Michigan Department of Conservation
Research and Development Report No. 67
Institute for Fisheries Research Report No. 1721, 1966
Progress Report on Brown Trout Removal From 4.2 Miles of the North Branch of the Au Sable River, 1964 and 1965
David S. Shetter and Gaylord R. Alexander
In studies by Beach (1937), Elson (1962), White (1937, 1957), Leonard and Shetter (1937), Salyer and Lagler (1940), and Alexander and Shetter (unpublished) it was well established that the winter diet of American mergansers (Mergus merganser) and brown trout (Salmo trutta) larger than 12.0 inches consisted mainly of smaller salmonids when these predators occupied trout waters. Furthermore, population studies which we have conducted on the North Branch in the fall after the close of the trout season and again the following spring show a large over-winter loss, most of which could be accounted for by these two trout predators. It was hypothesized that there would be a substantial increase in the anglers’ creel the following season if the between-season losses of trout were eliminated or reduced by control of predators.
In an attempt to test this general hypothesis, we have reviewed the creel census and population data for 1962-1965 for the 4.2 miles of the North Branch of the Au Sable River between Dam 2 and the County Line in Otsego County.
Estimates of angling pressure and the annual catch are obtained by counting and interviewing anglers under a schedule of stratified random sampling. Population figures are averages of estimates for three sub-sections of the experimental water. Fish are captured through the use of d-c electrofishing gear and Petersen-type population estimates are made.
Large brown trout were removed with electrofishing gear in the fall of both 1964 and 1965.
The numbers of American mergansers on the North Branch have been determined by weekly counts from an airplane from about December 15 to April 15 each winter since December, 1961. Pilot Peter VanValin made the counts which numbered 13 to 21 per winter (Table 1). Mergansers were nearly absent during the winter of 1964-65 (due to natural causes not apparent to us).
This paper reports the results of predator control measures carried on to date and some of the tentative conclusions reached after one complete year of reduction in numbers of the two main predators on the North Branch trout populations.