Michigan Department of Natural Resources
Fisheries Research Report No.1867, 1979

A Comparison of the Diet and Growth of the Trout From the Upper Au Sable and Upper Manistee Rivers, Michigan


James W. Strogen, Jr.


      Abstract.-Brook trout from the upper Au Sable River and brown trout from the upper Manistee River, the dominant species, were sampled from May through September 1976. Equal numbers of small (3. 0-5. 9 inches) and large (7.0-9. 9 inches) trout were sampled. Diet composition, various growth parameters, and population estimates were determined. These data were then compared to the results obtained by Stauffer ( 1977) on the brown trout populations of the South Branch and lower Au Sable rivers.

The diet of the trout sampled in each river was heavily dependent on one food item. Hexagenia limbata accounted for about 35% of the total consumption in the upper Au Sable brook trout diet, while trichopterans, especially Brachycentrus accounted for about 45% of the total upper Manistee brown trout diet. Fish became an increasingly important food source in both rivers as the trout attained greater lengths.

Some food items were more important at certain times of the year. Hexagenia limbata consumption by all trout on the upper Au Sable occurred almost entirely in June. Trichoptera and other Ephemeroptera consumption remained relatively constant from month to month in the smaller trout diet, but these insects were of varying degrees of importance to the larger trout throughout the summer. On the upper Manistee River, Ephemeroptera supplemented the diet early in the season, while Diptera became more important later in the season. Diptera consumption was highest in the month of June for the larger upper Manistee brown trout.

The upper Au Sable brook trout ate more than the brown trout from the upper Manistee, South Branch, and lower Au Sable in the smaller size range, but had less food in their stomachs than the brown trout at the greater lengths. The upper Manistee and South Branch total stomach volumes were essentially identical for the entire range sampled, yet the South Branch brown trout were found to be longer and heavier than the upper Manistee brown trout at any given age. There was no significant difference found in the lengths and weights of the upper Au Sable brook trout and the upper Manistee brown trout

The upper Au Sable was estimated to contain 276.8 trout per acre and the upper Manistee to contain approximately 1148. 9 trout per acre. Density of trout was not found to be a factor in determining trout weight on these rivers or the South Branch. Density was thought to be a factor in the lower Au Sable River where the number of brown trout in the stream apparently influenced the weight of the brown trout in the stream.