Research reveals ideal conditions for salmon runs

West Coast salmon runs are in trouble, according to Professor of Earth & Climate Sciences Leonard Sklar. But a new study by Sklar and a team of researchers could help identify ideal habitats for salmon spawning.

Female salmon lay their eggs after digging small divots into the riverbed with their tails. These underwater nests are known as redds.

Photo of a male and female pink salmon.

Pink salmon reside in the fresh water rivers of the northern United States and Canada. The female (front) and male (behind) can be differentiated by the male’s humped back.

Though large salmon carry more eggs, they also dig larger redds, taking up more space in the riverbed. Researchers found that intermediate-sized fish maximize the number of eggs a riverbed can host.

According to California’s Department of Fish and Wildlife, salmon habitats have been degraded by around 90 percent since 1940 due to logging, construction and other disturbances that escalate natural erosion. To help restoration efforts, the research team looked at the conditions of riverbeds where salmon still spawned.

For salmon to dig their redds, the river rocks must be small enough so that the fish can easily move them, but large enough so that their small eggs don’t lose access to the oxygen-rich water or get scoured away by floods.

“The conventional knowledge was that fish want rocks to be 10 percent of their body length,” said Sklar. “Until now, that’s all we knew. No one had taken into account the wide distribution of rock sizes.”

Upon investigation, researchers found that a range of middle-sized rocks was most advantageous for spawning habitats. Uniform-sized rocks did not attract spawning salmon. 

“Now we’ve developed a recipe for mixing up the rock sizes we think will be most useful to the fish,” said Sklar. “Salmon have a better chance of being able to move some of the rocks when there’s a range available to them.”

The research team carried out observations of pink, sockeye and Chinook salmon in California’s Shasta River, Washington’s South Prairie Creek and British Columbia’s Scotch Creek. In addition to Sklar, the team included researchers Clifford Riebe of the University of Wyoming and John Wooster of the National Oceanic and Atmospheric Administration Fisheries.

Recognizing ideal riverbeds for salmon spawning is important for environmental protection and restoration efforts.

“We’ve created a tool that restoration managers and practitioners can use to optimize the number of fish born in a given area,” said Sklar. “There’s only so much money allotted for habitat restoration each year. We want those efforts to be as effective as possible.”

“Optimal reproduction in salmon spawning substrates linked to grain size and fish length” is available online and will appear later this year in Water Resources Research.

-- Gianna Devoto