Seasonality in diet and feeding habits of the endemic Chala tilapia ( Oreochromis hunteri ) and two introduced tilapiine cichlids in Lake Chala, East Africa

Oreochromis cf. korogwe and Coptodon rendalli , two tilapiine fishes introduced to Lake Chala (Kenya/Tanzania) ~ 40 years ago, may negatively affect the endemic Chala tilapia ( Oreochromis hunteri ) by competition for food. However, the actual threat posed by the introduced cichlids cannot be assessed without data on the local feeding habits of all three species. This paper describes the diet of O. hunteri and both introduced species, focusing on seasonal changes in food-source availability. Microscopic analysis of gut content in 35 fishes collected over a 20-month period was compared with the composition of littoral food sources and seasonal variation in the limnetic phytoplankton and complemented with exploratory stable-isotope analysis of fish tissue. We found that all three tilapiines in Lake Chala are herbivorous, and during most of the year, they feed on algae and organic detritus on and between rocks in the littoral zone. However, in July–September they migrate to open water to feed on blooming phytoplankton. Interspecific differences in gut content and stable-isotope composition suggest that O. hunteri may have a competitive advantage over the two newcomers by making better use of this seasonal extra food resource. However, this advantage may erode when continuing climate change starts affecting the bloom’s predictability.