Experimental Assessment of the Influence of Zooplankton Size and Density on Gizzard Shad Recruitment

Abstract

Available prey influence fish recruitment, especially when size-dependent processes, such as starvation or predation, dictate larval survival. Yet, for zooplanktivorous larval fishes. quantitative assessments of how zooplankton density and quality (i.e., size or taxonomic composition) influence foraging success, growth, and survival are rare. With small-scale experiments, we tested whether high densities of small zooplankton promote recruitment of age-0 gizzard shad Dorosonia cepedianum from the larval (<25 mm total length, TL) to the juvenile (≥25 mm TL) stage. In our first experiment, in which larval and early juvenile gizzard shad (10–29 mm TL) were exposed to a range of densities of small zooplankton, foraging success varied substantially but generally increased at zooplankton densities over 200/L for larvae less than 20 mm TL and at densities over 100/L for individuals 20 mm TL or longer. In a second experiment. larval gizzard shad (15 mm TL), after 2 weeks in 1-m3 enclosures, survived more than 300% better in enclosures containing small zooplankton (mean = 0.33 mm) than in enclosures containing large zooplankton (0.39 mm), even though growth was similar between treatments. In a third experiment, early juveniles (24 mm TL), after 2 weeks in 1-m3 enclosures, grew significantly faster, but survived no better, in high (600 zooplankton/L) than low (90 zooplankton/L) densities of small zooplankton. Interactions among zooplankton size, zooplankton density, and gizzard shad size early in life should strongly affect larval gizzard shad success, which in turn should mediate their impact as juveniles on reservoir zooplankton assemblages and ultimately on fish communities.