Niche variation between fish communities as reconstructed from stable isotopes in two ecologically different Ethiopian Rift Valley lakes

Pieter Lemmens, Fassil E. Teffera, Maarten Wynants, Lynn Govaert, Jozef Deckers, Hans Bauer, Feleke Woldeyes, Luc Brendonck, Steven Bouillon and Luc De Meester

The concept of species niches – the role they play in the ecosystem – helps us to understand the structure and dynamics of biotic communities and food webs. An increasing number of studies indicates that variation in niches among individuals of the same species is an important determinant of food web characteristics.

In the present study, we compare the feeding niche of the fish community in two iconic Ethiopian Rift Valley lakes, Lake Abaya and Lake Chamo. These lakes differ strongly in ecology. Lake Abaya is a turbid lake with low primary production, whereas Lake Chamo is a clear-water lake with high primary production. Using stable isotopes of carbon and nitrogen, which allow us to determine what fish are eating, we investigate how far differences in fish feeding ecology between the two lakes are mediated by changes in species composition or rather by variation in feeding niche between individuals of the same species.

While we do not observe significant differences in fish community composition between the lakes, our results show that the feeding niche of the fish community in Lake Abaya is larger and more diversified compared to that in Lake Chamo. Fishes in Lake Abaya feed mainly on less nutritious sediment organic material, while zooplankton is a major food source for fish in Chamo. We show that observed differences in feeding ecology result from differences between lakes in the niches of individuals of the same species rather than from differences in fish species composition.

We argue that the observed differences in fish feeding ecology between both lakes likely result from the strong difference in primary production between the lakes. Fishes in Lake Chamo rely mainly on local primary production by feeding on zooplankton, whereas fishes in Lake Abaya seem to be forced towards consumption of alternative sources due to the much reduced primary production. Our study highlights the importance of niche variation among individuals of the same species for the feeding ecology of fish communities inhabiting two large Ethiopian Rift Valley lakes with distinct environmental conditions.

Image caption: Satellite image showing Lake Abaya and Lake Chamo (left and right respectively) (Landsat-8 image, 2016).
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