Switches between zoophagy, phytophagy and detritivory of Sarotherodon mossambicus (Peters) populations in twelve man-made Sri Lankan lakes

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.l.) of S. mossambicus ranged between 5–1 and 15–2. The m.r.i.l. decreased significantly with increasing body length: m.r.i.l. = 14–1 –0.018 T.L. ( r =–0.89; P <0.001).