Michigan Department of Natural Resources
Institute for Fisheries Research Report No.1754, 1969
Results of Predator Reduction on Brook Trout and Brown Trout in 4.2 Miles of the North Branch of The Au Sable River
David S. Shetter and Gaylord R. Alexander
Abstract.-During the autumns of 1964, 1965 and 1966, brown trout 12.025.1 inches in length were removed from 4.2 miles of the North Branch of the Au Sable River (Dam 2-County Line, and American mergansers were harassed in the same area each winter as time permitted. The objective was to decrease losses among the smaller salmonids which would otherwise be eaten by the large trout and mergansers. Secondarily the objective was to provide angling opportunities over a numerically larger population of trout. Effects of control efforts were assayed by comparison of 1965-1967 angling results and population estimates with similar data accumulated on the test section and two other stream sections of the North Branch during 1961-1964 when predator manipulation was not carried on. In 1964, 561 large brown trout (12.0-25.9 inches) were removed; the estimated population of large brown trout was 848, thus 66% of the estimated total population was removed. In 1965 and 1966, the actual removals were 346 of an estimated 546 and 104 of an estimated 261, or 63 and 40% removals respectively. The only major change noted was an increase during 1965, 1966, and 1967 in numbers of brook trout larger than 9 inches in the population estimates. However, this population increase did not affect the anglers' catches of brook trout during 1965-1967. No significant changes in the catch or population of brown trout could be demonstrated. Average lengths of the age groups of both brook trout and brown trout, before and after predator manipulation, were investigated. No significant differences which might be ascribed to predator manipulation were found. It was concluded that predator reduction would have to be conducted at a considerably higher level of intensity to induce major changes in trout populations and the subsequent angler’s catch.