Effects of stocking density on the performance of the Australian freshwater silver perch (Bidyanus bidyanus) in cages

Silver perch fingerlings (range mean weights, 109.3–115.4 g) were stocked at densities of 12, 25, 50, 100 or 200 fish/m3 in cages (1 m3) in an aerated, 0.32-ha earthen pond, with four replicate cages for each density. Fish were fed a formulated diet containing 32% crude protein and 13 MJ/kg energy and cultured for 210 days. Water temperatures ranged from 20.6 to 29.8 °C. There was aggression between fish in cages stocked at 25 or 50 fish/m3 and survival (50% and 64.5%, respectively) was significantly lower (P < 0.01) than at the other densities (87.5%, 98.5% and 97.4%). Stocking density did not affect final weight (454.6–471.1 g), specific growth rate (0.65–0.70%/day) or absolute growth rate (1.6–1.7 g/fish/day), but feed conversion ratios (FCR) of fish stocked at 25 or 50 fish/m3 (4.4 and 3.7) were significantly higher (P < 0.01) than FCRs (range, 2.3–2.5) at other densities. Coefficients of variation in weight (CV) at 25 or 50 fish/m3 (24.4% and 28.3%) were significantly higher (P < 0.01) than at other densities and the lowest CV of 16.1% was at 200 fish/m3. Production was significantly affected (P < 0.01) by stocking density, with the highest mean rate of 88.5 kg/m3 in cages stocked with 200 fish/m3. One infestation of the ectoparasitic ciliate, Chilodonella hexasticha, and two infestations of the monogenean gill fluke, Lepidotrema bidyana, were each treated successfully with an application of 30 mg/l formalin. The high survival, relatively fast growth, low variation in weight and high production rates of silver perch stocked at 100 or 200 fish/m3 demonstrate that cages are a viable alternative to ponds for the commercial production of silver perch.