Males and female California sea lions respond differently to lack of food
In Southern California hundreds of starving sea lion pups are washing up on beaches, filling marine mammal care centers that scarcely can hold them all.
Meanwhile thousands of adult male
California sea lions are surging into the Pacific Northwest, crowding onto docks and jetties in coastal communities.
How can animals from the same population be struggling in one region while thriving in another? The answer lies in the division of family responsibilities between male and female sea lions, and the different ways each responds to an ever-changing ocean.
“We’re seeing the population adjust to the environment as the environment changes,” said Sharon Melin, a sea lion biologist with the
Alaska Fisheries Science Center in Seattle.
The environmental changes affecting the sea lions can be traced to unusually weak winds off the West Coast over the last year. Without cooling winds, scientists say, the Pacific Ocean warmed as much as two to five degrees (C) above average. What started as a patchwork of warm water from Southern California to Alaska in 2014 has since grown into a vast expanse, affecting everything from plankton at the bottom of the food chain to sea lions near the top.
“The warming is about as strong as anything in the historical record,” said Nathan Mantua, who leads the Landscape Ecology Team at the Southwest Fisheries Science Center.
Female sea lions struggle to find food for pups
The Channel Islands rookeries where nearly all California sea lions raise their young off Southern California sit in the middle of the warm expanse. Female sea lions have strong ties to the rookeries. They take foraging trips of a few days at a time before returning to the rookeries to nurse their pups.
But the unusually warm water has apparently shifted the distribution of their prey, making it harder for females to find enough food to support the nutritional needs of their pups.
Their hungry pups, it now appears, are struggling to gain weight and have begun striking out from the rookeries on their own. Many do not make it and instead wash up on shore dead or emaciated.
Since the early 1970s the California sea lion population underwent unprecedented growth. The species is protected by the 1972
Marine Mammal Protection Act and is estimated to number about 300,000 along the U.S. West Coast. But the growth has slowed in recent years as ocean conditions have turned especially unfavorable for juvenile survival. That could lead to population declines in coming years, biologists say.
“We are working on data to look at whether the population might be approaching its resource limits,” Melin told reporters in
a recent conference call.
Sea lions serve as an indicator of ocean conditions because they are visible and are sensitive to small environmental and ecological changes, Melin said. The warm temperatures may well be affecting other species in less obvious ways.
“There are probably other things going on in the ecosystem we may not be seeing,” she said.
Male sea lions live like bachelors
Unlike female sea lions, males have no lasting obligations to females or young. After mating at the rookeries in midsummer, they leave the rookeries and roam as far as Oregon, Washington and Alaska in search of food.
“They’re bachelors,” said Mark Lowry of the Southwest Fisheries Science Center in La Jolla, California. “They just go wherever they can to find something to eat.”
Male sea lions search out prey with high energy content, especially oily fish such as herring and sardines, said Robert DeLong, who leads a program to study the California Current Ecosystem at the Alaska Fisheries Science Center. Increasing numbers have found their way to the mouth of the Columbia River to feed on increasingly strong runs of
eulachon, also called smelt, and have taken up residence on docks and jetties near Astoria, Oregon.
“More sea lions learned last year and even more will learn this year that this is a good place to find food,” DeLong said of the Columbia River. “They’ve learned these fish are there now and they won’t forget that.”
DeLong and Steve Jeffries, a research biologist with the Washington Department of Fish and Wildlife, attached satellite-linked tracking tags to 15 sea lions feeding on salmon near Bremerton, Washington, in November and December. Four of those sea lions are now at the mouth of the Columbia, Jeffries said.
Counts around Astoria rose from a few hundred in January to nearly 2,000 in February, exceeding numbers in previous years at the same time. The count includes some animals from the eastern stock of
Steller sea lions, removed from the List of Endangered and Threatened Wildlife in 2013. The California sea lions also feed on spring Chinook salmon and steelhead. Some of the Chinook and steelhead stocks are listed under the Endangered Species Act and
NOAA Fisheries is working with state officials to address sea lion predation.
By the beginning of May, the male sea lions depart for the summer breeding season at the rookeries in Southern California.
“It’s like flipping a switch,” DeLong said. “Suddenly it’s time to go.”
Poor feeding conditions may continue
The warm expanse of ocean extends to depths of 60 to 100 meters, Mantua said, and will likely take months to dissipate even if normal winds resume. Biologists expect poor feeding conditions for California sea lions will likely continue near their rookeries while warm ocean conditions persist. A more typical spring and summer with strong and persistent winds from the north would cool the water and likely improve foraging conditions along the West Coast.
tropical El Nino just declared by NOAA is one wild card that may affect West Coast ocean conditions over the next year. If the El Nino continues or intensifies through 2015, it would favor winds and ocean currents that support another year of warm conditions along the West Coast.
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