Beavers excel at making dams and that can lead to flooding of roads, property or trails, such as the ones near the Mendenhall Glacier Visitor Center.
Anyone who has spent time fishing and exploring the streams around Southeast Alaska has witnessed first-hand the ability of beavers to alter a stream. Dams built by beavers and subsequently, the ponds created by them, have claimed more than one dry boot and have angered property owners nationwide.
Historically, beaver dams were perceived as barriers to salmon migration and were frequently removed. Headlines, as recently as the 1990s, shouted out the message that beaver dams had to be removed, by whatever means possible.
But the story is a little more complex. A study led by Mason Bryant at the Forestry Sciences Lab suggests that beaver dams may in fact provide access to essential overwintering habitat for juvenile salmon during fall floods.
To test this theory, juvenile coho salmon populations in a beaver pond, slough and a stream complex within the Kadashan watershed on Chichagof Island were intensively studied for more than two years. Estimates on the total number, size and age of salmon were measured in early spring, mid-summer and in fall before high water flows.
During the fall sample, fish were marked to identify the location of capture: stream, slough or pond. The following spring, fish were recovered in each location to determine movement. Preliminary results show little or no differences during the summer as equal numbers of fish were found in the pond, slough and stream. Differences appeared during the fall sample as less than 100 juvenile salmon were found in the stream section, and nearly 3,000 juvenile salmon were found in the slough and beaver pond.
In all, more than 150 fish marked in the stream section during the fall were recaptured in the pond and slough in the spring. At the same time, less than five fish marked in the pond were recaptured in the stream. The movement pattern, from stream to pond, suggests fish movement is closely linked to fall floods which temporarily connect the main stream to off-channel habitat such as beaver ponds and sloughs.
Results from the Kadashan study suggest that removing beaver dams to allow upstream access is, in most cases, unnecessary. Adult coho easily move upstream of most dams, and ponds provide excellent rearing habitat for young salmon. Beaver ponds not only serve as significant winter habitat, but also provide productive habitat for juvenile salmon throughout the year. In addition, seasonal floods provide an important link to this productive habitat, and maintaining these links should be an important consideration when the "nuisance" beavers begin to flood the neighborhood.
In the future, understanding the role of beaver ponds in aquatic systems may be crucial for forest planning as beavers can be a used as a natural agent in successful salmon restoration programs throughout the beavers' range in the Pacific Northwest.
Mark Lukey is a fisheries technician at the Forestry Sciences Lab. He spends summers working on research projects and winters as a snowboard instructor. John Hudson of Friends of Berners Bay will present images and human and natural history of the bay when the Juneau Audubon Society meets 7:30 p.m. Thursday, Sept. 11, at Dzantik'i Heeni Middle School.