A landslide blocks a portion of the Chilcotin River in B.C. on Aug. 2. As the river began flowing through the area once again, officials are now watching to see how debris in the river will impact the migration of salmon to their spawning grounds.HO/The Canadian Press
B.C.’s Chilcotin River is flowing once again after breaching a kilometre-long landslide that had barricaded it for five days, unleashing a surge of water laden with silt and timber that poses an uncertain new threat to beleaguered salmon runs currently sniffing out their spawning grounds.
During a Tuesday briefing, provincial officials said they have yet to determine any effects on salmon runs, but are prepared to intervene as soon as the river is safe.
“We’ll assess and do the habitat restoration and the fish passage works that we need to do once we get a better sense of the impacts of the river flows,” said James Mack, assistant deputy minister of land, water and resource stewardship.
The emptying of the temporary 11-kilometre lake that formed behind the landslide dam was slower than worst-case scenario modelling, averting the need for mass evacuations downriver.
About 30 kilometres downstream from the dam, the Chilcotin flows into the Fraser River. Once called the world’s biggest producer of sockeye salmon, the Fraser’s bounty has declined severely in recent years. It reached a record low of 288,000 sockeye returning to the river in 2020 – down from around 30 million a decade earlier.
The federal Department of Fisheries and Oceans had been forecasting a historically low return again this year, and conditions in the lower Fraser had alarmed scientists weeks before the landslide. By mid-July, water temperatures along the lower Fraser had reached 21 C, an important threshold for the fish.
“That’s lethal,” said Scott Hinch, associate dean at the University of British Columbia’s Pacific Salmon Ecology and Conservation Laboratory. “They can cope with that for a period of time, but they can’t migrate at 21 C. They can exist, they can remain motionless, but they can’t migrate.”
He said that temperatures have levelled off since, but remain far higher than historical averages. The debris can’t help, he added, posing physical barriers and possibly disrupting the scents that salmon rely on to navigate a route to their spawning grounds.
The Fraser salmon have already adapted to environmental extremes along the river, which wends its way through dense industrial and residential build-up along the greater Vancouver shoreline and past the farms of the Fraser Valley, before ascending through the whitewater rapids of the Fraser Canyon. The fish quit feeding throughout their river journey and essentially starve as they expend calories up the river.
Unlike most fish, Fraser salmon are semelparous, meaning they die while spawning. Their bodies begin to change and break down while their immune system shuts down, making them susceptible to pathogens. If river conditions are unfavourable, they can wait it out – but not for long.
“They have limited energy to use, so a delay of even two or three days can have a serious impacts,” said Greg Taylor, fisheries adviser for the Watershed Watch Salmon Society. “When you factor in this delay, the temperature, the low water level and all this debris coming down, it’s going to compromise migration success.”
The long-migrating Fraser salmon are uniquely adapted to meet these challenges. Studies have found that they have a streamlined body, efficient swimming motion and a high density of lipids in tissues at the start of migration compared with populations that don’t travel as far to spawn.
“There’s a suite of energy-saving behaviours and physiological and morphological adaptations that these stocks have acquired over the last 8,000 years since the deglaciation,” Dr. Hinch asid.
During a Tuesday press conference, First Nations leaders based in the region around the slide called on governments to show leadership in protecting the salmon, even urging them to shut down commercial and recreational salmon fishing if they have to.
“We are dependent on wild stock salmon,” said Tl’etinqox Chief Joe Alphonse, who also represents five other local nations as tribal chair of the Tsilhqot’in National Government.
He said this year’s Chilko sockeye run is the first full life cycle to return to the region since a 2019 rock-slide along the Fraser north of Lillooet effectively trapped migrating salmon.
“Now they have another big hurdle,” Mr. Alphonse said. “We don’t know how many are going to get through.”
Dr. Hinch is optimistic, saying the Chilko sockeye have the best swimming ability of any of the Fraser sockeye and has a broad thermal tolerance. Studies have found they also have more efficient hearts than other salmon populations.
“There’s a tremendous ability there to continue to survive and get to spawning grounds,” he said. “We just don’t know yet how bad that migratory route is.”
With files from Xiao Xu