Effect of carp, Cyprinus carpio L., an exotic benthivorous fish, on aquatic plants and water quality in experimental ponds

The effects of carp, Cyprinus carpio L., on water quality and functioning of aquatic systems were investigated in two experiments in ponds (~90 cm deep) outdoors at Griffith, New South Wales. The experiments represented contrasting conditions of high and low impact, defined by stocking density and food availability, with stocking densities chosen to be above and below 450 kg ha-1, the stocking density suggested as a critical threshold for damage. Under high impact conditions, carp had a significant effect on water quality, habitat structure and pond physical characteristics. Turbidity increased from approximately 7 NTU to 26 and 73 NTU by Day 4, there was a complete loss of two out of five plant species tested (Chara fibrosa and Vallisneria sp.) by Day 6, and surface water temperature in ponds with carp was significantly greater by Day 7. Plant loss was attributed to uprooting rather than herbivory, as sometimes reported. Under low impact conditions the uprooting rate of Vallisneria was reduced to a third. Contrary to the results of previous studies, there was no evidence of increased nutrients or greater algal biomass in ponds with carp, but this may have been because the sediments were relatively low in phosphorus. A crude nutrient budget based on water concentrations and tissue analysis showed substantial growth of carp in 20 days that could be accounted for only by considering either sediments or terrestrial inputs (ponds were not covered) as an important food source.