The Effects of Pool Size and Beaver Activity on Distribution and Abundance of Warm-water Fishes in a North Missouri Stream
by Willis D. Hanson and Robert S. Campbell
American Midland Naturalist 1963 69: 136-149
This study of fish on Perche Creek comes to some happy conclusions for beavers. From February 1954 through September 1955, Hanson and Campbell counted fish and measured plankton in the creek and in the ponds created in the creek's watershed by beavers. One section of the creek had four beaver pools and one natural pool. Of course, the beavers pools were larger, averaging 2 feet in depth and 15 feet in width while the natural pool was no more than a foot deep and 5 feet wide. The researchers found that "the beaver pools in the headwater sections supported a standing crop ranging from 44 to 256 pounds per acre. In the natural pools in the same area fishes larger than fingerling size were seldom found. These headwater sections tend to become intermittent in flow in late summer and fall with consequent loss of fishes. By contrast, the pools continue to support many fishes including numerous green sunfish and bullhead of catchable size."
Given my interest in river otters ( see Tracking Otters ) I lick my chops more at this study than your average fisherman. The brown bullhead, in my experience, is a prized food for otters. This study shows how they can abound in a beaver pond, counterintuitively, outnumbering common shiners. Here is what the researchers found in the "Upper Pool" they studied:
|pounds per acre||144.4|
The researchers found that "the standing crop of plankton in the beaver pools... was approximately five times as great as in the stream proper.... Seasonal plankton peaks were always more pronounced in the beaver pools." Relying on other research, they suggest that plankton needs time to mature, 6 to 10 days for it even to appear in water and then 20 days "for a fully formed plankton biota." This is difficult to achieve in moving water. Beaver ponds slow water down. These researchers found the beaver ponds were rich in zooplankton which takes longer to form than phytoplankton. "Zooplankton was especially conspicuous in the beaver pools making up to 40 per cent by number of the total plankton. These zooplankters were primarily copepods and cladocera [water fleas]."
by Bob Arnebeck
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