Variable Gizzard Shad Recruitment with Reservoir Productivity: Causes and Implications for Classifying Systems

Achieving sustainable prey fish assemblages that support sport fish predator populations is a fundamental challenge to fisheries managers. Among Midwestern and Southeastern (USA) reservoirs, gizzard shad, Dorosoma cepedianum, have been widely stocked to improve predator growth. However, these stockings have yielded highly variable effects on sport fish, due in part to highly variable recruitment of gizzard shad. To determine whether reservoir productivity can be used to classify reservoirs according to recruitment of gizzard shad, we quantified gizzard shad recruitment along a mesotrophic to hypereutrophic productivity gradient. We sampled 12 reservoirs during May through June 1993, to evaluate the hypothesis that larval gizzard shad foraging success and survival increase with reservoir productivity. Both hatch abundance and survival of larval gizzard shad correlated positively with total phosphorus concentrations (TP), an indicator of reservoir productivity. Abundance of 15-mm (total length) larval gizzard shad survivors, an indicator of age-0 year class strength, increased by two orders of magnitude across TP concentrations. Larval gizzard shad foraging success increased with availability of preferred, small zooplankton prey. However, abundance of small zooplankton did not increase with reservoir TP concentrations, and larval survival did not increase with foraging success. These results provide mechanistic understanding for the relative lack of gizzard shad in mesotrophic reservoirs, and the dominance of gizzard shad in hypereutrophic reservoirs. In hypereutrophic reservoirs, negative effects of gizzard shad on sport fish may be alleviated by reducing phosphorus loading from the watershed, suggesting a watershed approach to this fishery and water quality problem.This work was funded by the Department of Zoology at The Ohio State University, Electric Power Research Institute grant 91-07, National Science Foundation grants DEB 9107173 and DEB 9407859, and the Federal Aid in Sport Fish Restoration Project F-69-P, administered jointly by the U.S. Fish and Wildlife Service and the Ohio Division of Wildlife

Prey Selection by Larval Fishes as Influenced by Available Zooplankton and Gape Limitation

Feeding success during the first weeks of life is critical to determining survival and ultimate year-class strength of fishes. To compare the relative influence of gape limitation and available zooplankton on prey size selection among the larvae of three species of freshwater fishes, we gathered data on fish gape size, prey size, and size-specific prey selection in lakes and reservoirs. These variables were compared among black crappies Pomoxis nigromaculatus from a lake that contained large zooplankton as prey and white crappies P. annularis and gizzard shad Dorosoma cepedianum (a potential competitor of white crappie) from reservoirs that contained small zooplankton. In three Ohio reservoirs (i.e., small-zooplankton systems), available zooplankton and larval stages of white crappies and gizzard shad were collected once per week during April through September 1987 and 1988. Although mean prey size of white crappies continued to increase with fish size, mean prey size of smaller-gaped gizzard shad did not. However, as documented for black crappies in north-temperate lakes, white crappies in reservoirs continued to consume prey that were smaller than other available prey, even when they were no longer gape limited. Thus, although the potential for gape limitation differed between large- and small-zooplankton assemblages, prey selection did not differ as expected. Given between-species prey size selection, gizzard shad (that prefer small zooplankton) should be relatively more successful in reservoirs with small zooplankton, whereas white and black crappies (that prefer large zooplankton) should have better success in lakes with large zooplankton.This work was supported in part by DEB- 9108986 and DEB-9410323 to D.R.D., and by NSF BSR-8705518, DEB-9107173, DEB-9407859, and Federal Aid in Fish Restoration, project F-57-R to R.A.S., administered through the Ohio Division of Wildlife

Linking Adult Reproduction and Larval Density of Invasive Carp in a Large River

Eeotogists increasingly recognize the need to understand how landscapes ami food webs interact. Reservoir ecosystems are heavily subsidized by nutrients and detritus from surrounding watersheds, and ofren contain abundant populations of gizzard shad, an omnivorous ftsh that consumes plankton and detritus. Gizzard shad link terrestrial landscapes ami pelagic reservoir food webs by consuming detritus, translocating nutrients from sedimctn detritus to the water column, and consuming zooplaukton. The abundance of gizzard shad increases with watershed agricuhuralization, most likely through n variety oj mechanisms npeniting on ttuvat and adult life stages. Gizzard shad have myriad effects on reservoirs, including impacts on nutrients, phytoplankton, zooplankton, and fish, and many of their effects vary with ecosystem productivity (i.e., watershed land use). Interactive feedbacks among watersheds, gizzard shad populations, and reservoir food webs operate to maintain dominance of gizzard shad in highly productive systems. Thus, effective stewardship of reservoir ecosystems must incorporate both watershed and food-web perspective