A rash of dying and dead sea lion pups found on Southern California beaches last year was probably simply a result of their inability to get enough good meals — rather than a mass infection or disease, scientists announced Tuesday after months of collaborative research.

Analysis of the “unusual mortality event” that caused more than 1,600 California sea lions to strand along the coast in 2013 found that the marine mammals couldn’t reach enough of at least one of their favorite prey — sardines.

Large numbers of starved pups are expected to inundate rescue centers in the next few months, but not to the extent that they did last year, experts say.

National Oceanic and Atmospheric Administration scientists collaborated to try to understand whether sea lions were contending with a virus, infection or other microscopic cause, such as domoic acid poisoning that has depleted sea lion populations in previous years. Domoic acid is naturally occurring in certain algae blooms and it causes central nerve damage and seizures in sea lions.

But, this time, it appears that sea lion mothers abandoned their pups before they were weened because there wasn’t enough food.

“A single epidemic or infectious cause was not found,” said Sarah Wilkin, a fishery biologist with the NOAA’s marine mammal health and stranding response program. “There was a failure in nursing of some of the mothers to pups, and we still don’t understand exactly why.”


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Fishers, too, had trouble finding sardines last year, with their usually plentiful numbers dropping dramatically. In December 2013, West Coast fishers were required to lower their take of sardines from 18,073 metric tons to 5,446 metric tons for the season to ensure the dwindling supply of the species wouldn’t be over-fished.

In addition to declining numbers, Wilkin said scientists believe Pacific sardines spawned further offshore than usual in 2012 and 2013, making it hard for young sea lions to reach them for meals.

In the absence of the fat-rich coastal pelagic sardines, sea lion mothers munched on low-fat choices, including Pacific hake, juvenile rockfish, market squid and octopus, according to NOAA’s diet studies.

The investigation into whether California sea lions are facing some other unknown hazard continues, NOAA scientists said.

“We’ve been looking at it throughout the past year and over the past couple months,” said NOAA strandings coordinator Justin Viezbicke. “We narrowed it down, but it’s a work in progress.”

A molecular diagnostic investigation of the nasal passages and feces of unhealthy sea lions found a variety of illnesses, but nothing linking all the dead, NOAA officials said.

Among other things, they detected intestinal inflammation, pneumonia, herpes virus, San Miguel sea lion virus, salmonella species and astroviruses.

“Astroviruses and other pathogens may have contributed to the pups’ poor condition,” NOAA officials said in a written statement. “More analyses are underway to better understand sea lion astroviruses and their impacts on intestinal disease.”

Last year, a total of 227 sick sea lions were admitted to the five marine mammal rehabilitation facilities from Santa Barbara to San Diego, with a peak of 42 intakes on March 19, NOAA officials reported. Animals treated at the centers responded well, and more than 50 percent of them survived and were released.

Now, sea lions born last summer are showing up at rehabilitation centers, such as the Marine Mammal Care Center at Fort MacArthur in San Pedro with the same starvation symptoms as those found last year.

Since sea lions are normally weaned in May and June, scientists expect a jump in sickly animals soon.