May 14 2014

Sea-lion miseries tied to sardine reduction

Nursing the mammal population back to health raises another question: Do we let nature take its course if there are now too many?

Published: May 6, 2014

Mass beachings of malnourished sea lions in 2013 are likely linked to a drop in sardine populations near Channel Islands rookeries where thousands of sea lions are born each year, federal officials say.

More than 1,600 sea lion pups washed up on beaches from San Diego to Ventura between January and April 2013 – starving, dehydrated and suffering from a variety of diseases.

The mass stranding led the National Oceanic and Atmospheric Administration’s fisheries service to form a task force of scientists to research what could have caused the unusual mortality event.

In the past two months, 650 sea lions have been found on beaches in similar conditions.

“Although the pups showed signs of some viruses and infections, findings indicate that this event was not caused by disease, rather by the lack of high-quality, close-by food sources for nursing mothers,” said Sarah Wilkin, coordinator of the Marine Mammal Health Stranding and Response Program for the National Marine Fisheries.

The task force’s scientists considered prey, oceanic conditions, viruses, bacteria, toxins and even radiation from Japan’s Fukushima nuclear plant. The lack of sardines has been the only clear indicator of the mass stranding.

Sardines, a fatty, silvery fish, provide a high level of nutrients not only to sea lions but also to seals, elephant seals and humpback whales, many of which are also found along the Channel Islands and compete for food with the sea lions.

Sardine numbers are in steep decline, and those that are available have shifted spawning grounds, previously surveyed within 50 miles of the sea lion rookeries, to deep water up to 120 miles from shore even as sea lion numbers are booming.

Scientists say the absence of sardines near the rookeries likely created challenges for mothers in feeding their pups and forced juveniles to swim farther to find other forage fish like market squid and juvenile rock fish. Most sea lions hunt within 60 miles of their rookeries.

In the 1940 and ’50s, sardines were heavily fished off the coast of Monterrey in Northern California. Their numbers drastically declined for 30 years and then rebounded in the 1980s, according to task force member Sam McClatchie, an oceanographer with Southwest Fisheries Science Center in La Jolla who has conducted fish number studies on sardines.

In 2006, sardine numbers again began to crash, McClatchie said. Last December, the fishing quota was dropped to 5,446 metric tons for California, Oregon and Washington from January to June. In the same time period the previous year, the quota was 18,073 metric tons.

Sardines and other pelagic fish such as anchovies, market squid, rock fish and hag are known to fluctuate in populations and locations. The fish are mobile, migrating from Baja to Vancouver each year.

But sardines, while flexible, are easily affected by changes in ocean temperature. A dip of water temperatures in the south-flowing California current in the last decade could be reducing their numbers off the Southern California coast.

There are big differences in temperature between Baja, Vancouver and the California central coast along with seasonal and regional differences that cause volatile swings in sardine populations. While those conditions have brought a boom in some species, like market squid, they have pushed out sardines.

“In a year where conditions are good, we can get a lot of fish,” McClatchie said. “They often live five to eight years and in a good pulse, a lot of eggs can be produced related to the number of mothers there are. How many survive depends on environmental conditions and how many are eating them.”

Last year’s stranding was not the largest in the California, but the timing and location made it unique, said Justin Viezbicke, California stranding network coordinator for National Marine Fisheries.

Sea lions began washing ashore in January, much earlier than usual. Sea lion pups are born in summer and stay with their mothers for 10 to 11 months, so rescue centers don’t usually see strandings until June.


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