Abstract:
The introduced species Oreochromis niloticus in Sri Lankan reservoirs contributes
significantly towards the inland fish production which accounts for over 60 percent of inland
fish. However, the existence of O. niloticus in Sri Lankan reservoirs is often debated for
threatening the indigenous fish fauna excluding the opinion, “O. niloticus as a species for
filling the vacant niches in Sri Lankan reservoirs”. In this context, the present study aims to
understand the food and feeding habits of introduced cichlid O. niloticus and two indigenous
fish species Wallago attu and Dawkinsia singhala in Nagadeepa Mahawewa, Badulla, Sri
Lanka. A total of 91 middle size fish from Oreochromis niloticus (n = 38), Wallago attu (n =
16) and Dawkinsia singhala (n = 37) belongs to three fish species groups were sampled and
analyzed for stomach content analysis. The stomach contents were identified and analyzed
using the frequency of occurrence methods. The trophic niche breadth was calculated using
Levins' measure of the standardized trophic niche breadth and Pianka's index was used to
calculate dietary overlap between the species. Length-weight relationship of W. attu and D.
Singhala was also determined. The length-weight relationship of O. niloticus and W. attu
indicated that isometric growth (b~3) and the condition factor (K) (O. niloticus;1.89 and W.
attu; 0.53) indicate the healthiness of both species. Diverse food items ranging from
phytoplankton to macrophytes in the fish’s stomach were recorded. Food items of the O.
niloticus and D. singhla indicated that the omnivore feeding behaviour while the W. attu
exhibited carnivore. The standardized niche breadths of the three species were ranging from
0.94-0.18 indicating the highest niche breadth for O. niloticus and the lowest for W. attu.
Pianka’s index revealed that, high dietary overlap (1.43) between O. niloticus and D. singhala
and lowest dietary overlap (0.32) between O. niloticus and W. attu. The higher dietary
overlap between O. niloticus and D. singhala and may be linked to increase the intraspecific
competition under resource depletion which is highly unlike in Sri Lankan reservoirs.
Investigations through more accurate and advanced methods (i.e., stable isotope analysis)
may be important for further delineating the trophic relationship among the species.
