Eighteen O.l-acre ponds at the Auburn University Fisheries Research Unit, Auburn, Alabama, were used from April 5 through November 20, 1967. Both species of fishes were stocked together randomly at a rate of 4,000 fingerlings per acre. The experimental design consisted of three control ponds without fertilization or hyacinths; three control ponds without fertilization, but with hyacincths; three ponds with 0-8-0 (N,P,K) fertilization, but no hyacinths; three ponds with 0-8-0 fertilization, but with hyacinths; three ponds with 8-8-0 fertilization, but no hyacinths; and three ponds with 8-8-0 fertilization, but with hyacinths. The fertilizers were applied to stimulate the growth of hyacinths and fish-pond organisms. Greater numbers and dry weights of fish-food organisms were associated with roots of water hyacinths in control ponds than in fertilized ponds. Snails and odonate numphs were dominant in control ponds but were not important in fertilized ponds. Dry weight of amphipods and midge larvae did not differ significantly between the control and two fertilizer treatments. Low numbers and dry weights of fish-food organisms in fertilized ponds containing hyacinths was probably due to heavy predation by the sunfishes. Upon draining the fish production was greater in fertilized ponds containing plants compared to control hyacinth ponds. In the unfertilized ponds the mean standing crop of fish at draining time was approximately the same in both hyacinth and non-hyacinth ponds. The mean standing crop from fertilized ponds was at least twice as great in non-hyacinth ponds as in hyacinth ponds. The reduced fish production in hyacinth ponds was probably a result of competition for nutrients between the water hyacinths and plankton food-chain, and of the reduction of "edge" for fish-food organisms on the roots.
