Food and micro-habitat partitioning among cyprinids in Sri Lankan hill-stream pools

Abstract

Cyprinid fish assemblages in three stream pools in southwest Sri Lanka, were investigated with the objective of analysing species segregation, niche complementarity and degree of resource partitioning. Feeding ecology was studied by gut contents analysis and by direct underwater observations. Dietary data were related to the micro-distribution of the fish and to their horizontal distribution, the latter taken from previously published literature. Diet overlaps were calculated for each pool community and for the community as a whole. Resource partitioning among assemblage members was by diet, micro-distribution and horizontal macro-distribution. Among the three feeding substrates (rocky substrates, sand, dead leaves) rocky substrate was the most preferred, but other substrates, especially dead leaves, were used as well. Only Rasbora daniconius, which mainly fed high in the water column, fed always on rocky substrates at the relative rare occasions that it chose a firm substrate. Pethia nigrofasciata was the only species with a clear preference for sand. The assemblage could be divided into three guilds: (1) the surface feeders (Devario malabaricus, Rasbora daniconius) fed largely on terrestrial arthropods, (2) one generalist Systomus pleurotaenia which fed on all five strata, largely on terrestrial arthropods, ephemeropteran and chironomid larvae. (3) the bottom feeders (Dawkinsia singhala, Pethia cumingii, Pethia nigrofasciata, Puntius dorsalis) who fed preponderantly on chironomid larvae. Of the three niche dimensions, horizontal macro-distribution, micro-distribution and diet, horizontal distribution may be the strongest segregating factor among these fish species. The contribution of diet was substantial only between feeding guilds. A relatively high degree of endemism (ca. 40 % is endemic to Sri Lanka) and habitat partitioning suggests that the cyprinid assemblages studied are predictable, co-evolved systems with competition serving as an importing structuring factor.