Salmon, Bears, and the Forest

Text by Holly Reisner, Executive Director of the North Shore Black Bear Society. Photos by Roland Smith. Editor’s Note: This is a longer version of an article that appeared in the April 2025 Blueridge Bulletin

Since about the year 2000, there have been many studies done on the relationship between salmon, bears, and the forest. These were started by Tom Reimchen of the University of Victoria and continued by many others, including John Reynolds of Simon Fraser University.

We know that bears eat salmon, both grizzly bears and black bears. This happens on Haida Gwaii, throughout the Great Bear Rainforest, and on the North Shore too. Every fall, for example, bears are active in the Maplewood area of North Vancouver when salmon are spawning. This is when many populations of salmon return to our streams to spawn, and it coincides with bears undergoing a hormonal change called hyperphagia. Hyperphagia causes bears to hyper-focus on eating in order to put on enough body fat to get them through the winter without food and for females to gestate, give birth to, and nurse their cubs. During this period, black bears will be seeking up to 20,000 calories per day and can be active 20 hours per day to get them. The lucky bears live near salmon-bearing streams!

Some of the less lucky ones find these calories in our garbage bins.

What Reimchen discovered is that salmon providing nutrients to bears isn’t simply a one-way relationship. Salmon get something out of it, too, as does the forest and the entire ecosystem.

The studies have focused on a heavy isotope of nitrogen, N15. As salmon grow and feed in the ocean, they accumulate this form of nitrogen. As they return to their natal streams to spawn, they convey N15 from the ocean into the riparian environments. The salmon die after spawning, and their carcasses mostly remain within about 50 metres of the stream banks.

Nitrogen is often a limiting factor in our temperate rainforests, and so the effects of this nitrogen fertilization can be measured in the trees and plants that surround the salmon-bearing streams. Interestingly, there is a larger effect seen on certain plants such as salmonberry (well-named), false lily of the valley, and stink currants, which need more nutrients than others such as salal and red huckleberry.

Tom Reimchen studied the growth rings of Sitka Spruce by taking core ring samples of large trees close to salmon-bearing streams, as well as control trees further away, where salmon carcasses were not found. He analyzed the width of the growth rings and the amount of N15 in each year’s growth. Historical data of large salmon pulses in the late 1940s and early 1960s correlated with wider growth rings and more N15 in the trees. He concluded that up to 50 percent of the trees’ nitrogen comes from salmon and that the trees adjacent to salmon streams grew three times faster than the control trees, which showed low to trace levels of N15. The trees in the salmon carcass areas, although they were the same age, had larger circumferences and higher levels of N15. The highest levels of N15 were seen in trees in the major bear-foraging areas.

It turns out that, even though bears have enormous calorie requirements during the fall, they can also afford to be very picky when there is a lot of food available, i.e., during high salmon run years. They prefer larger fish, they prefer female fish, and they prefer the fatty parts of the fish, which are the brains, the roe, the dorsal muscles, and the skin.

Some bears have been observed setting themselves up in their favourite fishing spots below spawning grounds. This way, they have access to females that still have roe. They will sometimes catch fish, examine them, and throw them back if they aren’t large enough or don’t come complete with roe. However, Reimchen found that most female salmon consumed by bears had already spawned out, so bears didn’t have a significant impact on salmon reproduction.

When bears do decide to keep a fish, they often don’t eat it there in the stream but carry it off into the forest for a picnic. Reimchen studied tagged sockeye and found that up to 50% of the tagged fish caught were taken away from the stream for consumption. Studying in Bag Harbour on Haida Gwaii, he found that bears took 80% of the salmon run into the forest. In 1993, he found that 8 black bears took 3100 salmon (10,700 kg) into the riparian zone and only ate about half of each carcass.

One of the reasons bears do this is that since salmon spawn in certain specific areas and at certain specific times, bears are forced to congregate to fish. Taking their catch into the forest allows them to avoid more dominant bears and scavengers and keep more of their catch for themselves.

Interestingly, this bear/salmon effect is also seen much further away from the streams, up to 500 metres away. Bears are not carrying salmon this far, but this does correlate with bear trails and therefore urine and feces. A study by Gende in Alaska with grizzly bears estimated that bear urine deposits together with salmon carcasses made up 10-25% of the nitrogen needed by the forest.

But how do the salmon benefit from this? Salmon need cool, shady streams for fry to hide in and slow backwaters to spawn and to prevent fry from being washed downstream. The trees provide this environment by growing near the streams, casting shade, and dropping large, woody debris into the water. Tree root systems help prevent erosion. Overhanging tree branches drop insects into the streams where coho fry are feeding.

So, while individual salmon lose their lives, either before or after having the chance to spawn, the bears help to ensure that the cool, shady stream the next generation of salmon needs will be there. Not to mention the myriad other animals and plants that benefit from these nutrients.

Sources: