Michigan Department of Natural Resources
Fisheries Research Report No.1828, 1975

Growth, Survival, Production and Diet of Hatchery-Reared Rainbow and Brook Trout Stocked in East

Fish Lake, Under Different Stock Densities, Cropping Regimes, and Competition Levels


Gaylord R. Alexander


      Abstract.-Hatchery-reared brook and rainbow trout were stocked in East Fish Lake in mid-October during the falls of 1958-1968 and 1972. No trout were stocked during 1969-1971. All plants of trout were size-selected for an average length of 8.9 inches (range 8.5-9.5). The various plants were fin-clipped for identification. The trout population size, survival rate, growth rate, production, angler cropping, and diet were monitored. The standing crop of invertebrate benthos was also monitored.

Rainbow trout survival averaged 86%, compared to 41% for brook trout, from mid-October planting until mid-April just prior to the opening of the trout fishing season. Survival was low for both rainbow trout and brook trout from mid-April to mid-October, mainly because of successful angler exploitation or experimental cropping by research personnel. Anglers were responsible for 76% of the loss of rainbow trout, and 80% of the brook trout, during this period. During seasons of public fishing, only 3 to 10% of the rainbow trout and less than 1% of the brook trout survived for more than 1 year in the lake.

The proportion of trout dying of natural causes increased substantially with an increase in the stocking rate, particularly for rainbow trout the first 6 months after stocking. Further, during the first 2 years of no public fishing, total mortality was less, resulting in high standing crops of trout. Survival was much less with the advent of a white sucker population to the lake.

Trout growth was excellent during early years of the study, with rainbow trout attaining average sizes of 16. 5 inches (1.75 pounds) after 1 year's growth and 19.9 inches (3.07 pounds ) after 2 year's growth. Brook trout growth was good, but not so good as that of rainbows. A few surviving brook trout grew to 13.6 inches (1.06 pounds) in 1 year, and to 14.8 inches (1.36 pounds) after 2 years. Rate of growth of trout deteriorated badly with the increase of a white sucker population, and with an increase in the biomass of standing crop of the trout themselves. Trout condition "C" also decreased proportionally with the decline in trout growth. Flesh color and fat content of trout decreased with loss in condition factor.

Production of trout flesh in the lake varied from a high of 37.2 pounds per acre per year to a low of only 4.6 pounds per acre per year. High production resulted when angler cropping was reduced and when the lake was relatively free of suckers. Better than 80% of the production was made by rainbow trout, compared to 20% by brook trout. Trout production in 1973, following a 3-year period of no trout stocking in the lake, did not improve, apparently because of sucker competition.

The average quantity of food found in trout stomachs was strongly correlated with rate of trout growth. Rainbow trout ate more food per day than did brook trout. Rainbow trout in their second year after planting ate more food per day than did first-year rainbows, but they were poorer converters of food into trout flesh. Crayfish, cladocerans, midges, mayflies and forage fish comprised most of the rainbow trout diet; crustaceans were the most important. Brook trout diet was more varied than that of the rainbow, but was mainly composed of forage fish, crustaceans, mayflies and dipterans, in that order of importance. Suckers were rarely eaten by either rainbow trout or brook trout. Food composition stayed relatively stable during the study, but amount of food per trout stomach was much reduced when suckers entered the lake.

Trout food conditions were reflected in the benthos populations. When benthos populations were high, trout growth was good. Benthos populations declined considerably when suckers entered the lake, and also when high stocks (biomass) of trout were being maintained. Benthos populations were lowest during the year after the lake was treated with rotenone to eradicate sucker and minnow populations.