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Salmon runs with more diversity provide best catches, study finds

Given a choice between setting their nets for a large run of salmon, or a smaller run with greater population and species diversity, most fishers would likely choose the former. More fish should mean better catches.

But a new study by researchers at Simon Fraser University, who examined decades of data from First Nations fisheries on British Columbia’s Fraser River, has found that salmon runs offering the most diversity also provide the best catches over time.

“The fisheries that had more diverse salmon portfolios were more consistent from year to year – three times more consistent, roughly – and they also had way longer fishing seasons, three to four times more weeks where there was fresh fish coming in,” said Jonathan Moore, Liber Ero chair of Coastal Science and Management at SFU. He oversaw the research done by master’s student Holly Nesbitt.

The results were surprising but irrefutable, he said Thursday in an interview, prior to publication of the study on Friday in the Journal of Applied Ecology.

“By looking at 30 years of catch over 21 different fisheries, we can sort of deconstruct what matters in terms of the consistency of the fishery from year to year,” Dr. Moore said.

He said the research was able to clarify which fisheries had boom-and-bust cycles and which had stable and predictable returns year after year.

“And we could also figure out, well, is that due to them catching different species of salmon, or is it due to them catching lots of different populations? And that was one of the really cool parts of the study, because it seemed like what was the most important was that fine-scale diversity was really driving the patterns,” he said.

Ms. Nesbitt said she examined First Nations fisheries all the way from the mouth of the Fraser to the Nechako River, a tributary some 800 kilometres upstream. The data showed that the most productive fisheries were based on runs that not only had a diversity of different species, but had population diversity within species.

“One of the interesting things that we found was that population diversity had a stronger effect than species-level diversity did. And we found that surprising,” she said.

Chinook, coho, chum, sockeye, pink salmon and steelhead are all different species that return to the Fraser. Within each species there is also population diversity, which refers to behavioural attributes of different groups of fish, such as run timing or spawning location. For example, some populations of chinook return in early April, but others come back months later, in September; some spawn in big tributaries, such as the Thompson River, while others head for tiny streams, such as Blue Creek, in the Pitt River Valley.

“Species diversity is the number of species … whereas population diversity is the number of populations within a species,” Ms. Nesbitt said. “Each species is made up of different groups that spawn in different areas, and they come from all over the watershed and so they make up the finer-scale diversity that is spread throughout the watershed.”

She said managing fisheries down to that finer scale is extremely important. But that isn’t always done. In mixed-stock fisheries, for example, nets are set for one species of salmon, but other species are often caught because different runs sometimes mix together. Nets set for a prolific run of chum, for example, can end up killing endangered steelhead.

To avoid that, Ms. Nesbitt said, fisheries can be moved to areas where the species and different populations separate, such as further upstream rather than at the river mouth or in the ocean.

“In the Skeena, we’re seeing a push … to target specific populations by going to more terminal locations within a watershed. … That way you are not targeting the mass, sort of the big pool [of salmon], and you can focus on [catching] populations that are doing better and protect populations that are doing poorly,” Ms. Nesbitt said.

The study underscores the importance of protecting salmon habitat, she said. “It demonstrates that fine-scale diversity is really important – and you get fine-scale diversity by having a lot of habitat diversity throughout the watershed.”

Bob Chamberlin, vice-president of the Union of British Columbia Indian Chiefs, urged the federal government to take note of the research. “Protection of the broadest genetic diversity of wild salmon stocks will help [government] provide for our future generations,” he said in a statement.

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