TURBIDITY GENERATION AND BIOLOGICAL IMPACTS OF AN EXOTIC FISH CARASSIUS-AURATUS, INTRODUCED INTO SHALLOW SEASONALITY ANOXIC PONDS

Over the last decade, goldfish Carassius auratus L., have been introdu ced into lakes and ponds in the Montreal region. Controlled experiment s compared the impact of goldfish in both mud- and gravel-bottomed out door pools, with either caged or non-caged vegetation. High levels of turbidity were generated by foraging goldfish in mud pools (P < 0.01). All non-caged submersed plants (Potamogeton) showed extensive grazing damage and all of the unprotected submersed and emergent forms were u prooted. Data from Stoneycroft pond where a feral goldfish population reached 15-17 000 individuals ha(-1), confirmed the experimental findi ngs showing goldfish to be benthic herbivores, causing a visible incre ase in turbidity and decrease in aquatic vegetation. Gut analysis reve aled goldfish to have little diet overlap with native amphibians. High turbidity should reduce the success of predators feeding on goldfish, and may decrease the foraging success of other competitors. The spora dic distribution of goldfish in Canada may be linked to site character istics, specifically the susceptibility of the area to turbidity gener ation. (C) 1995 The Fisheries Society of the British Isles.