Researchers have been investigating the cause of a recent surge in the number of starving sea lion pups washing up on beaches in California, and claim to have found “compelling evidence” that declining availability of certain food sources is driving this tragic phenomenon. However, the reasons behind this shift in the composition of local prey species are likely to be varied and complex, making it difficult to predict how long the trend will continue or devise strategies to alleviate the situation.
Over the first half of 2015, more than 3,000 undernourished baby sea lions were found stranded along the Californian coastline – a figure that is more than 15 times higher than the normal rate. Since pups usually stay with their mothers for the first year of their life, the appearance of so many unaccompanied infants led scientists to suspect that local females may have been having difficulty nursing their young, causing them to strike out alone in search of food.
To try and determine why this was occurring, researchers decided to monitor the feeding habits of female sea lions in the region around the San Miguel rookery, located in the Channel Islands off the coast of Southern California. This they achieved by tracking the movements of six tagged females in order to establish their foraging territories, and cross-referencing this information with data regarding fish populations in these areas, obtained from the Southwest Fisheries Science Center’s Rockfish Recruitment and Ecosystem Assessment Survey.
Publishing their findings in Royal Society Open Science, the study authors reveal how populations of certain common prey fish such as sardines and anchovies had remained depleted throughout the sea lions’ foraging regions for the past decade. In contrast, other species such as market squid and rockfish had increased in abundance over this period.
Sardines, which are high in oil and fat, provide a vital source of nutrients to nursing sea lions. Rich Carey/Shutterstock
Because sea lions are opportunistic feeders that readily alter their diet in accordance with availability of prey, the researchers propose that the animals had largely switched from sardines and anchovies to squid and rockfish over the past 10 years. However, since squid and rockfish contain much less fat and other nutrients than sardines and anchovies, it is likely that the nutritional value of nursing sea lions’ milk has deteriorated as a result of this dietary shift. This, they claim, is probably the main cause of the recent increase in starving pups.
The study authors suggest numerous possible causes for the change in fish populations around the Channel Islands. Previously, sardine and anchovy numbers have been found to decline in El Niño years before bouncing back, yet the fact that populations have remained depleted for a decade rules this out as an explanation. Rather, the researchers propose that this sustained fluctuation may be the result of more permanent environmental changes.
However, they state that this effect has probably been exacerbated by other contributing factors, including recent increases in fishing of sardines and anchovies by humans. Additionally, the implementation of conservation efforts in the mid-1970s has seen sea lion populations expand from around 50,000 to 340,000 over the past 40 years. Subsequent over-hunting of high-nutrient fish by these sea lions may have driven numbers of these species down, resulting in the present crisis.