Turbidity generation and biological impacts of an exotic fish Carassius auratus, introduced into shallow seasonally anoxic ponds

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Abstract
Over the last decade, goldfish Carassius auratus L., have been introduced into lakes and ponds in the Montreal region. Controlled experiments compared the impact of goldfish in both mud‐and gravel‐bottomed outdoor pools, with either caged or non‐caged vegetation. High levels of turbidity were generated by foraging goldfish in mud pools (PPotamogeton) showed extensive grazing damage and all of the unprotected submersed and emergent forms were uprooted. Data from Stoneycroft pond where a feral goldfish population reached 15–17 000 individuals ha‐1, confirmed the experimental findings showing goldfish to be benthic herbivores, causing a visible increase in turbidity and decrease in aquatic vegetation. Gut analysis revealed 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 sporadic distribution of goldfish in Canada may be linked to site characteristics, specifically the susceptibility of the area to turbidity generation.