More fall-run Chinook salmon are returning to spawn in the Mokelumne River this year than in any year since 1940, when wildlife officials first began tracking the population, according to the East Bay Municipal Utility District.

EBMUD officials announced Thursday that more than 20,000 fish have so far returned to the river to complete their life cycle.

"This year's historic return highlights EBMUD's longstanding collaboration in the region and our deep commitment to sustaining this vital fish hatchery and protecting the river's habitat," said EBMUD Board President Andy Katz.

The river is the primary drinking water source for 1.4 million customers served by EBMUD in Alameda and Contra Costa counties and the water agency is responsible for balancing the needs of the river's human dependents with environmental and wildlife protections.

Salmon populations are counted every spawning season, which runs from August through January, when the fish return to their Mokelumne birthplace to breed and die after living for two to five years in the Pacific Ocean.

Some of the strategies for protecting the health of the salmon run include habitat improvements, increased river water flows from the upstream Camanche Reservoir, hatchery management and the cancellation of the state's commercial and recreational salmon fishing seasons, according to EBMUD officials.

"This year's large run allows us to meet our hatchery goals as well as improve natural production numbers in the river by maximizing the use of the available spawning habitat we worked so hard to build. Decades of work to improve the natural habitat are paying off," said Michelle Workman, EBMUD manager of fisheries and wildlife.