Why grizzly bears are quietly abandoning salmon streams near people

Grizzly bears across parts of North America are eating noticeably less salmon in river valleys touched by human activity, according to two new studies examining how noise and development change predator behaviour near salmon streams.

In British Columbia, Dr Megan Adams of the University of Victoria and the Raincoast Conservation Foundation led a team that studied 226 grizzly bears across 22 watersheds. They found that even modest human activity in river valleys sharply reduced how much salmon bears consumed, regardless of how much industrial development was nearby. A separate long-term analysis, using chemical signatures from bear hair collected between 1995 and 2014 across 88,000 square kilometres, showed that human structures affected bear diets more than the actual number of salmon in the rivers. Female grizzlies in disturbed areas reduced their salmon intake by up to 59%.

‘Valley bottoms are very important travel corridors and feeding areas for bears, but these are also places where human disturbance is often concentrated,’ Adams said. ‘We need to consider how our activities, whether changing valley landscapes or reducing salmon numbers in the ocean, can affect the important relationship between bears and salmon.’

A parallel experiment in Alaska’s Héen Latinee Experimental Forest, near Juneau, backed up the pattern. Research ecologist Philip Manlick and colleagues at the Pacific Northwest Research Station set up cameras and speakers along riverbanks, playing sounds of off-highway vehicles, human voices, and natural seabird calls. Bears and bald eagles were nearly 10 times more likely to flee at the sound of human voices than at natural noise, and shifted much of their feeding to nighttime in disturbed areas.

The consequences reach beyond the bears themselves. Fewer salmon carried into the forest by feeding bears means less nitrogen reaching the soil from leftover carcasses and bear waste, a process that normally fertilises trees and plants along the riverbanks. Researchers also warn that reduced feeding can lead to smaller litters and fewer bears surviving over time.

In British Columbia’s Great Bear Rainforest, the Kitasoo Xai’xais Stewardship Authority is already using the findings to shape forestry policy, in what researchers describe as the first work connecting land disturbance directly to the bear-salmon relationship in that territory. Scientists are recommending larger protected zones and seasonal road closures during the autumn salmon run to help restore the balance.

Wikimedia Commons/by Dmitry Azovtsev