Small streams play critical role in survival of brown bears, study finds

New research is revealing just how important small streams are to the survival of brown bears in Alaska, which are some of the world's biggest bears. 

The new study published in Conservation Letters, the journal of the Society for Conservation Biology, says while small streams are home to only about 20 per cent of the salmon in the Wood River basin watershed in southwest Alaska, they account for half of the salmon brown bears eat. 

"The important result was that it's not all about salmon abundance," said Johnny Armstrong, a professor and ecologist at Oregon State University, noting the diversity of fish and where they spawn also plays a key role.  

The study says fish in streams are easy for bears to catch, and because the water is cold, salmon in streams spawn early. This means fish are plentiful in the first week of July — six weeks earlier than rivers and lakes — making small streams the first places bears fish after they emerge from hibernation.

"Having that diversity of spawn timings across the salmon populations, it's critically important for the bears because it means they can feed on salmon for several months instead of just several weeks," Armstrong said. 

Oregon State Univeristy

He pointed to the brown bear Holly, who was crowned "Queen of Corpulence" when she won Katmai National Park and Preserve's Fat Bear Week contest in October, as an example. The viral internet sensation highlights the massive size brown bears, which include grizzlies, can grow to by gorging on salmon. 

"What allows her to get so fat is that she ... can exploit the diversity in salmon and go to some small, cold stream in July and start pigging out, then she can move to some warmer spawning sites where salmon spawn later," Armstrong said.

He explained they call this "surfing the wave of salmon," something Holly would likely finish around late October, when the last salmon population spawns. 

The findings of the new study come from a mathematical foraging model based on data collected from the University of Washington's Alaska Salmon Program. Researchers altered factors in the simulation, such as the abundance or diversity of salmon, to see how they could affect foraging opportunities for bears.

"It's like a video game almost," Armstrong said. 

Submitted by Johnny Armstrong

Researchers found having more salmon in lakes and rivers over smaller habitats, like streams, meant bears' total consumption of salmon dropped faster.

"Bears can have all the salmon in the world but if there's no diversity in those salmon, if they all spawn at the same time, then bears just aren't going to be able to eat that much because they're just going to get full and have to sit there and watch all these salmon spawn," Armstrong said. 

While the findings of the study are new, the information likely isn't news to Alaskan Indigenous groups. 

Armstrong said as a graduate student, he worked closely with Indigenous people in Bristol Bay, Alaska, where one of the traditional foods is spawning salmon. 

"They were keenly aware of the importance of timing of different food sources," he said. "For them, just because there's lots of salmon in a place doesn't mean it's going to support them and provide a resource for the entire year. They need the resources to be scattered in time, just like bears do." 

Armstrong said the study's findings have implications for conservation decisions, which tend to focus on lakes and rivers with larger fish populations, over smaller habitats. 

"In Alaska, in the future, we're going to have to make some really hard decisions about what we want our watersheds to do for people," he said, adding there can be conflicts between supporting fisheries and wildlife versus resource extraction.

"There is no easy answer because we need these resources."