Scientists: Warm waters, scarce prey likely cause of California sea lion strandings
An intensifying spate of sea lion strandings on the California coast is likely caused by a shift in winds that has warmed coastal waters, making prey scarce for sea lion mothers and interfering with their ability to feed their pups, federal scientists said Wednesday.
The announcement marked the clearest answer yet to what might be affecting the sea lions, hundreds of which have come ashore malnourished and severely underweight in recent months.
With more than 940 animals, mostly pups, already admitted to rehabilitative care over the past several weeks, the state’s marine mammal centers are nearing capacity and running through resources, said Justin Viezbicke, California Stranding Network Coordinator for the National Oceanic and Atmospheric Association.
Many sea lions won’t be saved.
Yet NOAA scientists said the situation is less alarming than would appear and doesn’t look to be tied to a disease or new malady. Instead, it likely reflects the sea lion’s acute sensitivity to a change in ocean circulation patterns. The altered winds have bathed the coast in warm water - 2 to 5 degrees warmer than usual - and made foraging for redistributed fish species more of a challenge.
“It’s unlikely to have any really critical drop in the total population,” said Sharon Melin, a wildlife biologist with NOAA’s National Marine Mammal Laboratory at the Alaska Fisheries Science Center in Seattle.
There remain many unknowns, however, including how bad the situation will get before it starts getting better. Experts said they do not expect the situation to turn around for at least a few months.
Sea lions serve as an indicator species, Melin said, and are sometimes among the first and most visible marine creatures to reflect something amiss in the ocean environment. Their recent plight is one in a series of die-offs and stranding events beginning in 2009 on the heels of a rapid ocean warming. The population of their core prey has been somewhat diminished over the same time period, she said, so scientists will be looking for longer term implications.
January strandings were more than five times the historic average and more than twice the rate observed in 2013, when NOAA declared an Unusual Mortality Event, or UME. More than 1,100 sea lion strandings were recorded that year.
This year has not been declared a UME yet, though observations so far suggest it may be an even worse year, with data collected from sea lion rookeries in the Channel Islands during September and February showing that pups born last summer were already severely underweight, and in many cases, continued to lose weight over the winter, Melin said.
Most turning up on the coastline now are around 8 months old and should still be with their mothers, but appear to have weaned early, leaving the Southern California colonies in search of food, she said. Many are starving, too young to have developed the necessary skills to survive.
Scientists believe the root problem is the inability of their mothers to find sufficient food to nourish their young, most likely because the large area of warm water off the coast has driven fish and other marine life to other areas.
Yet satellite tags on some of the female sea lions who bore pups last year indicate they are staying within their usual foraging grounds, suggesting they may be having to dive deeper and work harder to feed, and thus are leaving their pups for longer periods, Melin said.
Pups left long enough will be so hungry they go off on their own to seek food, she said.
If so, this year’s event is similar to 2013, in which unavailability of prey was determined at least partly responsible. Though sea lions are opportunistic feeders, their core diet includes species rich in fatty acids like Pacific sardines, northern anchovies, rockfish, Pacific hake and market squid, some or all of which may be in short supply, Melin said.
Many pups coming ashore have secondary infections, like pneumonia, but testing on those that have not survived has not revealed evidence of an infectious disease outbreak or harmful algae blooms, which also are potential risks, scientists said.
Nate Mantua, a NOAA climatologist, said a period of strong southerly winds and weak northerly winds has spread warm water north and depressed the upwelling of cold water from deep ocean levels toward the surface.
He said the shift reflects the vagaries of weather - not more permanent climate change.
The warmer off-shore currents have coincided with a variety of shifting marine populations, causing some species to turn up in areas that are not part of their traditional habitats, Mantua said.
“There’s just a whole suite of different animals - some are really good swimmers and some are really weak swimmers - that have changed their distribution,” he said.