Comparative feeding efficiency of indigenous and introduced phytoplanktivores from Lake Victoria : Experimental studies on Oreochromis esculentus and Oreochromis niloticus

Abstract

The ngege, Oreochromis esculentus Graham (Perciformes;Cichlidae), was a numerically important phytoplanktivore inLakeVictoria, East Africa prior to its extirpation and subsequentreplacement following the introduction of its sister species,O. niloticus Boulenger (Cichlidae). The disappearance of O. esculentus has been attributed to predation, competition,andintrogression by introduced species and to overfishing. Anoverlooked factor is a shift in algal community dominance inLakeVictoria, from large filamentous diatoms (a preferred fooditem ofO. esculentus) to smaller colonial bluegreen and greenalgae.We performed a series of experiments to test the hypothesisthatfeeding performance of the two species differs sufficientlythatalgal shifts could have contributed to the speciesreplacement.Adults of the two tilapiine species were fed individually on amonospecific culture of the small green alga Scenedesmusacuminatus, which grows in a range of colony sizes. Clearingrateswere compared at 4 algal food densities and at the 4 colonysizes.The results indicate that O. niloticus has the abilitytofeed on either smaller or less bulky particles than O. esculentus such as prevail in the postdisturbance LakeVictoria. O. niloticus is also a more efficient filterfeederoverall, with a differential that is inversely proportional toparticle size. Additional data would be required to show thatfeeding performance and limnological shifts were primary tothereplacement of O. esculentus by O. niloticus.Researchers should however, be cautious about invokingcompetitionas the proximal cause of exotic impacts when simpler,extrinsic mechanisms are equally plausible.