The effect of supplementary inorganic fertilization on juvenile walleye (Stizostedion vitreum) reared in organically fertilized ponds

Abstract The effect of supplementary inorganic fertilization on juvenile walleye growth, production and food availability in organically fertilized rearing ponds was examined. The six 0.04-ha ponds used in the experiment were continually aerated. Three control ponds were fertilized with a soybean meal slurry (27 g solid equivalent/m 3 weekly). The other ponds were given the same dosage of soybean meal, supplemented with an 8-32-16 inorganic fertilizer, dissolved in floating containers, and administered at the rate of 2.8 g P 2 O 5 equivalent/m 3 weekly. Two-day-old walleye were stockel in the ponds (30/ m 3 ), and reared for 41–42 days. Walleye in the inorganically fertilized ponds grew faster, and were almost twice the weight of those in the control ponds at harvest. Biomass at harvest was 42% higher in the ponds treated with inorganic fertilizer, but this difference was not statistically significant. Survival did not differ significantly between treatments. Mean levels of chlorophyll a concentration, total zooplankton abundance and macroinvertebrate biomass did not differ significantly between treatments, whereas total phosphorus concentration and biomass of Daphnia in walleye stomachs were significantly higher in the inorganiconds. Although density of Daphnia was not significantly different between treatments, Daphnia populations persisted longer in the inorganically treated ponds despite higher rates of fish predation. Improved walleye growth in the ponds given an inorganic supplement was apparently due to higher production of Daphnia , the major planktonic prey eaten by the fish.