Effect of larval walleye ( Stizostedion vitreum) and fertilization on the plankton community: implications for larval fish culture

Plankton abundance which greatly affects larval fish growth and survival can be regulated by fish predation and nutrient input. In this study, we examined community effects of larval walleye and fertilizer type on plankton in ponds involving three densities (0, 250 000, and 500 000 fish/ha) of larval walleye ( Stizostedion vitreum) and two fertilizer types (inorganic or organic). Walleye survival was not affected by fish density or fertilizer type, and more fish yield was gained in high fish density ponds. The best fish growth was obtained in ponds with low fish density and inorganic fertilizer. As fish density increased, the abundances of Daphnia, Diaptomus and nauplii, Secchi depth, NH 3-N, NO 3-N and chironomids were reduced, whereas primary productivity, Scenedesmus, flagellates, colonial algae, Anabaena, rotifers and pH were enhanced. Compared to organic fertilizer, inorganic fertilizer increased the levels of primary productivity, algal abundance, dissolved oxygen, pH and total phosphorus. Inorganic liquid fertilizer with N:P ratio at 20:1 and a stocking density of 250 0005̄00 000 fry/ha is recommended for larval walleye culture. The stocking and fertilization protocol for larval walleye culture could be applied to the larval culture of other planktivorous fish species.