The size of grass carp as a factor in the control of hydrilla

Abstract

Grass carp ( Ctenopharyngodon idella Val.) were restocked into a 5.42 ha central Florida lake in November, 1978, to study the effect of fish size and number on the control of hydrilla ( Hydrilla verticillata (L.f.) Royle). The results were compared with those from a previous two-year study on the lake which indicated a loss of hydrilla control during the third year. The hydrilla biomass in October 1979 approached that of October 1976, when the grass carp were initially stocked. The mean weight of the fish at the end of the previous study was 5.42 kg, while at the end of this study they had reached a mean weight of 11.37 kg. Since no significant loss in numbers of grass carp occurred between the second and third years (8%), the reduction in hydrilla control during this study was suspected to be due to the increased size of the grass carp. This supposition was tested utilizing three size-classes of grass carp in matched experimental ponds. A significant linear relationship ( P = 0.05) was determined between the size of the grass carp and the consumption of Egeria densa Planchon by the fish (kg Egeria fish −1 day −1 = 2.42−0.17 kg grass carp; r 2 = 0.9997), while at the same time the growth rate of small grass carp was found to be much higher than that for larger grass carp. It was concluded that only small grass carp should be stocked in hydrilla-infested waters, and that one can expect the fish to become ineffective as a means of weed control as they approach a body weight of 14 kg, due to their reduction in the absolute, as well as the relative, amount of vegetation consumed with increasing size.