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2013-2017 California Sea Lion Unusual Mortality Event in California

Elevated strandings of California sea lion pups have occurred in Southern California since January 2013. This event has been declared an Unusual Mortality Event (UME).

The UME is confined to pup and yearling California sea lions. Many of the sea lions are:

  • emaciated
  • dehydrated
  • very underweight for their age

Report a stranded sea lion

Report a live or dead stranded sea lion (or any marine mammal) to the local stranding network.

Comparison of monthly strandings for California sea lion pups and yearlings in 2013-2017 versus the average stranding rate (2003-2012).

Why are these California sea lion pups stranding?

Our findings to date indicate that a likely contributor to the large number of stranded, malnourished pups was a change in the availability of sea lion prey, especially sardines, a high value food source for nursing mothers.  Current data show changes in availability of sea lion prey in Southern California waters was likely a contributor to the UME, the exact mechanism is still under investigation.  Sardine spawning grounds shifted further offshore in 2012 and 2013, and while other prey were available (market squid and rockfish), these may not have provided adequate nutrition in the milk of sea lion mothers supporting pups or for newly-weaned pups foraging on their own.

Although the pups showed signs of some viruses and infections, findings indicate that this event was not caused by disease rather by the lack of high quality, close-by food sources for nursing mothers.  Current evidence does not support that this UME was caused by a single infectious agent, though a variety of disease-causing bacteria and viruses were found in samples from sea lion pups.  Several different kinds of one sort of virus (astroviruses, including some new species of astrovirus) were identified in a high percentage of the samples; however, the importance of this finding is still under investigation.

Investigating and identifying the cause of this UME, which left malnourished pups stranded along the coast, is a true public-private effort with many collaborators. The investigative team, assembled with considerable support from public-private partnerships, examined multiple potential explanations for the high numbers of malnourished California sea lion pups observed on the island rookeries and stranded on the mainland in 2013. The examination of the root causes of the California sea lion UME fall squarely within NOAA Fisheries' core missions to protect and conserve marine mammals.  The UME investigation is ongoing.

What is being done to help the California sea lion pups?

The pups that were rescued and rehabilitated had a high rate of survival.  After stranding network member rehabilitation centers provided malnourished, stranded pups with nutrition and hydration, over 50% survived to be released. For the animals outfitted with a satellite tag, tracking data show most survived following release.


FAQs are available for the 2013-2016 California sea lion UME:

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Updated: August 15, 2017