Unusual Mortality Event: California Sea Lions  


This 2013 Pacific sea lion pupping season has been a dramatic one. Rehabilitation centers have been inundated with over a thousand emaciated and dehydrated pups since the beginning of 2013, making it a record year for rescuers. NOAA has declared this an Unusual Mortality Event (UME) and this is the 6th overall UME for California Sea Lions.  According to NOAA, The Working Group on Marine Mammal UMEs lists 7 criteria to qualify something as a UME and an event has to meet one or more of these criteria to qualify as unusual:

  1. Marked increase in the magnitude or change in the nature of morbidity, mortality or strandings when compared to prior records.
  2. A temporal change in morbidity, mortality or strandings is occurring.
  3. A special change in morbidity, mortality or strandings is occurring.
  4. The species, age or sex composition of the affected animals is different than that of animals usually affected.
  5. Affected animals exhibit similar or unusual pathologic findings, behavior patterns, clinical signs, or general physical condition (e.g., blubber thickness).
  6. Potentially significant morbidity, mortality or stranding is observed in species, stocks or populations that are particularly vulnerable (e.g., listed as depleted, threatened or endangered or declining). For example, stranding of three or four right whales may be cause for great concern whereas stranding of a similar number of fin whales may not.
  7. Morbidity is observed concurrent with or as part of an unexplained continual decline of a marine mammal population, stock, or species.

This event most closely matches with item 1. above, however it likely qualifies under other criteria as well.

While the cause is currently undetermined, there are a few theories as to what is causing this mortality. The most publicized hypothesis is that due to less prey availability for these pinnipeds that the mothers are travelling further and for longer in search of food, making pups more likely to wander in search of their own sustenance. This is not only alarming for the health of the sea lion population but for the fisheries as well. Where did these fish go? What happened to cause such a drastic drop in population size? Answers to these questions are currently being sought out by the National Oceanic and Atmospheric Administration (NOAA) and the National Marine Fisheries Service (NMFS), and two other organizations have already gained preliminary results to the driving force behind this mystery.

Researchers from Australian Antarctic Division and Woods Hole Oceanographic Institute (WHOI) provide some insight to this conundrum whilst studying climate change in Antarctica. With global temperatures rising, there have been substantial changes to phytoplankton abundance, which is an integral source of food to fish and krill. They suggest that the trophic level have been and will be affected soonest, causing a chain reaction up the food chain from microorganisms to large cetaceans. This would be in agreement with what is being witnessed in California with less fish present for the sea lion population.

Thank You to OCEAN Researcher Nicole Smith


For more information on California Sea Lions and other UMEs visit this link:


"2013 California Sea Lion Unusual Mortality Event in California." NOAA Fisheries. NOAA, 30 May 2013. Web. 4 Oct 2013. http://www.nmfs.noaa.gov/pr/health/mmume/californiasealions2013.htm

Barlass, Tim. “Polar melt shakes up food chain.” The Sydney Morning Herald 7 April 2013. Web. 7 April 2013. http://www.smh.com.au/environment/climate-change/polar-melt-shakes-up-food-chain-20130406-2hdlx.html

Hillard, Gloria. “Starving Baby Sea Lions Flood Southern California Shores.” Npr.org 9 April 2013. Web. 9 April 2013. http://www.npr.org/2013/04/09/176586940/starving-baby-sea-lions-flood-southern-california-shores?ft=1&f=1001&sc=tw&utm_source=twitterfeed&utm_medium=twitter