in captivity. Researchers determined astrovirus might have contributed to the severity of the impact, and are still investigating.

During the mortality event, specifically in April 2013, 1,300 malnourished pups were taken into rehabilitation centers across California — twice the amount reported in April 2014 (650). San Diego alone had 381 live reported sea lion strandings in 2013, up from their five-year average of 67. Further, stranding occurred earlier than usual.

“What’s unique about the 2013 event is that we were seeing stranding in January, February and March, which is unprecedented,” Wilkin said. Stranding is expected during May and June, when pups would be weaning and learning to forage on their own.

Wilkin reported that the sea lions that were taken in for rehabilitation had a high survival rate and over 50 percent survived to be released. Of the 14 that were fitted with a satellite tag, most survived following their release and behavior was consistent with other sea lions their age.