66 JUNE 2015 • WORLD AQUACULTURE • WWW.WAS.ORG Stocking Density Study An experiment was designed to investigate the effects of stocking density on growth, survival and feed conversion of juvenile red claw crayfish in a nursery. Crayfish were first size graded (Fig. 2) to mitigate cannibalism, as suggested by Barki et al. (2006). Juvenile red claw crayfish (260 ± 80 mg) were stocked into nine 0.5-m² tanks that were part of a recirculating aquaculture system (Fig. 3). There were three stocking densities — 120, 240 and 360/m² — based on tank bottom area, with three replicate tanks for each density. Artificial substrate made from gardening Vexar® added about 7.68 m² of additional surface area to each tank (Fig. 4). This surface area provided sufficient space to juvenile crayfish to discourage confrontation and mitigate cannibalism. Crayfish were fed a highprotein shrimp feed (Fig. 5) twice daily at 6 percent body weight daily for the first 40 days of the trial. Feeding rate was reduced to 3 percent body weight daily due to concerns about water quality degradation. System maintenance, such as removing excess feed via siphon, was performed twice weekly to maintain optimal conditions. Water quality parameters were monitored and maintained to ensure environmental conditions did not invoke stress among crayfish. Alkalinity, dissolved oxygen, pH, flow rate, temperature, total ammonia-nitrogen, unionized ammonianitrogen and nitrite-nitrogen were measured twice weekly. Red claw crayfish are sensitive to high ammonia and nitrite levels. Water quality was optimal and did not cause significant stress that affected the study outcome. Each tank was harvested at the conclusion of the 60-d trial. Each crayfish was individually weighed (Fig. 6) and the rostrum to tail length was measured (Fig. 7). Average weight at harvest, length at harvest, survival and feed conversion ratio (FCR) were calculated for the three density treatments. Production parameters for the study are summarized in Table 1. Survival was not significantly different among the three densities tested. For final weight, final length and feed conversion, there was a significant difference between crayfish at 120 and 240/m2 but no significant difference between 240 and 360/ m2. These results are consistent with a study involving adult red claw crayfish (Pinto and Rouse 1996), indicating that growth was affected by stocking density but survival was not. Management Implications This article summarized the results of an experiment that investigated the effect of stocking density on production parameters of juvenile red claw crayfish in indoor nurseries. All things being equal, nursery managers might prefer greater stocking densities to keep production costs low. The results showed that stocking density affected growth of red claw crayfish in nursery tanks. Growth (indicated by weight at harvest) was inversely related to stocking density, consistent with many studies about the effect of stocking density on aquatic animal FIGURE 3. The recirculating aquaculture system serving as a red claw crayfish nursery. FIGURE 4. Vexar substrate used to promote good survival of red claw crayfish juveniles. FIGURE 5. Feeding red claw crayfish juveniles with a high-protein shrimp feed.
RkJQdWJsaXNoZXIy MjExNDY=