Michigan Department of Natural Resources
Fisheries Research Report No.1827, 1975
The use of Large Daphnia as Indices of Fishing Quality for Rainbow Trout in Small Lakes
Merle G. Galbraith, Jr.
Abstract.-Twenty-seven lakes (3 to 128 hectares), currently being managed for rainbow trout (Salmo gairdneri), were subjectively classified by the district fisheries supervisor as having "good" or "poor" fishing quality. During the summers of 1966 to 1968 plankton was collected periodically from these northern Michigan lakes with a small Wisconsin-type plankton net. Daphnia larger than 1.34 mm were counted and their volumes were determined. An analysis of the collection of large Daphnia revealed that a combination of two parameters--average number of large daphnids and a volumetric index--gave accurate assessments of the trout lakes categorized as "good" or "poor. " The study shows (1) that plankton should be sampled once per month for two different summer months, (2) that a trout lake is poor if the average number of large daphnids is less than 100 per haul and if their volumetric index is less than 0.65, and (3) that if the number of large daphnids is more than 150 per haul and their volumetric index is greater than 0.80, the lake is good for trout.