Effects of Beaver on Trout in Sagehen Creek, California by Richard Gard

This July 1961 article in the Journal of Wildlife Management (volume 25 number 3) assesses the impact beavers had on trout in a high mountain creek in the northern Sierra Nevada. Beavers were introduced to the stream in 1945 and had not been trapped. Gard studied one of four beaver colonies on the creek from the winter of 1954 to the summer of 1957. The colony maintained "a complex system of 14 dams." Gard sought to see what the dams did to the creek and the bottom fauna in it on which the trout foraged, and if the dams effected trout spawning migrations.

Written before abstracts were used to introduce an article, this one somewhat keeps you in suspense. Gard begins with the obvious: beaver dams slowed the stream to "no measureable velocity," and that "permitted the deposition of silt on the pond bottoms," and that in turn influenced "kinds and standing crops of bottom organisms." Beavers also influenced vegetation above the stream, and here Gard quickly notes a benefit of beavers: "downed aspens formed an interlacing tangle over some ponds and provided ideal trout cover." Winter played out differently in the pond and stream areas. Although the ponds were iced and snowed over, anchor ice did not form and thus there was not a complete freeze up. Anchor ice did form in the streams and some nights the streams "froze almost solid." And the breaking up of anchor ice "greatly disturbed the stream substrate."

Water temperatures in the beaver ponds were higher than in the stream. Gard noted that warmer temperatures "may kill trout directly or provide a habit more suitable for other fish...." Gard also measured water chemistry, especially changes in oxygen content of the water and found that in the beaver ponds "all water quality data determined were well within the lethal limits for trout." He concludes that save for a lack of "spawning gravels," the ponds provided superior conditions for trout in comparison to the stream.

Gard then reports on the foods available for trout in the ponds and stream. The stream had twice as many organisms than the pond, 81 to 42, which didn't surprise Gard because the ponds were a more homogeneous environment. However, "the average total number and weights of organisms" were greater in the beaver ponds. He provides a list of all the organisms and the numbers that were found at four "stations" in the stream and two in the beaver pond. Of course the organisms are all listed by scientific name, but then again some of these critters have no other name!

In the ponds "immature midges" (Chironomidae, Diptera) were the most abundant organism and were most abundant in the trout stomachs. The midge larvae live in the silt at the bottom and the trout "were observed rooting in the bottom silt and eating midge larvae as they floated away." The clam Pisidium was next abundance but only three were eaten by the trout. Gard found that brown and brook trout did well feeding off the pond bottoms. Rainbow trout did better feeding off the fauna in the stream bottom.

Then Gard turns to fish populations. He drained a narrow 221 foot long beaver pond and counted every fish in it, and then diverted an equivalent sized section of the stream into pools, drained that and counted the fish. He found that fish populations were about equal, but that the trout were bigger in the beaver pond. However, in a few years the average size of the pond trout declined but always remained higher than the stream trout.

A flood in December 1955 washed out one of the dams and afforded an opportunity to study what effect pond removal had on trout. Gard found a seven fold reduction in the weight of bottom organisms. As for trout "a pre-flood population of 103 trout totaling 8.3 lb. decreased drastically to only 19 trout totaling 2.7 lb. after the washout changed the pond to a stream." Also there were less brown trout and more rainbow trout.

Then he turns to the suggestion by many that beaver dams block trout migration. 443 trout were tagged, and at least 37 of them "were known to cross one or more dams. No doubt many more trout crossed dams, but were not detected." Gard speculates on how beavers got over dams. Since they were more successful during high water in the spring, he assumes they simply used the over flow. However, wiggling through the sticks in the dam is also a possibility.

Gard concluded that "beavers are of substantial benefit to trout in Sagehen Creek." However since there is not a small literature reaching the opposite conclusion in other areas of the country, Gard suggests that where trout are more scarce in the East and Midwest, beavers might be detrimental. I'll review that literature in time, but for now suggest that another phenomena might be at work. When fish populations don't suit the needs of sports fishermen, the usual suspects are rounded up from beavers to birds to explain why. It's all an effort to let people off the hook.

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