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Limits on Napa Fish
Why aren’t there more Chinook salmon, steelhead and California freshwater shrimp in the Napa River than there used to be? A two-year study shows reveals how major changes to the river and its tributaries have played a role in limiting the populations of the three rare species, and puts more real science on the table in the ongoing debate over the impacts of hillside vineyards on local forests, flora and fauna.
The salmon, steelhead and shrimp each have their own individual story, says Mike Napolitano of the S.F. Bay Regional Water Quality Control Board, which funded the $400,000 study (conducted by U.C. Berkeley and Stillwater Sciences) along with the California Coastal Conservancy and the Napa County Resource Conservation District. Chinook primarily use the main stem of the Napa River. Before the valley was intensely developed for agriculture and residences, the river was allowed to meander, forming shallow pools and backwaters, where the fry could linger and grow. "It was really excellent habitat if you were a young salmon," he says. But the river has been straightened and leveed, resulting in "a widespread and dramatic simplification" of its channel, he says. Long, deep, featureless pools were created, which favor exotic predators, such as smallmouth bass. Now the youngsters "have to run the gantlet" to survive, he says.
Turbidity caused by sediment proved to be less of a problem than expected. The study found it clouds the water for just a few hours after major storms, not enough to seriously interrupt the fishes’ feeding regimens. In the tributaries, however, fine sediment clogs gravel beds where steelhead lay their eggs, which reduces available oxygen and decreases the successful hatching rate, possibly by 50%. But steelhead population decline "cannot be explained by too much fine sediment alone," says Napolitano, citing other factors such as dams, diversions, water temperature and lack of wood. Little data on the shrimp emerged. The study says more research is needed to determine how cutting back riparian vegetation might be impacting shrimp habitat.
The study isn’t likely to diminish the ongoing controversy over hillside vineyard and residential development in the valley. Napolitano says that its purpose was to "provide a detailed description of what the streams look like." The next stage of the research will more closely examine the sources of sedimentation.
Vintner David Graves praises the scientists’ "even-handed approach" and sees reason for optimism. Valley residents are becoming more aware of the consequences of their actions, and of the importance of improving conditions in the streams. "We shouldn’t be afraid of the truth," he says.
The study, Draft Napa River Limiting Factors Analysis, is available at the board's web site: www.swrcb.ca.gov/~rwqcb2.
Contacts: Mike Napolitano (510)622-2397 or Bruce Orr (510)848-8098 O'B
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