After Four Years in Hot Water, Sea Lion Pups Are Faring Better
Years of unusually high ocean temperatures off the West Coast of the United States have left sea lion pups in bad shape. The fish that sea lion mothers catch and bring back to shore to feed their young had moved far from traditional foraging spots, leaving pups emaciated and often stranded.
But pups born this year at Channel Islands rookeries off Southern California’s coast appear to be healthy, according to researchers from the National Oceanic and Atmospheric Administration and Hubbs–SeaWorld Research Institute.
“We finally have pups that are actually about normal size this year,” said Sharon Melin, a wildlife biologist at NOAA’s Alaska Fisheries Science Center Marine Mammal Laboratory. “That’s really good news after a lot of years of bad news.”
Field teams from NOAA and Hubbs monitor sea lion populations at San Miguel and San Nicolas islands, where thousands of pups are born every summer. Since 2013, the teams have found underweight and malnourished pups. Strandings of sea lions also rose sharply over that time, peaking in 2015, when more than 3,000 pups washed up on the California coastline in the first five months of the year—a rate 15 times higher than normal. Births subsequently fell about 41 percent.
Brent Stewart, director of ecology at Hubbs, said the September pup weights could mean normal birthrates are returning. Since 2013, with the formation of the “warm blob” in the northern Pacific and El Niño’s influence from the south, warm water temperatures have persisted from Alaska to Baja California, averaging 1.5 degrees Celsius higher than average.
But in the past few months, water conditions off Central and Southern California have started to return to normal temperatures, which means the fish that sea lions eat should return.
“The condition of pups we recorded in September was good—similar to what we would see in a non-unusual mortality event year or non–El Niño year,” Stewart said. “Our next trip out to the rookeries will be in December, and that’s when we’ll be able to determine if the females have been doing well foraging and getting enough nutrients to transfer to the pups.”
While the drop in water temperature is a welcome reprieve for California’s marine mammals, it’s not likely to stick around. Increasing levels of carbon dioxide and other greenhouse gases in the atmosphere are pushing sea surface temperatures to rise at a rate of about 0.2 degree Celsius per decade.
That warmer-than-normal water is a problem for more than just sea lions. In 2015, a massive algal bloom that swept the West Coast resulted in record levels of a neurotoxin called domoic acid in fish, clams, crabs, birds, sea lions, and even whales.
The bloom stretched from Santa Barbara to Alaska, closing mussel, clam, and sardine fisheries in Oregon, as well as Washington’s $20 million Dungeness crab fishery.
Now scientists have linked the severity of the domoic acid outbreak to the warmer-than-average water temperatures that persisted in the region. Kathi Lefebvre, a wildlife algal toxins researcher at NOAA’s Northwest Fisheries Science Center, and her colleagues published a study in September that found the warm blob of 2015 facilitated the algal bloom’s extent and potency.
“Algal blooms are not uncommon, but one of this size and this duration are,” Lefebvre said, “and this warm-water period allowed for a certain phytoplankton to persist and flourish.”
That phytoplankton, known as Pseudo-nitzschia, produces domoic acid. Filter feeders such as clams and mussels trap in their digestive tracts high concentrations of the toxin, which works its way up the food chain. By the time sea birds and sea lions are chomping on anchovies, the domoic acid–laced fish basically poison the top predators.
Sea lions experiencing seizures were reported in record numbers in 2015—a symptom of domoic acid poisoning. Lefebvre said 229 sea lions were found with domoic acid poisoning just at The Marine Mammal Center in Sausalito in Northern California—more than tripling the annual average.
“We don’t have a warm blob anymore, but what that blob has done is kind of given a window into the future of what our oceans could look like,” Lefebvre said. “The natural seasonal algal blooms over smaller regions in the ocean could become these massive long-term events unlike anything we’ve seen that could really damage marine animals.”