|March 12, 2020|
Glacial retreat good news and bad news for Pacific salmon species
|The 16,000-year retreat of glaciers in North America left vast and diverse freshwater habitats for Pacific salmon species, but for some watersheds the golden age of salmon is about to end.|
As some glaciers finally melt out of existence, the volume and cooling effect of runoff water will disappear along with them, creating significant challenges for salmon runs in the southern half of B.C., according to a new study from Simon Fraser University.
Species such as sockeye and chinook are particularly "picky" about water temperature as they migrate upriver to spawn, said lead author Kara Pitman, a PhD candidate. Warmer water can lead to exhaustion and death on a journey that can be 50 to 1,000 kilometres long.
Around 85 per cent of watersheds in western North America still have some glacial coverage, but those ice sheets are predicted to lose 80 per cent of their volume by the end of this century.
"The retreat of glaciers has accelerated with climate change. So that becomes a bit more alarming," said Pitman.
The Earth's temperature has been rising generally since about 1850 --- the end of the Little Ice Age and the last glacial maximum --- but warming has recently accelerated.
The extinction of glaciers in the upper Thompson and Fraser River watersheds could lead to higher summer water temperatures that negatively impact adult spawners and young salmon that rear in fresh water, the study says.
"Salmon still exist in parts of the world where there are no glaciers," she said. "They aren't defined by glaciers, but they are enhanced by glaciers."
All the Pacific salmon species evolved during a non-glacial period and survived the last ice age by exploiting whatever small refuges they could find, explained Pitman.
"If given enough time, salmon are well-adapted to cope with the landscape changes associated with glacier retreat," she said. "Once glaciers aren't the main drivers of stream flow, juveniles can find offshoots and eddies to rear in."
Nearly all the glaciers in B.C. have retreated to the point where they are no longer revealing new habitat suitable for salmon, but that isn't the case everywhere.
In places that are still dominated by glaciers, such as south-central Alaska, glacial retreat is likely to open up new salmon habitat in low-lying valleys, rivers and lakes, she said.
The vast majority of salmon return to spawn in the streams where they were born, but pink salmon and chum are more prone to straying and could be first to colonize new habitat.
A post-glacial future, won't necessarily be bad for salmon as a species --- their abundance is at a historical peak in the Pacific Ocean --- but the proportions of pinks and chum compared with sockeye, chinook and coho could be altered.
An earlier study by SFU professor Brendan Connors found that in years when the number of pink salmon returning to the Fraser River dramatically increased, sockeye returns suffered.
American scientists have long noted that strong pink returns had a negative effect on Bristol Bay sockeye and Puget Sound chinook.
In fact, pink returns have been increasing steadily since the 1980s, apparently thriving in climatic conditions that are unfavourable to other species.
Chum are the most widely distributed species of salmon and may be insulated from some of the effects of climate change as they spend less time in fresh water than coho, chinook and sockeye.
Restoring habitat in freshwater systems is the only lever of control we have to help salmon stocks that are suffering as the climate changes, said Pitman.
"We should be looking at ways to restore habitat in ways that make it better for chinook and sockeye, which are more sensitive," she said.