Observations on the abundance of the exotic cichlid Sarotherodon mossambicus (Peters) in relation to fluctuations in the water-level in a man-made lake in Sri Lanka

Abstract. The fishery of Parakrama Samudra, an ancient man-made lake of 2662 ha in Sri Lanka (8°-10°N; 80°-82°E), as in most other man-made reservoirs in the island, is dominated by the exotic cichlid Sarotherodon mossambicus (Peters), introduced in 1952. The fluctuations in catch per unit/fisherman over the period 1960 to 1982 are shown to be dependent on the yearly fluctuations in the water-level, the changes in the water-level effects being manifested in the fishery after a period of 3 years. The above results were arrived at by auto-correlation analysis of yearly catch per unit effort against the fluctuation of the water level at the main sluice. The changes in yield in a particular year are correlated to the changes in the water level and are described by the equation;
where Y = yield in kg fisherman^-1 month^-1 in year t and X = fluctuations in the water-level in year t - 3. The available growth rates in natural and semi-natural impoundments also indicate that the size at which S. mossambicus enter the fishery in Parakrama Samudra corresponds to their size in the third year of life (age 2+). The significance of the above observations is discussed on the basis of available knowledge on the reproductive cycle and the nesting requirements of S. mossambicus which are achieved when significant fluctuations in the reservoir level take place by making available optimal water depths in areas with a suitable substratum. The possible use of the above criteria as a management measure is also briefly discussed in the light of possible irrigational and fisheries conflicts in the utilization of the reservoir resources.