Coho Salmon Thrive On Fellow Fish

Michelle Nelson at work on a salmon stream in British Columbia's Great Bear Forest (Credit: Alison Page)

Death in nature is never a matter of existential concern, but rather a means of resource reallocation. A new study from Simon Fraser University and the Raincoast Conservation Foundation shows how juvenile coho salmon participate in this reuse of resources by gaining nutrients derived from their deceased distant cousins.

University researchers found that juvenile coho populations are three-times greater in streams with pink and chum salmon populations. The coho’s consumption of pink and chum salmon eggs and remains account for some of the impacts on the population. However, bolstered coho populations were also observed in waterways where coho have no direct interaction with other salmon.

The study is a chapter in the thesis of Michelle Nelson, lead author and doctoral student at SFU in British Columbia.

“We thought it was really interesting in terms of fishery management because pink and chum aren’t highly commercially valued… whereas coho are,” Nelson said. She suggested that showing a link between the less desirable species and the prized coho might redefine management practices.

Correlation vs Causation

Establishing a correlation between the booming coho and the spawning pink and chum salmon required the researchers to eliminate other possible biological advantages. Between 2006 and 2009, SFU grad students and First Nations volunteers studied coastal waterways in the Canadian province.

“We would go out in the morning by boat… in these near-pristine watersheds, and collect data on habitat variables,” Nelson said. “We looked at pretty much everything we could think of that might affect the fish.”

That included basic hydrological characteristics such as connectivity, turbidity, dissolved oxygen and pH levels. Additionally, researchers examined substrate composition, woody debris and other physical features.

“It’s always challenging to do fieldwork in a remote location,” Nelson said. “You have to worry about things like the weather and wildlife… not that that would keep you from doing things, but you still think about them.”

Michelle Nelson at work on a salmon stream in British Columbia’s Great Bear Forest (Credit: Alison Page)

Really Cool Results For Coho Salmon

After accounting for environmental variables, the research revealed that pink and chum salmon that die during spawning indirectly contribute to coho nutrition. However, the converse was not true. Nelson said this is because coho stay in freshwater for up to two years before migrating to the ocean, giving them plenty of time to reap the benefits of the salmon-enriched food cycle. Pink and chum juveniles head to the sea as soon as possible and miss out on the extra nutritional pathways.

Nelson said one pathway, in particular, was “really cool.”

“When the spawning salmon come into the freshwater, wolves and bears come and drag the carcasses into the forest,” Nelson said. “A lot of carcasses.”

Flies lay eggs in the salmon remains, which eventually hatch into larvae. Some of the larvae end up in nearby streams, where they’re gobbled up by the young of the same salmon that hosted their eggs.

Hope For The Future

Nelson said she and her colleagues carried out the study with hopes of providing useful science for legislative and management decisions. But fishery management legislation in Canada has changed in recent years, and money for research is harder to come by.

“In Canada… funding for these types of problems has really been cut,” Nelson said. “I think that’s a challenge going forward.”

With the rise of ecosystem-based management in Canada, Nelson sees an opportunity to expand and implement ecological research. For her, the bottom line is simple:

“We need to understand how the ecosystems work,” she said.