High fish mortality rates possible after record-breaking B.C. floods

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Several species of fish could suffer high mortality rates as a result of catastrophic flooding in southern B.C. last week, warn some experts and observers.

Pink salmon are at particular risk, said biologist Marvin Roseneau, as they spawn at the main stems of the now-swollen Fraser and Vedder/Chilliwack rivers.

Most of their eggs were laid in the fall and hidden in the gravel to incubate until spring hatching.

“The floods would have scoured the gravel out almost certainly,” he explained.

“Once the embryos are washed out of the gravel, predators eat them, they get smothered — it’s over for them.”

Renee Coghill of Hope Search and Rescue has already found 100 dead fish on dry land off the Coquihalla Highway in the aftermath of catastrophic floods in southern B.C. on Nov. 14 and 15, 2021. Courtesy: Renee Coghill

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Roseneau, who teaches fish ecology and management at the BCIT, said Chinook, coho and steelhead salmon may be impacted to a lesser extent as well.

“We will quantify it a year and a half from now when the adults come back. We’ll see whether or not those runs have absolutely collapsed as a function of this event.”

He estimated salmon mortality rates in the low millions for juveniles and the billions for embryos, while clarifying that not all fish and eggs would have survived anyway, due to other natural causes.

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Ron Houniet, a co-director of the Fraser Valley Angling Guides Association, added chum salmon to the list of species whose eggs may have been “flushed out” by the fast-moving flood water.

“It will be total devastation for the future of those salmon stocks,” he said in an interview.

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“We have other issues here with losing fish over commercial fishing, climate change, and with this happening here it will be just another impact.”

He, too, estimated billions of eggs were scoured by the water and have “no chance.”

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In addition to wiping gravel away, Simon Fraser University earth sciences professor Brent Ward said floods can deposit dangerous amounts of gravel that can impact fish eggs.

“The fish need a certain grain size to make the reds to lay their eggs in right, and if there’s so much sediment that the water becomes very turbid, then you know it can actually seriously affect the fish and kill them,” he explained.

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B.C. Agriculture Minister Lana Popham said land fish farms have been impacted by the disaster too, but the damage hasn’t been quantified.

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“The losses haven’t been entered yet, but we believe that all fish are presumed lost,” she said in a Thursday news conference.

“These are tilapia and barramundi fish. We also have, unfortunately, a commercial Chinook hatchery on Vancouver Island that also has been damaged due to the flood.”

The floods of Nov. 14 and 15 have had devastating impacts across southern B.C., killing five people, displacing thousands, and destroying critical infrastructure. More than 20,000 farm animals have also been lost.

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