Management options for a more complete utilization of the biological fish production in Sri Lankan reservoirs

Stock dynamics of the introduced tilapia Oreochromis mossambicus and co-occurring indigenous cyprinids were studied in a typical lowland reservoir in Sri Lanka. All species exhibited peaks in reproductive activity during the rainy seasons in November and May. O. mossambicus was estimated to grow to 14 cm in its first year and to 21 cm in its second. A high natural mortality (M) was found, both for the cyprinids (M = 1.3/yr to 4.7/yr over a range of species) and for O. mossambicus (M = 1.9/yr and 1.7/yr for females and males). The fishing mortality (F) of O. mossambicus, estimated for the length range from 15.0 to 18.5 cm, was 5.5/yr for females and 4.7/yr for males. The total fish biomass was estimated at 1 829 kg/ha, with 128 kg/ha O. mossambicus and 1 098 kg/ha small pelagic cyprinids, mainly Amblypharyngodon melettinus. The total biological fish production was around 5 400 kg/ha/yr, with O. mossambicus and the small pelagic cyprinids accounting for 400 kg/ha/yr and 3 600 kg/ha/yr respectively. The commercial fishery yielded 242 kg/ha/yr (including 70% or 169 kg/ha/yr O. mossambicus), indicating an incomplete utilization of the total biological fish production. It was concluded that there is scope for an approximately 100% increase of the total yield. Re-implementation of a 76 mm minimum mesh size, to prevent overfishing of O. mossambicus, is recommended as a management measure. Additional options are direct exploitation of the untapped resource of small pelagic cyprinids, or indirect exploitation by means of a culture-based fishery on Lates calcarifer, a controllable predator. [KEYWORDS: Sri-Lanka; freshwater-fish; stocks-; reservoirs-water; biological-production; fishing-mortality; yield-; population-dynamics