Beavers Eat Fish. Wait. What?

Beavers do not eat fish. They are herbivores specializing in the bark of a number of different tree species. Period. End of story. Nothing more to see here. Move along.

Except that it isn’t. On my Facebook page last week, my friend Jim told me about watching a beaver in Montana Creek, a popular fishing stream in the Matanuska-Susitna River Valley north of Anchorage. The big aquatic rodent was pulling the carcasses of recently filleted salmon from the bottom of the river, dragging them to the shore, and proceeding to bite the back of their heads and consume the brain. (Lovely mental image, no?)

Had I gotten this question from someone else, I might have been tempted to blow it off, but Jim is a skilled naturalist

Had I gotten this question from someone else, I might have been tempted to blow it off, but Jim is a skilled naturalist who I’ve been in the field with a few times. He knows his stuff. So I looked into it.

First, his observation kind of made sense. These weren’t dead, spawned out salmon, but fresh fish, recently discarded by fishermen. Fresh salmon brains are mostly fat. Despite being an odd source of nutrition for an herbivore, a beaver could get a lot of useful calories from that resource. But, as Jim asked, “How did learn to do that?” I was curious too.

Google searches and academic paper search engines initially led nowhere. I kept adjusting search terms and finally came up with one paper from the journal “The Canadian Field Naturalist” from 2005. The paper is brief, but documents observations of beavers diving for discarded salmon carcasses, dragging them to shore and consuming them. Where? None other than Montana Creek in the Mat-Su Valley. The same place Jim made his observation earlier this summer.

The authors of the paper hypothesize that the behavior could be widespread throughout Alaska since fresh salmon, discarded by bears or humans, are a widely available. But, the authors are unable to provide any documentation of the behavior elsewhere.

I find this fascinating. For at least 10 years, Beavers have been feeding on discarded salmon in this particular creek. I returned to the internet for a little more searching, and found a YouTube video (posted below) of a beaver eating a dead salmon in Lake Creek, Alaska. The comment stream below the video, (always interesting) has a number of people noting similar observations from elsewhere in North America. If these comments are accurate, the phenomenon is wide-spread, but almost undocumented in the scientific literature.

Is it just an opportunistic resource exploited only occasionally? Or do fish, actually make up a small but important part of a beaver’s diet.

Working on the assumption that this is, in fact, a wide-spread, if rare, phenomenon how important is it to the beavers? Is it just an opportunistic resource exploited only occasionally? Or do fish, actually make up a small but important part of a beaver’s diet. I don’t know the answer to that question, but I’d love to find out.

So how did beavers learn to do this? I have no idea. Once one beaver figured it out, it isn’t difficult to imagine that others in the neighborhood would catch on. But the initial discovery is a mystery. Perhaps a beaver was moving a dead fish away from its dam, or lodge and took a liking to the flavor? Stranger things have happened.

Have you ever seen anything like this or know more about this kind of behavior in other animals? If so, leave a comment or send me a message. I’d love to hear about it.

Thanks to Jim Gilbert for asking the question. And I invite you all to send inquiries that have been burning in your minds. You can leave them here in the comments, via the contact page, or on my Facebook or Twitter Feeds.

Here is the citation for the paper noted above:

Gleason, J. S., R. A. Hoffman and J. M. Wendland. 2005. Beavers, Castor canadensis, feeding on salmon carcasses: Opportunistic use of a seasonally superabundant food source. The Canadian Field Naturalist 119: 591-593.