Growth Depensation in Year-0 Largemouth Bass: The Influence of Diet

Abstract

Young-of-the-year largemouth bass Micropterus salmoides in Lake Opinicon, Ontario, displayed a divergence in size over their first summer. The range in total body lengths was 11 mm when year-0 fish were first captured on June 22–25, and this range increased to 26 mm by September 8–12. The size divergence was not due to a protracted or disrupted spawning period and, instead, appeared to be underlain by differences in diet and prey availability. Diets of small and large year-0 largemouth bass (less than and greater than the cohort mean length, respectively) differed progressively over the summer. Large year-0 fish maintained a diverse diet of which fish prey always constituted 25–30% by volume. In contrast, small year-0 largemouth bass shifted from a diet comprised of zooplankton, aquatic insects, and larval fish in July to one dominated by zooplankton, and including no fish, in September. The diet shift of the small individuals occurred when fish prey became too large to handle. However, some forage fish were always within the manageable size range for large year-0 largemouth bass and, consequently, large individuals were able to maintain fish in their diet. Analysis of the gross energetic value of the diets showed that larger year-0 individuals obtained a higher energy content in their diet, relative to the small individuals. Received March 22, 1984 Accepted December 17, 1984