The Delta's signature fish species has seen a promising rebound this year, thanks to robust river flows.

Scientists, however, say the population of the Delta smelt remains a long way from full health.

Data released Wednesday by the California Department of Fish and Game from an annual summer survey indicate the Delta smelt population has more than doubled compared with last year, and it is at its highest level since 2004.

The finger-length smelt is listed as "threatened" under the federal Endangered Species Act. It lives only in the Sacramento-San Joaquin Delta and is a key barometer of the estuary's health because each fish generally lives for just one year.

Despite the increase, smelt numbers remain well below the highs seen in the 1970s. The species is still considered at risk of extinction, suggesting a worrisome trend in water quality for the entire Delta.

It has long been known that greater river flow to the ocean benefits smelt, and this year's field survey results reflect that, said Marty Gingras, a supervising biologist at Fish and Game.

Higher flows generally push the smelt population farther downstream, where habitat is more favorable and the fish don't risk getting killed in the state and federal pumps near Tracy that divert water from the Delta.

"Beyond that, we don't yet know exactly what the mechanism of the Delta smelt increase was, whether it was improved survival of eggs or greater numbers of eggs produced to begin with," said Gingras. "That type of thing is the object of ongoing research."

The smelt has been a focus of derision by some water diverters and politicians, especially in the San Joaquin Valley, because its fortunes control a Delta water supply that serves 25 million Californians and more than 1 million acres of farmland.

Federal rules restrict water diversions from the Delta under certain conditions to protect the species. Those rules have been less noticeable this year because of huge river flows brought on by winter rains and snowfall far above average.

"It demonstrates that when freshwater is flowing through the system, the fish can respond from very low numbers," said Jon Rosenfield, a conservation biologist at the Bay Institute. "At this point, anything that's not extinction, we'll count that as good."

Some observers project water diversions from the Delta will set records this year, yet relatively few smelt have been killed by that pumping. Both are benefits of ample precipitation.

Rosenfield cautioned that this year's water abundance is an anomaly. Continued protections are needed not only to let the smelt recover, but also to ensure Delta water and habitat are healthy for people as well as fish, he said.

"We are nowhere near out of the woods," he said.

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