To determine the carrying capacity and overall performance of the Nile tilapia Oreochromis niloticus (L.) in small volume cages, 5,850 sexually inverted fingerlings, averaging 18.85 g of live weight, were stocked in 1.3 m3 cages, at densities of 75, 150, 300, and 600 fish/m3 (T75; T150; T300, and T600). The 196-day long trial was set up in a totally randomized design, with 4 treatments and 3 replicates. Cages were set up in a 4-ha, 2 m average depth reservoir. Twenty percent (20%) of each cage stock were weighed and measured every 30 d to determine growth rate as weight gain (WG), and feed conversion rate (FCR). Fish were fed twice-a-day (09:00 and 17:00 h) an extruded, floating, 28% crude protein commercial diet. Feed consumption, mortality and water temperature were recorded at feeding time, and pH, dissolved oxygen (DO) and water transparency were recorded monthly. The first evaluation (30-d) showed FCR values between 0.68 and 2.40:1, and WG between 11.19 and 21.52 g. Best FCR values were recorded for T600. However, the stock could not repeat or maintain the initial performance level, and final results showed: FCR = 6.31:1 and WG = 71.52 g for T75; FCR = 4.08:1 and WG = 65.14 g for T150; FCR = 3.57:1 and WG = 59.60 g for T300; and FCR = 2.96:1 and WG = 69.17 g for T600. The best WG result was attained by T600, which was not significantly different of T75 (P<0.01). The FCR was better for T600 than for T75. The performance of both T150 and T300 was inferior to T600 and T75. Compared to literature data, the performance of Nile tilapia in this trial was very poor. Low cage effective depth (0.85 cm), causing thermal and brightness stress, poor quality of commercial feed and low sex reversal rate of fish stock could explain the unsatisfactory results here obtained.
Alevinos de tilápia nilótica Oreochromis niloticus (L.) com peso médio de 18,85 g, foram estocados em gaiolas de 1,7 m3 cada, em densidades de 75, 150, 300 e 600 peixes/m3 (T75; T150; T300 e T600), num delineamento inteiramente casualizado (4 tratamentos e 3 repetições), e criados por 196 dias, com ração comercial extrusada, flutuante, contendo 28% de proteína bruta. A cada 30 dias, 20% do lote de peixes de cada gaiola foi medido e pesado para determinar ganho de peso (GDP) e conversão alimentar (CA) de cada tratamento. Os resultados finais de CA e GDP para cada tratamento foram, respectivamente: para T75, 6,31:1 e 71,52g; para T150, 4,08:1 e 65.14g; para T300, 3,57:1 e 59,60g; e para T600, 2,96:1 e 69,17g. Em relação ao ganho de peso, T600 e T75 foram semelhantes (P > 0,01) sendo que T600 apresentou melhor CA que T75 (P < 0,01). Os tratamentos T150 e T300 apresentaram desempenho inferior a T600 ou T75. Os índices de CA e GDP obtidos no experimento, resultaram da interação de fatores como estresse térmico e luminoso, a baixa qualidade da ração comercial utilizada e a baixa taxa de reversão sexual do lote de alevinos adquiridos de produtores comerciais.