Record numbers of California sea lion pups have been starving and stranding on beaches by the thousands in recent years, and now new research finds that a decline in their mothers’ food quality is behind the disturbing trend.

High calorie sardines and anchovies are now harder for sea lion moms to find, causing them to eat more rockfish and market squid that can be great for people on diets, but aren’t as hearty a meal for hungry sea lions. The findings are published in the journal Royal Society Open Science.

“For human consumption, highly oily fish may actually be less desirable to consumers,” lead author Sam McClatchie told Discovery News. “In contrast, for predators with high energy demands, such as nursing female sea lions, eating fish with higher energy density due to higher content of calories and fats provides a more effective way to meet their nutritional demands.”

Photos: Mammals of the Sea

Human demand for anchovies, in particular, has been on the rise due to the savory fish’s popularity in Caesar salad dressing and in “junk foods” like pizza. The real junk food for breeding female sea lions, minus the high calories, turns out to be the other types of fish that are now more prevalent in their feeding areas off central California.

McClatchie, an oceanographer at NOAA Fisheries Service’s Southwest Fisheries Science Center, and his team studied both the sea lion pup strandings and population trends for fish that adult sea lions eat. Data for the latter came from surveys that are conducted each summer off the coast of California.

A perfect storm now appears to be in place that is hurting the sea lions. Due to conservation efforts, numbers of these marine mammals increased from about 50,000 40 years ago to around 340,000 now. No one knows what the historic populations were like, given that Native Americans also frequently hunted sea lions and there are no detailed sea lion population estimates prior to the 1970s.

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At a time when the sea lion population appears to be approaching what the researchers call its current “carrying capacity” for the region, sardine and anchovy numbers plummeted while market squid and rockfish abundance increased.

Several researchers, such as marine biologist Malin Pinsky of Rutgers, have attributed this and prior dramatic fish population plunges to overfishing by humans. He and his team note that such collapses started to occur more frequently in sardines and anchovies after the advent of efficient fishing vessels and techniques following World War II. Anchovies and sardines are important to the pet food and fish oil industries, in addition to their other mentioned common uses.

“Overfishing is a problem throughout the world and across all species, including slow-growing fish like sharks, many of which are in serious trouble,” said Pinsky. “But it turns out that fishery collapses are three times more likely in the opposite kinds of species — those that grow quickly.”

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McClatchie and his team, however, believe that the problem is “environmental,” and not because of overfishing. This distinction is important, as federal regulators are planning to do an official stock assessment of anchovies in the fall and will consider updating the 25,000-ton rule that now limits catches.

“Sardine and anchovy populations both show large inter-annual variability that is environmentally driven and prior to any fishing,” McClatchie said.

Joshua Lindsay of the National Marine Fisheries Service told Discovery News that populations of anchovies, sardines and other small fish “are linked to prevailing environmental conditions. NMFS has been working to better understand these environmental processes driving fish populations as well as the diet linkages between forage fish species and higher order predators to enhance the ecosystem science used in our fisheries management.”

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Marc Mangel, a professor at both the University of California at Santa Cruz and the University of Bergen, says the study “will help refocus the discussion about the causes of sea lion declines. More importantly, in my opinion, it is a terrific example of how we can use marine mammals and birds as sentinels or samplers of the environment.”

In the meantime, the short-term outlook for sea lions is worrisome, particularly for rescue centers that have been stretched to their limits. McClatchie and his colleagues say that they “expect repeated years with malnourished and starving sea lion pups,” but can’t predict when that will end.