Hypothesis Confirmed: Sea Lion Mass Deaths Caused by Malnutrition

by Alastair Bland
August 2, 2017
Originally published by Hakai Magazine

Meteorologists had never seen anything quite like it—a mass of abnormally warm surface water that overwhelmed much of the northeastern Pacific Ocean for three years starting in late 2013. They called it the Blob.

Within months, thousands of starving sea lion pups began washing ashore along the west coast of the United States.

At the time, scientists figured the two occurrences were related. But in a new study, a team of researchers describes in detail how warm water and related impacts on ocean productivity hurt the young sea lions.

Scientists believe the Blob began to form in the Gulf of Alaska after the winds that drive the upwelling of nutrient-rich cold water from the seafloor to the surface weakened or died, allowing surface water to grow unusually warm and slowing the production of phytoplankton. At the same time, farther south, El Niño was disrupting ocean circulation. The resulting masses of warm, nutrient-poor water being formed in the north and south joined up, and the so-called Blob settled in for a three-year stay off the coast of California.

The Blob hit the sea lions when reduced primary production sent forage fish, such as anchovies and sardines, elsewhere, says study coauthor Karina Acevedo-Whitehouse, a disease ecologist at the Autonomous University of Queretaro in Mexico.

Without their usual prey, hungry sea lions turned to rockfish, “which is like junk food for sea lions,” Acevedo-Whitehouse says. Oily forage fish pack a nutritional punch; white-fleshed Pacific rockfish do not.

After analyzing blood samples and the blood glucose levels of nursing sea lion pups off Baja California, the authors concluded that lactating sea lions were unable to provide adequate nutrition for their pups. This had a devastating effect on an entire generation of sea lions.

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