Southern California waters support growing populations of four species of pinnipeds (seals and sea lions). These robust and recovering populations are expanding into habitats that are now mostly occupied by humans. Accordingly, conflicts between human and animal populations are becoming more prevalent as common use of space and marine living resources intensifies. The only way to effectively manage the resource conflicts inherent in the tremendous growth of these populations is through a clear understanding of the population ecology of seals and sea lions. Drs. Brent Stewart and Pam Yochem have been working on the Channel Islands since the late-1970’s studying changes in seal and sea lion populations and attempting to identify the biological and physical factors that influence them. They conduct annual surveys, censuses at colonies at San Nicolas and San Miguel islands, extending a 30-year database on patterns of pinniped abundance and distribution. They also are documenting the foraging behavior of various life stages of seals and sea lions to understand their dietary preferences and habitat needs. Using instruments that record not only the location of the animals, but also the depths at which they are foraging and temperature of the water at those depths, they are tracking the geographical and vertical use of marine habitats by these top-level predators. This information, along with the census effort, will aid in the development of predictive models that can be used to establish proactive conservation and management strategies for California pinnipeds and for the North Pacific.
Returning from San Nicolas Island this past Monday, March 18th, 2013 Dr. Stewart reports that California sea lion pups (on average, 9-10 months old) are very underweight, and adults and subadults appear to be lean and spending more time foraging at sea and less time ashore. Dr. Stewart has also observed a noticeable increase in premature births. The sea lions don’t appear to be showing any outward signs of infectious disease but Dr. Stewart will continue to collect samples for evaluation.
Harbor seals appear to be so far unaffected but they prey in different habitats that appear less affected by broader oceanic variation in environmental conditions.
All of these symptoms are typical responses to sudden scarcity in food availability. Those changes generally have the most immediate and serious effects on young pups, that have been dependent to a large extent on their nursing mothers but suddenly find themselves weaned and now dependent only on themselves to find food.
What has been causing this food shortage is, however, a big mystery. Periodic warm water events, called El Niños, are known to result in substantial change in the food chain. Those
El Niño conditions generally result in a deepening of the warm surface layer and a substantial reduction in the availability of nutrient at the surface that would otherwise support healthy abundance of phytoplankton, zooplankton that feeds on those microscopic plants, and the small schooling fish and squid that eat those small animal plankton species which are key prey for California sea lions. But El Niño conditions have not been present in Southern California during the past year. Something has caused the usually abundant prey to leave Southern California waters and what that is… remains yet to be discovered.
Dr. Stewart expects that these recent findings will likely result in fewer births of sea lions this coming summer, as compared to the large numbers last year, but we will be watching it closely.