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FAQs on the 2013 California Sea Lion UME Investigation

Q: Do you know what caused the Unusual Mortality Event?

The likely cause is that some female California sea lions were unable to provide adequate milk to nourish their pups, resulting in premature weaning. The exact mechanism of why this happened remains unknown and will continue to be investigated.

Examination of prey distribution found that sardines, an energy rich prey species, spawned further offshore in 2012 and 2013 than in previous years, and that this was likely linked to oceanographic conditions including offshore winds. Diet studies of adult females show that the diet mostly consisted of Pacific hake, juvenile rockfish, market squid, and octopus, which are low-fat prey items.

Through the efforts of a strong interdisciplinary team pursuing many lines of investigation, we are able to examine and rule out other potential causes. A survey of potential infectious pathogens was conducted and no single virus or bacteria was found in a majority of cases that could have caused the observed patterns of strandings. Additionally, there was no sign of radiation detected in these pups from the Fukushima nuclear accident caused by the 2011 Japanese tsunami.

Q: Why is it unusual to find stranded sea lion pups in January, February, and March?

Typically, the stranding network begins seeing stranded pups in May and June as the young of the year are weaned and begin foraging on their own. California sea lions are born in large rookeries on offshore islands along California and Baja Mexico, with almost all pups born in June each year. Pups typically remain with their mothers for the first 10-11 months of their life and become independent in May of the year after their birth.

The 2013 stranding event is unique because most of the strandings were in the first four months of the year and were located in Southern California, but the annual total for 2013 does not represent the largest number of stranded California sea lions for the state of California.


Annual stranding rates of live California sea lions from 2008-2013.

Q: What is the scope of the investigation?

The investigation into the cause of the Southern California sea lion UME continues into 2014 and includes:

Q: What are the results of the study of pup births, condition, and mortality at the offshore rookery islands?

Many female sea lions that successfully reproduced in 2012 had difficulty supporting the nutritional needs of their pups, resulting in poor condition and high mortality of pups. Pup birth rate was considered normal in 2012, but about 35-55% below normal in 2013, suggesting that females also had difficulty supporting pregnancies during the 2012-2013 gestation period.

Population assessment and condition of California sea lion pups at San Miguel Island have been conducted since 1975. This long-term data set provided the foundation for evaluating the UME relative to long-term trends in pup births, pup mortalities, and pup condition. Data for this study were collected from July 2012 through January 2014 at two offshore rookery islands – San Miguel Island and San Nicolas Island.

Q: What are the findings in the stranded and rehabilitated California sea lions?

In 2013, over 1,600 California sea lions stranded alive along the Southern California coastline and were admitted to rehabilitation facilities, with unprecedented numbers of animals being admitted in January, February, March, and April. In one week in March, 227 animals were admitted to the five marine mammal rehabilitation facilities between Santa Barbara and San Diego Counties, with a peak of 42 individuals admitted in a single day on March 19th. Malnutrition was the predominant finding in stranded pups. When these pups were provided with nutrition and hydration, the recovery rate was high; over 50% of them survived and were released. Fourteen rehabilitated pups were released with satellite tags so their movements and diving behavior could be monitored. Twelve of the fourteen pups satellite-tagged are believed to have survived at least a few months following release, until their tags stopped transmitting due to molting or battery issues.



Live stranded California sea lions admitted to rehabilitation in California for 2013 by month.

Q: What are the findings from the post-mortem investigations of pups?

To assess common causes of death and findings among UME sea lion pups, organ tissues were evaluated from 26 sea lion pups between January and May 2013:

Many of the pups had mild intestinal infections and depleted lymph nodes that likely increased their susceptibilities to secondary bacterial infections in the lung and brain. We also analyzed some of the dead pups for radionuclides consistent with the Fukushima nuclear disaster in Japan, and no radiation was detected.  Given the natural history of these animals, and their limited range off the California coast, we expected that radiation would not be detected and did not contribute to the UME.

Q: What are the findings from the molecular disease diagnostic investigation? 

While a variety of bacterial and viral pathogens have been found in sea lion pup samples, current evidence indicates that this UME does not appear to have been caused by a single infectious agent.

The molecular diagnostic investigation focused on potential infectious pathogens that may have caused

To date, over 900 molecular diagnostic test results have been reported from fecal and nasal samples collected from 83 sea lion pups. We identified pathogens including:

Astroviruses were the most commonly identified virus in the surveys, with five different astrovirus types were identified. Because of the high genetic diversity of the viruses and previously documented high prevalence of astroviruses in surveys of healthy wild and captive sea lion populations, these astroviruses are currently not believed to have caused the UME. However, astroviruses and other pathogens may have contributed to the pups' poor condition. More analyses are underway to better understand sea lion astroviruses and their impacts on intestinal disease.

In the pups with pneumonia, no common lung pathogen was detected.

Q: What are the behavioral findings of the lactating California sea lion mothers?

Traditional foraging areas may not have contained sufficient prey for all sea lion females to support the nutritional needs of their pups during 2012 and early 2013 leading to increased pup mortality, early weaning or malnutrition of dependent pups at San Miguel Island. The presence of robust females nursing fat or skinny pups during this period indicated the event did not affect all females equally.

Thirteen adult females with pups were instrumented with satellite tags in April 2013 – seven with pups in good body condition and six with underweight pups.

There was no difference in condition of these females by weight, and no significant differences were detected in their foraging behavior. We also analyzed the diet of lactating female sea lions. It was comprised of a high frequency of market squid, juvenile rockfish, juvenile hake (low fat-content prey), and very low frequency of sardine and anchovy (high energy-content prey). Diet composition may be related to the strandings in southern California.


Figure 1: Spring density distributions of 13 satellite tagged lactating California sea lion females captured at San Miguel Island, California, in April 2013.  Credit: NMML

Q: What are the associations between the diet of the adult females and pup condition?

A retrospective study modeling annual summer diet of adult females and pup condition at 3-4 months of age between 1980 and 2011 at San Miguel Island found that the best predictors of pup weight were sea surface temperature (Temperature) and the frequency of occurrence of market squid and rockfish in the female sea lion diet. Increasing Temperature, particularly during El Niño events, and an adult female diet with high frequency of market squid and juvenile rockfish were associated with lower pup weights. Thus, pup weight is related to both prey consumed by their mothers and to ocean conditions.

Q: What are the findings regarding potential prey species for California sea lions, specifically small pelagic fish?

The sardine spawning grounds shifted further offshore in 2012 and 2013, which likely resulted in these fish not being available for weaned pups and lactating females at San Miguel Island, as demonstrated by the low levels of sardine detected in sea lion diets during 2012 and 2013. While other prey sources were available (i.e., market squid and rockfish), those prey may have not provided adequate nutrition for those lactating females to provide for their pups, or for pups that were prematurely weaned and foraging on their own. We will continue to investigate this hypothesis, particularly once 2012 and 2013 sea lion diet data are available from the San Nicolas Island and San Clemente Island rookeries, and as additional data on spawning distribution are analyzed.



Figure 2: Distance from shore of spawning for four forage fish species (Pacific sardine, northern anchovy, jack mackerel, and hake).
Sardine spawning distribution has moved offshore over time, as shown by the trend line.
Credit: SWFSC

Q: How was the response and investigation supported?

In coordination with NOAA's Marine Mammal Health and Stranding Response Program and the National Marine Mammal Stranding Network's efforts to respond to this UME, the following groups worked together to help support the immediate need to feed an unprecedented number of pups:

In the midst of the UME, these groups also teamed up to investigate the cause of malnourished sea lion pups. Funding from these groups provided critical and immediate funding to support an investigation by government and non-profit scientists:

Thanks to private-public partnerships, sea lion pup lives were saved, their deaths were investigated, and we have a better understanding of the ocean's health.

The Investigative Team consisted of many partners, including staff from:

Additional funding and support to further the investigation was provided by:

Updated: May 6, 2014