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Where have all the coho gone?

FEWER endangered coho salmon are spawning in Marin this season than at any time in the past dozen years – and biologists don’t know why.

What concerns fish watchers is that this year should have been a prime year for coho, based on their three-year life cycle. Yet the number of redds – clusters of eggs – is at an all time low.

“This year’s count has turned the best year-class into the worst,” said Todd Steiner, biologist and executive director for the Lagunitas-based Salmon Protection and Watershed Network.

Coho salmon have a three-year life cycle. The 2004-05 winter was a banner year for the return of coho to spawn in Marin, meaning this winter – three years later – should be just as strong.

Instead, the numbers have dropped precipitously.

Changes in ocean patterns possibly due to climate change, flooding in the county and even the Cosco Busan oil spill may be having an effect, experts said. But the trend is not only local – the numbers of returning fish are down throughout the region.

“We are seeing low returns across California,” said Paola Bouley, watershed biologist for SPAWN, which has kept track of coho returns for the past 12 years.

Marin’s salmon run is one of the more critical in the state. The
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Lagunitas watershed has one of the largest remaining populations of wild coho salmon in central California, and comprises 20 percent of the state’s total. Coho have gone extinct in 90 percent of California streams that once supported the species.

“We thought maybe the coho were late this year and we waited and waited, but the run is just about done,” Bouley said.

That has raised red flags among biologists.

“This year’s decline is extremely serious for Lagunitas coho salmon,” Steiner said. “When an endangered population is already jeopardized from decades of habitat destruction, its ability to bounce back from such a large decline is greatly diminished.”

The life cycle of the coho is rigid: In the winter, fish return to the streams in which they were born to spawn and then die. Young fish hatch from eggs in the gravel in the spring and then spend another year in the streams feeding and growing while seeking refuge in deep, cold pools. After enduring a summer and winter they then head out to sea in their second spring to feed along the productive California coast. Fish return to their streams from the ocean to spawn, die and continue on the cycle of life.

“Coho have a very fragile life cycle, they come back only once to spawn and then die,” said Bill Cox, biologist for the state Department of Fish and Game. “It doesn’t take much to wipe out a whole year’s class.”

This year’s findings compared with three years ago show:

– In the small tributaries of the San Geronimo Valley, spawning numbers were down 93 percent.

– In Olema Creek, which runs through Point Reyes National Seashore, numbers were down 82 percent.

– In the Lagunitas/Devil’s Gulch/San Geronimo Creek watersheds as a whole, populations have dropped 70 percent.

– In the Redwood Creek watershed in Muir Woods National Monument, no coho returned this year.

Only the 1995-96 winter had lower counts, and that may be because counting was not as thorough in that first year of data collection, officials said.

It is possible that this year’s low number may be in part due to the flooding that occurred in the winter of 2005-06, when raging waters may have flushed out many of the coho fry that were in creeks, Bouley said.

“There may have been changes in the ocean which warmed waters that could be caused by global warming, but it also may be a natural fluctuation,” Cox said. “Because the population is so low in the region, it doesn’t take a big fluctuation to do them in.”

The lack of spawning coho in Redwood Creek has raised fears that the Nov. 7 Cosco Busan oil spill may have driven the fish away.

The Redwood Creek coho salmon run comes from the Pacific Ocean and through Muir Beach – which was hit hard by the spill. Coho congregate there waiting for seasonal rains to break a berm at the beach. They can then travel up the creek and spawn. Visitors can usually see the spectacle at Muir Woods, but not this year.

“This severe decline demonstrates how fragile this population is and how important it is to re-double our efforts to protect and restore this population,” Steiner said.