GROWTH-RATE AND THE ROLE OF OREOCHROMIS-MOSSAMBICUS (PETERS) IN THE FISHERY OF A TROPICAL, UPLAND, DEEP RESERVOIR IN SRI-LANKA

The fishery for Oreochromis mossambicus (Peters) in Victoria reservoir was studied by monitoring the monthly catch for 18 months. The reservoir is situated 483 m above m.s.l. and has a surface area and a mean depth of 2370 ha and 31 m, respectively. The annual fish production of the reservoir was 70.13 kg ha-1. The main fishing gear was 9-15 cm stretched-mesh gillnets. Oreochromis mossambicus constituted 42.5% of the catch. Growth parameters of O. mossambicus, as estimated from the monthly size-frequency data, were L infinity = 39.3 cm total length and K = 0.48. It takes about 8 years to reach the asymptotic length, but the observed minimum and maximum lengths of 15.1 cm and 37.8 cm, respectively, would be reached in about 12 and 67 months. Ninety percent of the O. mossambicus catch were in the size range of 19.0-28.5 cm (131-439 g) and were second or third year fish. The mean growth rate, asymptotic length, and the growth performance index-phi of 2.60, of the population, as well as the exploitation rate of the fishery of 0.645, compare favourably with those of other populations and fisheries of O. mossambicus in the lowland irrigation reservoirs of Sri Lanka. The spawning of O. mossambicus in Victoria reservoir takes place throughout the year but appears to be enhanced during periods in which the water level fluctuations are relatively small.