New Zealand Journal of Marine and Freshwater Research abstracts

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

C. P. MITCHELL

Fisheries Research Division Ministry of Agriculture and Fisheries P. O. Box 951, Rotorua New Zealand

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~'), 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.

Keywords grass carp; Ctenopharyngodon idella; Retropinna; Gobiomorphus; weed control; forage fish; abundance; size; native fish; eutrophication; biological control; Parkinsons Lake

New Zealand Journal of Marine and Freshwater Research, 1986, Vol. 20: 219-230 Received 29 July 1985; accepted 21 October 1985

PDF file of entire paper: medium quality (1369K); (scanned from paper original: notes about this process)

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