• is a science journalist at the University of California, Santa Cruz.

Early Plankton Blooms, Not Dams, Killing Washington Salmon

21 February 2010 10:33 am

SAN DIEGO -- Dams may not be causing the steep drop in Pacific salmon populations in Washington and Oregon. Instead, earlier plankton blooms in the ocean may mean salmon smolt reach the sea too late and starve to death.

A comparison of salmon populations in a Pacific coast river with dams and one without revealed that dams did not affect how many salmon made it out to sea.

Instead, earlier plankton blooms caused by climate change may be key. The findings were presented by Ron O'Dor in the "How New Tracking Technologies Can Help Manage Sustainable Fisheries" session of the AAAS meeting on Friday, February 19.

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"It's not the dams that are causing the problems. It's actually happening in the open ocean as a result of plankton changes," said biologist Ron O'Dor of the Census of Marine Life in a presentation at the American Association for the Advancement of Sciences meeting in San Diego. "Less plankton means fish don't get to eat what they want to eat."

The salmon are born in river beds in Washington and Oregon, spend a few years growing into smolts and group together to migrate out to sea. They spend several years in the frigid waters near Alaska, then return to the rivers to spawn.

For decades, salmon stocks in the Columbia river, which snakes northwest through Washington and into Oregon have been plummeting.

"Traditional wisdom was that it was caused by eight dams on the Columbia," O'Dor said. "And if we just tore down the dams, all the fish would be happy and healthy."

In this theory, dams thin the salmon population by blocking their habitat, killing young salmon as they pass through turbines, and increasing how long it takes them to migrate to sea, making them more vulnerable to predation and stress.

But the team wanted to see whether the dams actually affected salmon survival. They compared survival rates in the Columbia River, which has eight dams along its length, and British Columbia's Fraser River, which has none.

The team tagged hundreds of salmon smolts with an approximately half-inch sized acoustic tracking devices. The miniscule devices could be inserted into salmon as tiny as three-quarters of an ounce. To detect the fish, they also laid down hundreds of tiny acoustic receivers in both rivers, which sensed the position of each tagged smolt as it passed by.

There were no real differences in how many salmon made it to the mouth of each river, O'Dor said. However, very few made it back in their migration two years later, suggesting the smolts were dying once they reached the sea.

In another study, they showed salmon that shoot off from the main Columbia river into a smaller tributary with half as many dams have the same survival rates as smolts that migrate along the entire length of the river, O'Dor said. Both findings suggest the dams are not the key to the salmon population decline.

Because the team does not have receivers throughout the ocean, they turn off tracking devices in the fish once they reach the ocean. What happens out there is still a mystery.

However, they suspect that earlier plankton blooms may be a culprit in the salmon decline. Satellite images of chlorophyll show surges in plankton production have happened earlier in recent years, likely because of global climate change. That suggests only the salmon who reach the sea earliest can pack in enough food to sustain their arduous migration back.

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