Scientists played human voices near Alaska salmon streams and found that bears and eagles fled

Playing recorded human voices near Alaska’s salmon streams causes bears and eagles to run away, disturbing the natural process that carries important marine nutrients deep into nearby forests.A series of experiments by the Pacific Northwest Research Station in the Héen Latinee Experimental Forest near Juneau, Alaska, found that even the sound of human activity changes when and where wildlife searches for food. Because important predators help move nutrients through forest ecosystems, their reaction to human sounds changes where key nutrients like nitrogen are left behind.This US research matches a large Canadian study published in Ecology & Evolution.

Led by Dr Megan Adams from the University of Victoria and the Raincoast Conservation Foundation, the Canadian team studied 226 grizzly bears across 22 watersheds in British Columbia. They found that human activity in river valleys greatly reduces the amount of salmon eaten by bears, even in areas with very little industrial development.Together, the studies show that human activity can change the natural relationships that help keep temperate rainforests healthy.

The sound experiment in Alaska

To understand how human sounds affect wildlife, Philip Manlick, a research ecologist with the Pacific Northwest Research Station, placed motion-activated cameras and speakers along Alaska’s riverbanks. The speakers played recordings of off-highway vehicles, people talking, and natural seabird sounds as a comparison. Researchers also placed salmon nearby to attract predators.The cameras recorded hundreds of visits from bald eagles, ravens, black bears and brown bears.

The results showed that animals were twice as likely to run away from off-highway vehicle sounds compared with natural sounds. They were almost ten times more likely to flee when they heard human voices.At places with human disturbance, predators changed their behavior. They took fewer salmon from streams and moved much of their feeding activity to nighttime to avoid people.

In a healthy, undisturbed forest, bears catch salmon and carry them inland.

When salmon are plentiful, bears eat only the richest parts of the fish, such as the brains, skin and eggs, leaving the rest of the carcasses on the forest floor. These remains, along with bear waste, break down and release nitrogen, which works as a natural fertilizer for trees and plants. By pushing predators away from streams or forcing them to feed at night, human noise changes where and when these important nutrients enter the forest.

The impact on Canadian grizzlies

Further south in British Columbia, Dr Adams and her team studied the long-term effects of human activity on this same relationship. Using chemical clues from grizzly bear hair collected between 1995 and 2014, researchers studied salmon consumption across an area of 88,000 square kilometres.They found that human structures and activity in river valleys, mainly linked to industrial resource development, had a stronger effect on a bear’s diet than the number of salmon available in rivers.

When human activity was present near streams, female grizzly bears reduced their salmon intake by up to 59 percent.Researchers explained that bears avoid these important feeding areas when they feel the risk from human activity is too high. Less food can have serious effects, including smaller litters and fewer grizzlies in the population. This also means fewer bears are available to carry salmon nutrients into the forest.“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.”

Protecting forests while sharing the land

The growth of industrial development since 2020 has placed more pressure on wilderness areas.

Current protections near streams are not fully preventing these disturbances, leading scientists to call for stronger land management.In British Columbia, researchers working with the Kitasoo Xai’xais Stewardship Authority are already using these findings to improve forestry policies.“This is the first work that looks at the connection between land disturbance and the bear-salmon relationship in Kitasoo Xai’xais territory and the Great Bear Rainforest,” researchers said.

“Because bears have major cultural and economic importance for the Kitasoo Xai’xais Nation, this research helps guide a more careful approach to land planning.”

Scientists say that simple protection zones along rivers may not be enough. They recommend larger protected areas and seasonal restrictions, such as closing wilderness roads during the peak autumn salmon spawning season.According to Manlick, the goal is not to stop people from entering forests completely, but to create a balance that allows ecosystems to remain healthy. Planning tourist routes and industrial areas while protecting quiet spaces can give wildlife the room they need to feed naturally. The aim is to protect recreation and local economies while keeping areas where “bears can be bears.”