California Salmon Can Fight (Some) Climate Change Threat By Eating More

Adult coho salmon. (NMFS/Southwest Fisheries Science Center; Salmon Ecology Team)

Climate change is coming for our homes, our perfect waves, our birds of prey, and now, our fish — specifically California's salmonids (salmon and trout).

The vast majority of the state's 32 species of salmonids are worse off than they were just a few decades ago. Populations have declined, with 15 of the 32 landing on the endangered species list. A comprehensive 2017 report predicted that over the next century, 74% of our salmon and trout could disappear from California.

The losses are associated with vanishing habitats (floodplains, estuaries and lagoons), as cities and agricultural operations have expanded. But the losses are also a result of warming waters.

Salmonids are ectotherms, which means that they need cool water to regulate their body temperature. If they're not sufficiently cooled, their growth can be stunted and their health can suffer, making them more vulnerable to predators and disease. If it's too hot they can also just ... die.

A new study from UC Davis, published in the Canadian Journal of Fisheries and Aquatic Sciences, offers a glimmer of hope, with a caveat.

It found that salmonids can likely thrive in slightly warmer waters, provided they eat like bodybuilders trying to bulk up for a competition.

"WE WERE STUNNED..."

The research team led by Robert Lusardi, a California cold water fish scientist at UC Davis, placed 150 baby coho salmon at 25 different enclosures throughout the Shasta river basin during some of the year's warmest months.

To test a range of temperatures, scientists placed groups of salmon different distances from natural springs, which tend to keep water temperatures lower.

Coho were chosen in part because they're one of the most temperature-sensitive species, stressed by water above 60.8 degrees Fahrenheit.

"We went in really thinking that, you know, these fish up in the coldest water near the springs are going to grow really well," said Lusardi.

Over 63 days, scientists monitored water flow and temperature, as well as the amount of food available to the fish. In the end, they found that some of the largest growth rates were among the salmon that experienced the highest average daily temperatures, around 61.8 degrees.

"We were stunned, really blown away when we started removing the enclosures and removing the fish and we found that the fish actually optimized the growth at much warmer water temperatures than we thought," said Lusardi. "The reason they could do that is because they were compensating for those temperatures with food."

While both groups of coho salmon started out the same size, those on the left experienced an average temperature of 55.4 degrees Fahrenheit and those on the right experienced 61.88 degrees Fahrenheit. (Robert Lusardi, UC Davis)

Higher temperatures mean higher metabolism, which means higher energy requirements. The fish that were able to eat enough, bulked up.

However, the team did see a temperature limit.

One group swam through a daily average water temperature of 64.58 degrees. Though they grew as much as the other fish in hot areas, a few of them died. A small change in temperature can make all the difference.

"I think we were surprised by the fact that salmon could grow under these really warm water conditions because they have an abundance of food," said Lusardi. "That's not to suggest that fish are going to be able to eat their way out of warmer water temperatures..."

Even though this study provides a snapshot of salmonid health in warming waters, it's by no means all encompassing.

It's unclear if those baby coho that gorged themselves to survive would have stayed in the warmer, more stressful conditions if they weren't caged, or if they would have tried to migrate elsewhere for cooler waters.

Or whether salmonid food — freshwater shrimp, mayflies and midges, in this case — will hold up as our world continues to warm.

The 2017 state of the salmonid report, of which Lusardi was a contributor, recommends rebuilding and maintaining habitats lost to development in an effort to offer salmonids as many places as possible to find optimal conditions.