Fewer sea lions have been stranded this year, but that’s a bad sign, scientists say
More than 3,000 sea lions were found stranded on Southern California beaches last year, a statistical surge that amounted to more than the previous five years combined.
So far this year, the number of strandings is down, but scientists say it’s a sign that the sea lion population is dwindling rather than recovering. An ongoing fish famine is preventing mothers from producing enough milk, resulting in smaller and less hardy pups.
As of Monday, there had been 375 sea lion strandings so far in 2016, still busier than usual. According to National Oceanic and Atmospheric Administration data, about 160 sea lions are found stranded during the first two months of a typical year.
Warmer waters and less available prey are hurting newly born sea lions the most, potentially slowing down the species’ population growth, scientists say.
“It’s going to decline,” said Sharon Melin, wildlife biologist at the NOAA Alaska Fisheries Science Center in Seattle. “We don’t know how much and we don’t know exactly when we’ll be seeing that signal, but it has to.”
Field researchers travel to the sea lion breeding grounds on San Miguel and San Nicolas islands to keep tabs on the population and its health.
During NOAA’s survey of the sea lion breeding grounds, Melin said, researchers saw more dead pups than usual. The increased mortality could cause fewer pups to become stranded because they’re dying before they can leave the islands, she said. The exact number of dead pups is not known.
Since 2009, the weight of 3-month-old sea lion pups has been below expectations. The pups that were born in June and first weighed in September should have gained about 20 pounds in the last six months, Melin said, but they haven’t grown at all.
“It still looks very grim this year,” Melin said.
Earlier this month, an 8-month-old sea lion pup was found malnourished and dehydrated in La Jolla. The pup was rescued by SeaWorld San Diego and has gained 4 pounds during the last several weeks. The pup, nicknamed Marina, is one of hundreds of underweight and underfed sea lions found stranded on the California coast.
SeaWorld San Diego typically rescues 200 marine mammals in any given year. Already this year, there are 147 sea lions in SeaWorld’s care, according to spokesman David Koontz.
Because sea lion mothers have to travel farther to feed, their pups are going several days without feeding. Like dolphins and seals, sea lions get their hydration from the food they eat. Not having enough food can cause the mammals to deteriorate fast.
“It’s a double-whammy,” Koontz said.
The expected average weight of a 3-month-old California sea lion is 33 pounds, NOAA says, but it’s been running around 26 pounds. In the last seven years, the birth weight of pups on San Miguel has fallen below the mean. These pups have a more difficult time surviving in open waters and lack the energy needed to forage for food.
At this age, pups are dependent on mother’s milk. The low birth weight suggests that there isn’t adequate prey near the island for the mothers to produce enough milk.
“It’s a clear sign that there is a mismatch between supply and demand,” said Nate Mantua, a NOAA climate scientist.
Warming waters hurt the prey that the sea lions eat. Some of the prey may leave for colder waters, or like the sea lions, slowly die off. Mantua said that the best evidence of warming is the arrival of large fish such as marlin and mahi mahi that have appeared in the Southern California waters.
Sea lions typically eat Pacific sardine, northern anchovy, Pacific hake, rockfish and market squid. In June, female sea lions typically give birth to one pup each. That pup is dependent on its mother for nearly a year.
The dearth of food across the ocean isn’t harming the adult sea lions as much as the pups. But the warmer waters are pushing the typical prey out of the mothers’ feeding range, forcing them to get creative.
“It may not be something they are familiar with or good at catching,” Melin said. “Or, they’ll try to follow it and sometimes catch it. They stay out too long, and their pup starves to death.”
Melin said more pups are leaving their mothers before they are ready, probably because of hunger. Already physically small and unskilled at foraging, the pups will often die in search of food or end up stranded on beaches.
Wheaton writes for the San Diego Union-Tribune.
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