Low salmon returns make for hormonal bears

News Science

A recent study has uncovered that coastal grizzly bears are more prone to hostile behaviour when salmon numbers are low. The study, conducted by the University of Victoria, the University of Calgary, and the Raincoast Conservation Foundation, traced stress hormones in bears’ fur during periods when the expected salmon run was lower than average. Less salmon means greater competition for food, and the higher competition means grizzly bears could be a greater risk.

The main hormone that experienced change during periods of greater competition was testosterone. Testosterone is mainly thought of as a reproductive hormone; however, according to the study, it also “facilitates behavioural and physical traits necessary to win social conflicts in fitness-enhancing situations.”

Dr. Heather Bryan, one of the researchers, said in an e-mail, “Testosterone, which we often think of as a male sex hormone, is also affected by the social competitive environment,” and that the study found it higher in both male and female coastal bears, as compared to other bears. Dr. Bryan said this “suggests that coastal bears have to compete more heavily for access to important resources such as salmon.”

Another hormone tested in the coastal bears was cortisol. Cortisol is a hormone that helps with long-term stress, recovery from stress, and coping with change. Although salmon has high amounts of glucocorticoids, a class of steroid hormones to which cortisol belongs, coastal bears did not have a higher amount of cortisol in their fur samples.

The fur samples were collected on a 5000-square-kilometre grid consisting of 73 area cells. Each cell was seven square kilometres. In each cell, researchers erected a barbed wire enclosure with a 25-metre perimeter. Inside the enclosure, the researchers placed some fish oil. This is called non-reward bait. Although bears are hopelessly attracted to the scent, when they arrive and there is nothing to eat or defend. Within a few short moments the bears leave the enclosure. When bears pass under the barbed wire, small tufts of hair are pulled loose, and from those samples the researchers were able to find the information they were looking for.

People are reminded time and time again that bears are not the kind of animal that would ever attack a human for sport or snack, but a starved bear can be dangerous. With loss of habitat and food, it could mean life or death for the bear. In November, two residents of Churchill, Man. were attacked by a starved polar bear on the way home from a party. Another man in Quebec barely survived a bear attack while on a wilderness excursion sometime in August. These occurrences aren’t common, but bears do attack out of a sense of necessity.

Salmon numbers are known to fluctuate. Pink, coho, and chinook salmon were plentiful this year along B.C.’s coasts, but this isn’t always the case. As Dr. Bryan points out, there “has been an overall downward trend in the last 60 years with several dramatically low returns in the last decade on the central coast.”  And while interior bears live off of a diet of mainly plants, the study points out, “Salmon allows bears to meet their energetic requirements more efficiently than a diet of plants alone.”

When asked about what could be done to increase the numbers of salmon available to the coastal bears, Dr. Bryan said, “Setting appropriate commercial fisheries so that salmon are shared among people and wildlife is important, but one of the challenges is that the exact number varies over space and time. Currently fisheries quotas are determined based only on the needs of people, and do not consider the needs of wildlife.” There is a national strategy called The Wild Salmon Policy that recognizes there are other species besides humans that have needs for salmon, but Dr. Bryan says, “There isn’t a lot of information on exactly how much salmon other species need, which makes it difficult to implement the strategy.”