Are bears picky eaters?
The motivation – Bears take advantage of spawning salmon as a significant source of food. Not only are their appetites satisfied, but their feast also benefits the local ecosystem. Bears transfer and discard partially-consumed salmon into the forest where the marine-derived nutrients of the carcasses become available to terrestrial organisms ranging from animal scavengers to plants to breeding birds.
As far as the bear is concerned, the larger the salmon the better, and the most energy-rich parts of the catch – the eggs and the brain – are the ultimate prize. This predilection for large fish and selective consumption could influence the salmon population in interesting ways. For example, a consistent preference for larger fish could lead to the evolution of smaller body sizes of salmon. Given the potential consequences to salmon of these bear hunting and eating practices, researchers at Simon Fraser University (SFU) embarked on a two-part study to identify factors that determine the extent of size-biased hunting and selective consumption of spawning salmon by bears on the Central Coast of British Columbia. Together, these two studies show how bears can influence salmon body size and nutrient transfer to aquatic and terrestrial habitats.
The discovery – Andersson and Reynolds hypothesized that characteristics that make a stream more complex can act as obstacles for predators and/or decrease visibility of prey and, therefore, decrease size-biased predation. To test their idea, they examined the influence of stream complexity – stream features such as the volume of large wood in the water and percent of the streambank that is undercut – on size-biased hunting behaviour of bears. Then, considering selective feeding after a fish is captured, they predicted that bears would consume the majority of each catch in streams where it was more difficult to capture prey. This was tested by examining how stream complexity and hunting difficulty may influence the selective consumption behaviour of the bears and, consequently, how much salmon carcass is left to the surrounding environment.
True to their prediction that size-selective hunting would be easier in less complex streams, they discovered that bears caught larger than average salmon at streams that contained less wood and lower percentages of undercut banks. Because this scenario represented a size-selective force against large salmon body size, they examined the size of the fish in the streams versus the magnitude of the size-biased predation. Indeed, streams that had the more size-biased predation had smaller fish.
The second study did not match the original prediction: when bears caught fish in more complex streams, they were more selective for energy-rich parts and more likely to abandon the majority of the catch and resume fishing. Their study showed that more fish were caught by bears in the more complex streams. As a result of these actions, significantly more salmon biomass is available to terrestrial organisms in the vicinity of streams with higher complexity.
In combination, the results of their research led Andersson and Reynolds to postulate that streams with more complexity may not increase the difficulty of catching fish, but rather, increase the difficulty of being selective towards the highly-prized larger fish. That is, streams with higher complexity may lower the visibility of the fish, making it more difficult for the predator to target prized individuals; nonetheless, the bears still catch salmon with relative ease. Under these conditions, salmon hunting would be a crapshoot in terms of the quality of the catch, with the more disappointing results abandoned and ultimately more catches made. In contrast, bears catch the prized larger individuals more often than not at lower complexity streams; and they consume more of each prized catch and therefore need to capture fewer fish.
Its significance – Historically, rivers have been cleared of wood and other complexity with the goal of increasing salmon spawning habitat. For several reasons, we now know that this practice was a mistake; expensive stream remediation projects are underway all along the coast to return complexity to streams. These studies demonstrate an added benefit provided by stream complexity, and they inform management decisions in these remediation projects. Namely, the selective foraging behaviour of bears causes stream complexity to indirectly benefit the surrounding environment through salmon nutrient transfer, and may even result in the evolution of larger fish – often prized by humans as well.
Read the papers –
“Effects of habitat features on size-biased predation on salmon by bears” by Andersson, LC and Reynolds, JD. Oecologia 184:101–114 (2017). DOI: 10.1007/s00442-017-3845-0
“Habitat features mediate selective consumption of salmon by bears” by Andersson, LC and Reynolds, JD. Canadian Journal of Fisheries and Aquatic Sciences (2017). DOI: 10.1139/cjfas-2017-0055
Website article compiled by Jacqueline Watson with Theresa Kitos