Production and Theoretical Equilibrium Yields for the Bluegill (Lepomis macrochirus) in Two Michigan Lakes

Abstract

Standing crop, production, exploitation, and theoretical maximum equilibrium yields were calculated for bluegill populations in Jewett Lake for 4 out of 6 years during the period 1957–1962, and in Lodge Lake for 1957–1958. The computation method for production and equilibrium yields utilized annual spring population estimates from trap-net data and seasonal instantaneous rates of mortality and growth. Fish less than 4.0 inches long (total length) were not included because of limitations of the sampling methods (angling and netting). During this time, spring standing crops of bluegills ranged between 5.2 and 65.0 lb. per acre in Jewett Lake; 21.3 lb. per acre for Lodge Lake. Production of new biomass by the populations ranged from 2.2 to 29.8 lb. per acre in Jewett Lake; 7.3 lb. per acre in Lodge Lake. Production by 4- and 5-year-old bluegills exceeded the weight taken by anglers in most instances, but among older fish yield outweighed production. Total production among these older age groups ranged from 44 to 48% of the standing crop in Jewett Lake, 34% for Lodge Lake. Fishing pressure varied from 70.5 to 115.0 hours per acre on Jewett Lake; 10.5–52.9 on Lodge Lake. There was no correlation between size of the spring populations and catch per hour. Exploitation rates, based on percentages of marked fish caught by anglers, ranged from 17.2–24.6% for Jewett Lake; 8.2–26.1% for Lodge Lake. These values were substantiated by rates computed on the basis of the proportion of available biomass (standing crop + production) harvested by fishermen. There was a significant tendency for anglers to catch or keep only the older individuals in 2 out of the 7 years. The calculations of maximum sustained yield revealed that, with one exception, these populations could have supported increases in fishing pressure ranging from 30 to 90% at the observed rates of fishing success. No appreciable change in growth rate would accompany this higher yield, an important condition for these calculations. The harvest from Jewett Lake in 1962 approximated the computed maximum equilibrium yield. It was suggested that the computational methods used in this paper will be useful in solving some of the problems associated with the management of prolific warmwater game fish by providing a rational basis for cropping overabundant populations.