Sea lion breeding shifts north to San Francisco Bay area islands
by NOAA Fisheries 25 Mar 2018 01:36 AEDT
Sea lions congregate in the Channel Islands, their longtime breeding ground. Many animals carry brands as part of population studies © Sharon Melin / Alaska Fisheries Science Center / NOAA Fisheries
Researchers tracking the population of California sea lions have for the first time documented hundreds of breeding sea lions shifting north from the Channel Islands off Southern California to small islands near the San Francisco Bay Area.
Although a small proportion of the sea lion population overall, the nearly 2,000 pups counted in July 2017 at the Southeast Farallon Islands and Año Neuvo Island reflect the largest shift of breeding sea lions away from longtime rookeries in the Channel Islands in recorded history, scientists said.
"This is definitely something new that's happening, and at this point we don't know why," said Mark Lowry, a research biologist at NOAA Fisheries' Southwest Fisheries Science Center in La Jolla, Calif., who leads annual aerial surveys of sea lions to track the population. "Is this just a temporary effect of the last few years, or is it the beginning of something bigger?"
The move adds to numerous marine ecosystem and species changes in the aftermath of the "warm blob," an expanse of unusually warm ocean conditions that emerged off the West Coast about 2014 and continued into an especially strong El Niño pattern in 2015. The continued warmth represented an extreme example of what scientists call a "marine heat wave." In many cases species shifted north as they followed their preferred temperatures, or their prey.
A recent NOAA Fisheries assessment found that California sea lion numbers rose from fewer than 90,000 in 1975 to an estimated 281,450 in 2008, roughly its carrying capacity at the time. The population hovered around that point in the following years, reaching a high of 306,220 in 2012, but then declined as the heat wave took hold and prey shifted.
Thousands of sea lions died, and NOAA Fisheries declared the elevated deaths an Unusual Mortality Event in 2013.
Scientists tracking effects of the blob and the subsequent El Niño documented female sea lions venturing farther north from their rookeries in the Channel Islands in search of prey such as sardines, anchovies, rockfish, and market squid, leaving their pups for longer periods. That led hungry pups to set off on their own in search of food, only to become stranded on Southern California beaches.
Some sea lions following their prey may have taken advantage of the islands as northern outposts by taking up permanent residence there and reproducing, Lowry said. Climate modeling has also suggested that a warming ocean may push marine species north in the long term.
The arrival of breeding sea lions in the Farallon Islands and at Año Neuvo may reflect some combination of their robust population and the scarce prey in the Southern California Bight in recent years, said Elliott Hazen, a research ecologist at the Southwest Fisheries Science Center.
He said humpback whales have similarly spent more of the winter months in Monterey Bay in the last few years, north of their typical wintering grounds off Mexico and Central America. The whales have likely been following dense schools of anchovies north, but their growing population has probably also contributed.
"With increasing populations, it is more likely that new breeding colonies and new foraging habitats will continue into the future," Hazen said.
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