A new study has examined the cause behind the recent deaths of juvenile sea lions, which have been flooding rescue centers in Southern California over the last few years, pointing to numerous different factors including the growth of the overall population and decline in high calorie prey in the most important feeding areas.
The team began by examining the abundance of four of the sea lions' main prey species: sardine, anchovy, rockfish and market squid. They looked at these species between the years 2004 and 2014 and found that both sardines and anchovies, known for their high fat content vital for young sea lions, have seen declines in their population since the mid-2000s in the most common foraging areas. This has forced female sea lions to prey on market squid and rockfish instead, which contain less fat and fewer calories.
"When you have hundreds of thousands of animals to feed and you have a fluctuating prey base that has trended downward, you are going to have starving animals," said Sam McClatchie, lead author of the new research. "Sardines and anchovies have both trended downward, and that compounds the problem for the sea lions."
Consuming a low calorie diet is fine for adult male sealions as well as females without pups, but lactating females and their pups are sensitive to changes to low calorie diets. Previous studies showed that during El Niño events and when female diets are low in calories, pup weight decreases.
Given the long-term and widespread nature of the changes in prey found, the team believes that the root cause is environmental shifts.
"The overall driver appears to be the natural fluctuations in fish populations," McClatchie said. "They do fluctuate up and down over time, and since 2004, they're doing it in phase."
The team believes that these environmental shifts stem from human activities, and despite conservation efforts leading to a resurgence in the sea lion population over the past half century, during the same time period humans have increased consumption of anchovies and sardines, decreasing important sealion prey populations.
While sea lions are not endangered, they are protected by the 1972 Marine Mammal Protection Act, and despite their growth from 50,000 to 300,000 over the last 40 years, the population's carrying capacity is expected to vary with climate and ocean conditions.
"Given the likelihood that the California sea lion population is approaching carrying capacity, density-dependent effects such as food limitation (and stranding) of pups may be a long-term consequence of a rebuilt sea lion population during periods of low abundance of high-quality forage," the scientists wrote.
The findings were published in the Mar. 2 issue of the journal Royal Society Open Science.
