46 SEPTEMBER 2019 • WORLD AQUACULTURE • WWW.WAS.ORG In the laboratory, each bag with fish was placed in a 1-m3 tank containing 500 L of rainwater and opened after fish had acclimatized to the tank water temperature. Mortality was determined by counting the number of dead fish in each tank. Mortality in each tank was monitored for 5 days and was then added to the mortality immediately after transport to obtain the cumulative mortality. Mortality of smaller fish (2-4 cm) was significantly greater during transport and after five days compared to larger fish (Table 1). Prior to sealing bags, water pH, dissolved oxygen (DO) concentration and temperature were measured with a multiparameter meter (YSI PRO 2030). Three water samples were collected from each bag and stored in ice for laboratory analysis of initial total ammonia nitrogen (TAN). Upon arrival pH, DO and temperature in each bag were determined and other water samples collected for TAN analysis. TAN was analysed in the laboratory using standard method (AOAC 2005). The TAN concentration after four hours of transport was significantly greater than the initial value of 0.11±0.01 mg/L. The TAN concentration reflected density, with the lowest concentration (0.21±0.01 mg/L) at 40 g/L, intermediate concentration (0.43±0.07 mg/L) at 80 g/L and highest concentration (0.69±0.78 mg/L) at 100 g/L. Growth Trials in Cages and Ponds Feeding of Nile perch started one day after stocking. Fish were fed with live feeds (catfish and tilapia fry) maintained at 10 fry/L in the culture tank for two weeks before transfer to cages and ponds. Catfish fry were obtained by artificial induction, fertilization and hatching using standard procedures previously established in the KMFRI laboratory and tilapia fry were obtained from KMFRI nursery ponds. Fish for cage trials were transported to the cage site in aerated tanks. Fish cages (Fig. 2) measuring 10 m × 5 m × 2.5 m were located in Mageta island (0° 7’ 59” N, 34°0’ 51” E). Cages were made with a steel frame holding a 5-cm mesh net at the bottom and located in an area sheltered from strong winds. Cages were placed 200 m from the lakeshore for ease of access and management and at a depth of about 5 m. Three cages were used for the experiment. Each cage was stocked with 2,000 fish, each with an average initial weight of 20 g. Fish were fed daily with live foods consisting of haplochromines and other small fishes obtained from the lake, dams and abandoned fish ponds. To maintain diet throughout in the cage, a solar lamp was fitted on the cage at night to attract wild fish fry and insects for the cultured Nile perch. Nile perch were stocked into three ponds at 1000 fish per pond. Ponds measured 30 m × 10 m × 2.2 m and were located at the KMFRI-Kegati station. Ponds also had 1000 mixed-sex tilapia that produced approximately 5000 fry per week that served as food for Nile perch in addition to haplochromines and small fish. The pond was fitted with solar lamps to allow fish to feed at any time as was the case with cages. Tilapia in ponds were fed formulated feeds with 40 percent crude protein. Biometric data on length and weight was collected weekly for the six-month culture period. At the end of the experiment, the specific growth rate (SGR) and survival rate (SR) were calculated for both fish in cages and ponds using the formulae: SGR= [ln (final weight) – ln (initial weight) ×100]/ number of days of the experiment; SR= (Number of fish at stocking / number at harvesting). The growth rate (SGR) of Nile perch in cages (0.83 percent/d) was significantly greater than that in ponds (0.52 percent/d) (Fig. 3). Survival was 100 percent in both environments. Water Quality in Cages Water quality parameters, dissolved oxygen, temperature, pH, and TAN and total phosphorus were tested biweekly throughout TABLE 1. Mortality and survival of Nile perch fingerlings after four hours transport and after five days. Fish size (cm) Stocking density (g/L) Mortality during transport (%) Mortality after 5 days (%) Survival after 5 days (%) 2-4 40 63±15 8±10 28 80 70±21 3±3 27 100 57±34 4±5 30 5-7 40 15±5 0 74 80 11±10 0 88 100 24±14 7±4 64 8-15 40 0 0 100 80 7 5±7 88 100 3±2 3±4 94 FIGURE 2. Experimental cages for Nile perch culture at Mageta Island.
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