California and federal wildlife officials are scrambling to figure out how hundreds of endangered salmon recently became stranded in irrigation ditches in the Colusa Basin, west of the Sacramento River.

Finding answers is a matter of some urgency, because tens of thousands of fall-run Chinook salmon are weeks away from their annual return from the ocean to the Sacramento River and also could become trapped.

“There has been some stranding in the past, but as far as I can tell, the numbers have been significantly lower than this,” said Jeffrey McClain, assistant supervisor at the National Marine Fisheries Service office in Sacramento. “It’s significant, and that’s why this is a serious thing for us to figure out.”

The rescues were led by the California Department of Fish and Wildlife. In 11 trips over a month, starting May 2, officials rescued 312 adult salmon headed upstream. Of these fish, at least eight were determined to be spring run, and the balance were winter run. Spring-run salmon are listed as a threatened species under federal law, and winter-run salmon are endangered.

Twenty-four of the rescued fish were missing their adipose fins, indicating they were winter-run salmon bred at the U.S. Fish and Wildlife Service’s Livingston Stone Hatchery near Red Bluff.

During the rescues, 18 salmon either died after handling or were found dead in the canals. Many more live salmon were seen but could not be rescued because of flow or terrain obstacles.

The water temperature at some rescue sites was 74 degrees. Salmon begin to suffer when temperatures exceed 68 degrees and will eventually die from prolonged exposure. Most of the rescued fish were taken to a release site on the Sacramento River.

Kevin Shaffer, salmon conservation manager at the California Department of Fish and Wildlife, said it is possible that many more stranded salmon went undetected in the hundreds of miles of irrigation canals in the Colusa Basin.

“Basically, the strandings went on for some time,” he said. “I can’t imagine all those animals would be able to get out themselves.”

Stranded salmon were first seen April 24 by an employee at the Sacramento National Wildlife Refuge, which is connected to irrigation canals in the basin. Most of the rescues occurred there, as well as at the Delevan National Wildlife Refuge.

The web of ditches eventually drains into the Yolo Bypass through a canal called the Knights Landing Ridge Cut. Officials suspect this is one way the salmon swam astray, probably when the Yolo Bypass flooded after heavy rains in December.

The balance of winter was odd. That wet December was followed by the driest January through June in the region in recorded history. This led to unusually low flows on the Sacramento River for most of winter. As a result, winter-run salmon may have gotten their primary migratory cue from the agricultural canals.

— Sacramento Bee