Technical and economical evaluation of periphyton-based cage culture of tilapia (Oreochromis niloticus) in tropical freshwater cages

摘要

A 3-month study (March-June) was carried out in the Meghna-Gumti River (Bangladesh) to assess the potential for periphyton-based cage aquaculture. The growth of genetically improved farmed tilapia (GIFT) strain (Oreochromis niloticus) was monitored during a trial conducted in nine 2.5-m(3) floating cages. The importance of periphyton in tilapia diets was assessed using three treatments. In the first treatment, the periphyton biomass was increased using additional substrates placed in the cage. In the second treatment, additional substrates were used and the diet was supplemented with a locally available compound feed. The third treatment had no additional substrate but received some supplementary feed. The effect of grazing on periphyton biomass was also assessed in a parallel experiment. Biomass was assessed by measuring ash, ash-free dry weight (AFDW), chlorophyll a, phaeopigments and respiration. Net production of fish ranged from 0.12 to 0.19 kg m(-3) month(-1) and specific growth rate (SGR) from 0.7% to 0.95% body weight day(-1). Jaw abnormalities affected 33% of fish and reproduction was observed in fish as small as 35 g. Productivity of a 1 m(2) of net located in the uppermost 50 cm of the water column, however, was estimated at 0.94 g fish day(-1). The periphyton biomass dropped in just 2 days from 1.12 to 0.34 mg/cm(2) after the introduction of the fish in the cage. The fish fed on plankton, artificial supplementary feed and periphyton growing on substrate in the cage. Filter feeding and grazing on the cage net appeared to be the most important source of energy. The effect of the additional substrates placed in the cages was marginal. The periphyton-based cage aquaculture described here was not economically viable and the main economic constraints were the costs of cage material (net and bamboo poles) and fry. Periphyton-based cage culture could be made more profitable by using local fish species, less expensive cage design, better substrate and by using adapted management techniques to improve periphyton productivity.