THE SURVIVAL AND GROWTH OF LARVAL WALLEYE, STIZOSTEDION-VITREUM, AND TROPHIC DYNAMICS IN FERTILIZED PONDS

Two fertilization methods, inorganic fertilizers (IF) versus combined inorganic and organic fertilizers (IOF), were compared in four ponds with two replicates of each treatment at a fish stocking density of 244 000 ha- 1. Fish survival and total length averaged 77.9% and 38.9 mm in the IF Ponds, and 2.5% and 49.2 mm in the IOF ponds respectively. No significant differences were found for the abundances of total zooplankton (P > 0. 1 5) and phytoplankton (P > 0.34) between the two treatments, but the mean dissolved oxygen at the bottom was significantly lower in the IOF ponds (6.8 mg O2 l-1) than that in the IF ponds (10.4 mg O2 l-1 (P < 0.021). The high fish mortality in the IOF ponds maly have been due to low dissolved oxygen, or to ammonia toxicity. We suggest that in hatchery ponds filled with lake water, the use of inorganic fertilizer alone (N : P = 20:1) will provide better results than the combined use of inorganic and organic fertilizers.