Mokelumne River salmon defy the drought

Despite extreme drought conditions, 12,118 Chinook salmon returned to the Mokelumne River last fall to spawn, becoming the river’s fifth-largest run in 74 years, East Bay Municipal Utility District announced this week.

This is the fourth consecutive year the number of Chinook salmon returning to the Mokelumne River exceeded 12,000 fish, nearly triple the river’s long-term annual average. Of the returning salmon, about one-fourth spawned in the river and the rest were collected at the Mokelumne River Fish Hatchery for egg production. The hatchery is a partnership between EBMUD and California Department of Fish and Wildlife.

“These positive results indicate that our efforts are successful and our partnerships are working,” said EBMUD Manager of Fishery and Wildlife Jose D. Setka. “The Delta is already an unwelcoming place for juvenile salmon and the drought makes those conditions worse. Salmon need all the help they can get.”

The Mokelumne River, the source of water for 1.3 million people in the East Bay, is the site of Pardee and Camanche Reservoirs. To contend with dry conditions, EBMUD biologists got creative.

EBMUD staff released reservoir water and pulsed it downstream to mimic storms. Even with limited amounts of water last year because of the drought, by modifying releases last spring, enough savings were achieved to pulse flows to attract adult salmon in the fall. Woodbridge Irrigation District, in Lodi, also on the Mokelumne River, changed its dam operations to deliver extra pulses.

To deal with warmer water temperatures, EBMUD biologists turned to Pardee Reservoir. Drawing cold water from the bottom of the reservoir lowered the temperatures to less than 60 degrees in the lower Mokelumne River, better for salmon spawning.

The Delta is also home to salmon predators such as largemouth bass and striped bass. To bypass the predators and the South Delta export pumps, another source of high mortality, staff trucked juvenile salmon from the hatchery and released them at Sherman Island, northeast of Antioch, with positive results.

Additionally, EBMUD is supporting a study which explores transporting juvenile salmon from the Mokelumne River by barge and releasing them in the San Francisco Bay. Barging may improve fish survival as they travel through the Delta and it may also help juvenile salmon learn the smell of the Mokelumne River through a process called imprinting, necessary for adult fish to re-trace their way to natal waters.

State and regional agencies met yesterday to review last year’s statewide salmon return results and develop forecasts for the 2015 season. EBMUD’s fishery program now is gearing up for what could be another dry, or worse, critically dry year. All eyes are set on helping the juvenile salmon produced this year survive and return to the Mokelumne River in two to three years.

The Department of Water Resources forecast, based on the April 1 snow survey, will determine how EBMUD and California Department of Fish and Wildlife manage the river and the hatchery later this year. The strategies are part of a 1998 agreement between EBMUD and federal and state agencies to promote healthier conditions and water flows in the Mokelumne River.

“Ensuring a healthy fishery is one way EBMUD is committed to environmentally sound management of natural resources,” Setka said. “We are working with over 20 partners to ensure the continued viability of the Mokelumne salmon population – a natural resource that Californians depend on.”


Tracie Morales
Public Information Representative