Salmon Making a Wrong Turn

March 7, 2024

Question: I remember a story late last year about salmon apparently getting lost on their way back to their natural spawning location. Why do fish sometimes get lost and choose the wrong stream or river?

Answer: In November, several salmon swimming upstream to spawn were found dying or dead in a small North Stockton (San Joaquin County) creek. The fish were likely trying to find their way to the Mokelumne River many miles to the north instead.

Usually, salmon raised in hatcheries find their way back to their native river for spawning season, but some take wrong turns along the way. There are several potential reasons for that. It’s believed fish raised in hatchery settings don’t have quite the same strong olfactory (smelling) skills that guide them to the river they were hatched. Also, many rivers in northern California are connected to the Sacramento-San Joaquin Delta and it’s possible through changing tides and flows that water from one river is mixing with water from other rivers, and that can confuse salmon returning upstream.

“There’s a natural instinct of fish to follow flow,” said Jason Julienne, the senior environmental scientist who oversees CDFW’s Sacramento Valley anadromous fish hatcheries. “And where you have a mixing of flows from different tributaries, as is the case in the Delta, it can confuse adult Chinook salmon coming back to the Central Valley that are depending on sense of smell cues to get where they’re supposed to go.”

Other factors that can contribute to fish straying include the trucking of fish downstream to a release point, and water delivery operations such as diversions, pumping and dam releases.

CDFW can determine the origin of raised fish thanks to a coded wire tag that’s implanted in a portion of all hatchery-raised fish at about five months old. The tag is uniquely coded to the hatchery where it was raised, it contains information indicating the age of the fish and the location where it was released.

It's also possible (and fairly common) for fish to make it all the way back to a different hatchery than the facility where they were raised.