Fisheries Research Report No. 1716, 1966

Michigan Department of Conservation
Research and Development Report No. 58
Institute for Fisheries Research Report No. 1716, 1966
Merganser Predation on Brook Trout in Lakes

William C. Latta

      Introduction.-Causes of natural mortality of trout stocked in lakes have not been definitely determined (Johnson and Hasler, 1954; Alexander and Shetter, 1961; Eipper, 1961; Hatch and Webster, 1961; Latta, 1963). The American Merganser (Mergus merganser americanus) becomes a likely suspect as a major source of mortality in lakes following the studies of Salyer and Lagler, 1940; White, 1957; and Elson, 1962, on merganser predation on salmonids in streams and, particularly, when mergansers are commonly seen on the lakes (Johnson and Hasler, 1954; Hatch and Webster, 1961).
       The objective of this study was to measure the effects of predation by American Mergansers on population structure and angling success for brook trout (Salvelinus fontinalis) in lakes.
       Four lakes at the Pigeon River Trout Research Station, Vanderbilt, Michigan, were used in the experiment. These lakes are considered by geologists to be limestone sinks; i.e., they were formed through the solution of underlying limestone by ground water, accompanied by a settling of the surface layer of sand and gravel which produced a cone-shaped pothole. The water surface is 40 to 60 feet below the surrounding terrain. Tanner (1960) gave a physical and chemical description of each lake. In the early 1950’s a toxicant was used to remove all fish from the lakes and since then trout have been planted annually. Trout do not spawn successfully in these lakes. In 1957-63 the lakes received about 100 brook trout per acre each November. North Twin Lake was stocked with 550 trout, South Twin with 430, West Lost with 400, and Lost with 460. Brook trout 5 to 6 inches long (average total length about 5.5 inches) were selected for each planting. Each year class was identified by a distinctive fin clip.
       Regulations for trout fishing in the lakes were: minimum size, 7.0 inches; creel limit, 5 fish; no restriction as to lure or bait, except that minnows could not be used. The fishing season extended from the last Saturday in April through the second Sunday in September. A permit-type creel census required anglers to report their catch after each trip to each lake, which guaranteed a nearly complete record of the harvest.