Long-awaited rain that arrived last month prompted the endangered fish to begin their annual rush into local streams and tributaries to lay their eggs, the San Francisco Chronicle reported Friday.
The return of the coho salmon was encouraging news for fisheries experts and watershed managers.
"It shows that these fish can survive for months without spawning, while waiting for the rains to come," Eric Ettlinger, aquatic ecologist for the Marin Municipal Water District, said.
"Three years ago people were discussing how coho were about to become extinct in Central California, and it seems like they are beginning to bounce back."
The salmon, he said, have been waiting since November for stream flows to be strong enough for them to swim up to their favored spawning locations.
The spawning run began Jan. 19 when the region received 10 inches of rain.
Coho salmon are born in cold freshwater rivers and streams and live there for a year before swimming to the ocean, typically returning at age 3 to the exact spot where they were born to lay eggs and fertilize them.
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