Competition between Larval Fishes in Reservoirs: The Role of Relative Timing of Appearance

Abstract

In small, hypereutrophic reservoirs (<100 ha; >100 μg total phosphorus/L), larval gizzard shad Dorosoma cepedianum and threadfin shad D. petenense (henceforth, shad) reach high densities in the limnetic zone, virtually eliminate zooplankton, and perhaps compromise success of other planktivorous larvae, such as bluegill Lepomis macrochirus. Because relative timing of appearance of shad and bluegills probably influences their relative success, we quantified densities of fish larvae and zooplankton during spring through summer in three reservoirs across 8 years (1987–1994), and we conducted three hatchery experiments with varying larval appearance times and gizzard shad densities in plastic bags (1 m³). When shad were abundant in reservoirs, bluegill abundance often peaked either at the same time (36% of reservoirs and years combined) or after (40% of reservoirs and years combined) shad peaks. When gizzard shad were placed in bags 2 weeks before bluegills (N = 1 experiment), they depleted zooplankton, reducing growth (∼0.075 g·g⁻¹·d⁻¹) but not survival of bluegills. In experiments (N = 2) in which both species were added simultaneously, zooplankton declined only slightly with gizzard shad, and there was little effect on bluegill growth (∼0.21 g·g⁻¹·d⁻¹) and survival; in general, gizzard shad growth declined with time and increasing gizzard shad density. Based on experiments, bluegill success should vary among reservoirs and years as a function of their appearance relative to gizzard shad. In reservoirs, zooplankton availability and bluegill abundances were consistently low during years when gizzard shad dominated reservoir fish assemblages. Because gizzard shad probably reduce bluegill success in hypereutrophic Ohio reservoirs, management strategies that reduce gizzard shad should improve bluegill success.