Effects of introduced grass carp on populations of two species of small native fishes in a small lake

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Abstract
Stocking Ctenopharyngodon idella (grass carp) for aquatic weed control into a 1.92 ha lake (Parkinsons Lake, 37°19'S, 174°41'E) altered the size and abundance of two species of small native fish, Retropinna retropinna (smelt) and Gobiomorphus cotidianus (bully). The response of the two species differed. R. retropinna recruitment apparently failed during the ‘high impact’ phase of C. idella stocking (44 fish ha‐1), when all aquatic weeds were removed. Immature fish disappeared and samples comprised 100% large females. G. cotidianus also increased in size, however, condition improved and the sex ratio did not alter markedly. Diet of both species switched from small zooplankton to Chironomus sp. Following reduction in C. idella stocking density, some return of population meristics toward prestocking values occurred. Loss of the weed bed habitat, water quality deterioration, and increased predation were considered the major environmental impact factors during the period when these changes occurred. The response of both fish populations paralleled the effects of cultural eutrophication, to which G. cotidianus appears to be more tolerant.

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