An Empirical Model of Size Distributions of Smallmouth Bass

Abstract

As part of a study of the life cycle of the smallmouth bass Micropterus dolomieui, young of the year were sampled in the Baie du Doré, Lake Huron, during six summers to determine growth after dispersal from nests. Length-frequency distributions were plotted approximately weekly each of the summers. The changes through time of the mean size of fish in each cohort were described by a mathematical model that assumed a size-dependent growth rate and no size-dependent mortality for the fish. The constraint on the model to fit the empirical mean fish sizes through the growing season automatically produced time-varying standard deviations of the size distributions that compared closely to the empirical standard deviations for 5 out of the 6 years. The poor fit to the standard deviations of the sixth year was explicable in terms of unusual data on one sampling date. These results suggest that size dependence of the growth rate was an important mechanism in the spread in the length-frequency distribution and that size-dependent mortality played no significant role in determining the changes in this distribution through the growing season. Because winter mortality of first-year smallmouth bass in northern waters is important in their population dynamics and is size-dependent, the model provides a link in the understanding of smallmouth bass populations and their management.