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Vancouver Island study disentangles river and marine stressors on salmon and trout

Decline in freshwater recruitment of three species strongly associated with watershed logging
Researchers sample fish in the Keogh River on Vancouver Island. Simon Fraser University.

A study by researchers from Simon Fraser University suggests environmental changes in both rivers and oceans may be driving declines of wild trout and salmon.

The study, published in Global Change Biology, examines fish population data collected over 40 years in the Keogh River of north Vancouver Island.

“The Keogh has all kinds of sea-run, migratory salmonids — Pacific salmon and trout, more generally,” said SFU professor Jonathan Moore. “It was a great opportunity to start investigating what are some of the big drivers affecting them.”

Concerns about wild salmon declines often focus on threats in the ocean, but stressors in rivers need to be studied too, said Kyle Wilson, researcher.

“It’s easy to point your fingers at different things, with regards to what part of the life cycle is actually driving patterns,” said Wilson. “But without studying it, you can’t know for sure.”

Sharp declines were seen in some species — including 80 per cent in steelhead and 70 per cent in cutthroat — an important finding in itself.

“Even that sort of basic knowledge is surprisingly hard to get,” said Moore.

But connecting these and other population patterns to environmental changes in both the ocean and freshwater is where the study could make its mark on management.

Reduced steelhead survival in oceans was associated with increased interactions with marine species, including seals.

The study also measured recruitment productivity, meaning the number offspring produced per individual, for five species and compared it to environmental conditions in the Keogh Watershed.

They found three distinct phases in freshwater productivity over time.

At first, recruitment was low to moderate when fish were abundant, but then it increased as fish numbers declined.

“Salmon and steelhead in general have this pretty amazing ability of doing well when at low abundances, allowing them to recover from population declines,” said Moore. “Basically, it’s a signal of competition.”

But then, starting in the mid-2000s, productivity declined.

“That’s kind of the scary thing with these three stages … now we’re seeing that despite variable numbers, they don’t seem to be compensating as well,” said Moore.

Unlike the other study species, pink salmon recruitment stayed relatively constant, likely due to their short life cycle.

“They don’t rely on freshwater quite as much and probably use the ocean differently,” said Moore. “Some people consider them the weeds of the salmon world.”

The researchers found that 97 per cent of the productivity decline seen in steelhead, cutthroat trout and coho salmon was associated with logging, which intensified in the 1990s.

“What we’re seeing is that over that 40-year span, logging was associated with an incredibly strong decline in productivity,” said Wilson. “Logging along watersheds is a potentially dangerous practice for salmon.”

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