Michigan Department of Conservation
Institute for Fisheries Research Report No.1740, 1967
Production and Theoretical Equilibrium Yields for the Bluegill (Lepomis macrochirus) in Two Michigan Lakes
Mercer H. Patriarche
Abstract.-Considerable information on the bluegill populations in two small lakes at the Rifle River Recreation Area was collected between 1957 and 1962. During this time, fishing pressure varied from 70 to 115 hours per acre on Jewett Lake; 10.5 to 53 hours per acre on Lodge Lake. Based on the catches of fish marked each spring, anglers caught between 17 and 24 5% of the bluegills in Jewett Lake; 8 to 26% of the Lodge Lake fish. These same exploitation rates also were obtained by another method of calculation. In only two instances was there a tendency for anglers to catch or keep only the older (larger) fish. Annual spring inventories in Jewett Lake showed that the bluegill population (fish at least 4.0 inches long) ranged between 5 and 65 pounds of fish per acre; one such census on Lodge Lake indicated there were 21 pounds of bluegills per acre. New growth put on by these fish during the year ranged from 44 to 47% of the weight of these spring populations; 34% for Lodge Lake. By weight, the new growth varied from 2 to 30 pounds per acre for Jewett Lake; 7 pounds per acre in Lodge Lake. For nearly all of these fish, the loss of weight from natural deaths greatly exceeded the weight of fish caught by fishermen. To determine how much fishing pressure these lakes could take without affecting the populations, a method of calculation was employed that is used mostly by marine biologists for commercial fisheries. This method makes use of information on growth and the rates at which fish die (both natural deaths and fishing deaths). These calculations disclosed that these bluegill populations could have supported increases in fishing pressure ranging from 30 to 90%. Usually bluegills (and many other species) reproduce so fast that they literally overwhelm their food supply and few can reach a desirable size. They have to be cropped off but usually there is no way of knowing how many must be removed in order to make the best use of the lake. The kind of calculations used in this study can provide the answer if there is enough information at hand.