The diet of 1200 adult Sarotherodon mossambicus [16-37 cm total length (T.L.)] in 12 man-made Sri Lankan lakes, belonging to five different irrigational systems were studied over four climatic seasons using volumetric analysis. Food differed between seasons and reservoirs but overall, the diet consisted of 2, 2, 6 and 15 genera of diatoms, blue-green algae, green algae and animal material respectively. However, none of the S. mossambicus populations maintained either a detritivorous, phytoplanktivorous or a zooplanktivorous habit through the year. Detritivory was most common during the rainy season while phytoplanktivory predominated in the dry season. Blue-green algae were not an important constituent of the diet. Out of the 12 reservoir populations. five were predominantly detritivorous, six phytoplanktivorous and one zooplanktivorous. Detritus was completely absent in the ingested material in certain populations when they devoured animal or plant material, indicating the ability of S. mossambicus to switch from deposit to suspension feeding. The mean relative intestinal length (m.r.i.1.) of S. mossambicus ranged between 5.1 and 15.2. The m.r.i.1. decreased significantly with increasing body length: m.r.i.l.= 14.1 -0,018 T.L. ( r = -0.89; P

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The nutritive quality of ingested material of adult Oreachrumis mossumhicus wild populations from 12 perennial. man-made lakes were evaluated for four climatic seasons. The seasonal variation in condition of each population, and the overall condition of each population, considered as the theoretical weight of a 20 cm individual, was computed from the length-weight regressions, which were curvilinear. The mean overall condition of the 0. mossumhicus reservoir population was 152.6 g (~0 . k 9 . 5 ) and ranged between 139.8 and 167.8 g. Distinct patterns in the seasonal changes in condition of an individual population were not apparent. The mean percentage of protein and total organic matter (TOM) and calorific content (kJ g -' ) of the ingested material were 20.7% (s.D. 3.7; range 9.6-35.2%), 46.0% (s.D. k9.8; range 20,9-73.7%) and I 1.6 (s.D. f 3.4; range 4.6-20.9) respectively. The amount of each of the above components for any one population varied seasonally in relation to the changes in feeding habit. The protein content of the ingested material of those populations devouring predominantly detritus also showed seasonal variability. The amount of protein (mg) in the digestible total organic matter (DTOM) and the calorific content (Cal) in the ingested material were rclated to the TOM content as follows: DTOM=3.21 (TOM)-47.8 (d.f.40; r=0,89; P

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.

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