Linking Bluegill and Gizzard Shad Prey Assemblages to Growth of Age-0 Largemouth Bass in Reservoirs

Abstract

Either gizzard shad Dorosoma cepedianum or bluegills Lepomis macrochirus dominate prey assemblages in many small (<100 ha) Ohio reservoirs. Because gizzard shad spawn early in the spring and their offspring grow rapidly, age-0 gizzard shad may be invulnerable to age-0 largemouth bass Micropterus salmoides thereby compromising this piscivore's growth and, potentially, recruitment. To test this hypothesis, we quantified growth, abundance, and diets of age-0 largemouth bass in reservoirs dominated by age-0 bluegills (one reservoir) or age-0 gizzard shad (two reservoirs) during June through early October 1992–1994. In the bluegill-dominated reservoir, age-0 largemouth bass grew slowly (about 0.04 g/d) during June through mid-August. Though age-0 bluegills became abundant after mid-August, contributing to rapid growth (about 0.2 g/d) of age-0 largemouth bass, these age-0 largemouth bass only reached small to moderate sizes by fall (range of mean wet weights, 3–7 g). In the reservoirs dominated by gizzard shad, summer growth and fall sizes of largemouth bass varied among systems and years. During one summer in one shad-dominated reservoir, an early rise in temperature plus small age-0 gizzard shad probably contributed to rapid growth (∼0.12 g/d) and large fall size (mean wet weight, 10.8 g) of age-0 largemouth bass. More commonly, age-0 largemouth bass grew slowly or moderately (∼0.06 g/d). However, fall sizes of largemouth bass always were equivalent to or exceeded those in the bluegill-dominated reservoir (range of mean wet weights, 3–11 g). Our results suggest that growth of age-0 largemouth bass should vary more in systems dominated by gizzard shad than in bluegill-dominated ones. Management efforts that increase gizzard shad vulnerability during early summer may reduce this variability, thereby enhancing first-summer growth and, potentially, recruitment success of largemouth bass.