Do they help or hurt TROUT STREAMS?


February 01, 2014

Many trout anglers have heard how good trout fishing is behind a beaver dam. While it is true that trout in a small stream can grow larger in a beaver impoundment, too many beaver dams can destroy a trout steam’s fishery. Beaver populations are at extremely higher levels than they were in pre-logging days. Original surveys indicated that many watersheds were almost entirely climax tree species which cannot support large numbers of beaver colonies.

Aggressive logging historically and again in recent years, converted many stream areas to a large forest component of aspen. Aspen is ideal food for beaver and this resulted in significant increases in the state’s beaver population. Historically, trapping pressure was a major influence on beaver populations. In recent years, the number of outdoorsmen trapping beaver has dropped off dramatically. This decline in trapping pressure on beaver has also resulted in the increase of beaver populations on many trout streams.

There has been extensive study of the impacts of excessive beaver dams on trout streams since the 1940s. Wisconsin DNR found that a beaver dam density of more than one per mile on a low gradient stream is problematic for trout habitat and trout populations. We initiated trout stream habitat aerial studies in 1992 in the South West UP and found the average beaver dam density on our best trout tributaries was over 10 per mile. A USDA study had earlier found an average increase of small impoundments (predominantly beaver impoundments) of over 3,000 acres per year between 1966 to 1980.

Extensive trout habitat studies show that beaver dams increase water temperature. This happens as shade trees are cut or die out from high water levels. The drowning out of conifer tree species like cedar along trout streams has a long-term negative affect on the stream’s riparian zone and deer thermal cover. The siltation of the bottom increases solar warming of impounded water. A summary study by Michigan DNR’s Hunt Creek Trout Researchers of over 150 Michigan trout streams revealed significantly lower trout population densities in streams where average July water temperatures exceeded 68 F. Many trout in trout mainstreams rely on migration to these cold trout tributaries as refuge from hot mainstream water temperatures. During these hot spells trout must move or die and the beaver dams block escape to cold water refuge.

Most brook trout migrate to headwater spring water areas or tributaries to spawn. Brook trout require ground water (springs) percolating up through clean gravel to spawn effectively. Beaver dams can block off the headwater and tributary spawning areas for brook trout. Siltation behind beaver dams also covers spawning and trout food production gravel areas. Decaying organics (pond vegetation and sediment) combine with increased water temperature to drive oxygen levels in the aging beaver impoundments below what trout can tolerate.

The reason why some trout anglers can catch a few larger trout in a beaver impoundment than in a small trout tributaries is that the reduced number of brook trout have access to larger numbers of minnows and suckers as food. Once trout become large enough to feed on these forage fish they can grow quickly. The down side is brook trout do not compete well with a high diversity of other fish species and the trout numbers dramatically decline over time.

A beaver impoundment follows the same well documented pattern as a man-made reservoir. When they are new they are most productive however, as they silt in and age their productivity declines. Streams in Upper Michigan are predominantly low gradient streams and old beaver dams on small low gradient streams are less likely to “blow out.” This results in many streams with extensive area of the stream left with silted ponds.

Faced with heavily degraded trout habitat and declining trout stream fisheries in the southwest UP, it was clear that a major effort was needed to restore trout habitat. At about the same time Wisconsin DNR launched an extensive and well-funded beaver dam control program Michigan DNR had not provided similar funding. The two outdoor related fund raising organizations of Wildlife Unlimited of Iron and Dickinson Counties both entered a partnership with Michigan DNR Fisheries in Crystal Falls. Since the budget was limited, the fisheries biologist had to limit the program to the most critical trout tributaries of the Blue Ribbon trout mainstreams of Iron and Dickinson Counties. This resulted in extensive trout habitat work on 18 tributaries out of about 190 trout steams in the two counties.

The first step was an aerial survey of the targeted trout streams which added up to over 250 miles of trout stream. Detailed maps were produced using GPS technology to determine the number and location of active beaver colonies and beaver dams. The Wildlife Unlimited groups paid contracted beaver trappers a subsidy of $15 per beaver for animals taken from the targeted streams. The maps aided trappers since many of these streams’ sections are very remote and have very poor road access.

Once the beaver were trapped the dams could be removed. Removing beaver dams without eliminating the beaver accomplishes nothing as the active dams are often rebuilt the next night. In this program the beaver dams were removed by public works crews from the local minimum security Michigan Department of Corrections work camp. The GPS location maps of beaver dams were crucial to work crew efficiency finding remote dam locations. We found that if beaver trapping were maintained on a stream, the number of active dams would typically drop after two to three successive years of sustained effort.

Over the 16 year duration of the project 3,094 beaver dams were removed in Iron County. In Dickinson county trout tributaries 2,031 beaver dams were removed. On a more specific basis using Cooks Run in West Iron County as an example; initially we removed about 99 beaver dams on this valuable 12 mile long trout stream. As the program continued we eventually reduced to a removal of six to 10 beaver dams per year at Cooks Run. During that time we conducted trout population surveys where we saw a marked increase in trout population. Trout researchers were surprised to find that Cooks Run had one of densest trout populations in the State of Michigan including the best of the “holy water” in the Grayling area. Trout population surveys confirmed significant trout population improvement on several other trout tributaries in Iron and Dickinson Counties. Wisconsin DNR Fisheries studies found it took the cumulative effort on trout mainstreams tributaries over a number of years to improve the mainstreams. Our evaluations indicated the positive trout population response in the treated tributaries was relatively rapid.

Although the Wildlife Unlimited /DNR Fisheries Crystal Falls trout habitat/restoration program was a big success on a very limited budget (about $5,000 per county), a more comprehensive approach is the long-term solution. This program was the medical equivalent to providing CPR and assisted breathing to the patient. In the long run adequate buffer strips (at least 300 feet on each side of the stream) on high quality trout tributaries is the answer. In addition, fisheries biologists from the DNR and US Forest Service need to work with foresters to convert stream riparian corridors to the long lived conifers that were so prevalent along good steams in pre-logging times. Maintaining healthy trout stream habitat and steam corridors (riparian zones) will lead to stronger trout populations and maintenance of quality trout fisheries.