California should leave more water in the state's most vital river delta to save crashing populations of native fish, state regulators said Wednesday in findings that could cut the amounts that cities and farms can take from the Sacramento and San Joaquin waterways.

The draft findings from the state Water Resources Control Board also could complicate Gov. Jerry Brown's proposal for building $15.7 billion giant water tunnels to carry water from the Delta.

The state currently takes about half of the Delta's normal flow for agriculture and cities each year. The new state report says current management of the Delta has caused the decline of almost all native species there.

It recommends the state leave more water in the river system and Delta for at least three seasons of the year.

The Delta provides water used by two-thirds of California's people and at least 3 million acres of California farmland.

Connecting to the San Francisco Bay, the Delta also is part of the West Coast's largest estuary. Native salmon in the waterway are an anchor species on the West Coast, supporting life forms ranging from Northern California forests to orca whales in the Pacific Ocean.

Five years of drought on top of decades of water withdrawals from the Delta have cut the numbers of native salmon and brought other species, such as the Delta smelt, close to extinction.

The state water board is now slated to consider altering or adopting recommendations in the report.

Wednesday's findings make no recommendation on how much normal flow to leave in the Sacramento River watershed, including the Delta. Instead, board members will analyze the outcomes of the state letting anywhere from 35 percent to 75 percent of the water flow out to the Pacific Ocean.

The report's recommendations could threaten the twin-tunnels proposal, which would send water around the delta to be delivered to south, because the tunnels project is based on maintaining the current level of water withdrawals from the waterways.

Kate Poole, a senior attorney at the Natural Resources Defense Council, which supports leaving more water in the rivers system, called it a "safe bet" that the recommendation for restoration of more natural flow would cut deliveries of water for people.

California farm groups adamantly oppose any such water cuts to agriculture.

"They tell you if we put more water in the river it'll be better for the fish," said Paul Wenger, president of the California Farm Bureau Federation. "I don't buy that for a second."