Title: Trophic levels of pelagic fish species from Lake Tanganyika
Author: Demmy Namutebi
Supervising Institution: IHE Delft - Institute for Water Education
The pelagic fish community of Lake Tanganyika comprises of four Lates species and two sardines all of which are endemic to the lake. Studies from the last two decades discuss the downward trend in fish catches at Lake Tanganyika (Van Zwieten et al, 2002; Van der Knaap et al., 2014). For this downward trend, two possible reasons are discussed; overfishing and climate change. In order to understand the influence of these two mechanisms, knowledge about the trophic ecology of the pelagic fish, life history and geographical variations in the food web are needed. In pursuite of such knowledge, I investigated stable isotopes of fish muscles and stomach content of fish. I calculated the trophic positions of the different pelagic fish species, tested whether the species shift habitat during growth and whether fish from the Southern basin show a distinct isotopic signature compared to fish from the Northern basin. I find variation in trophic positions of the same species between sites. Perhaps most strikingly, I find that the herring Limnothrissa miodon and the perch Lates stappersii previously considered to be prey and predator to occupy identical trophic positions in many but not all places. This suggests that these two species act more as competitors in some places but perhaps as predator and prey in other places. Additionally L. stappersii and Stolothrissa tanganicae spend their life entirely in pelagic waters and L. angustifrons shows a clear habitat change from the littoral to the pelagic zone with growth. Furthermore, I find a distinct difference in isotopic signatures between fish caught in the North and fish caught in the South and also between samples from different seasons. Overall, my results find local differences in the food web and isotopic signatures. I therefore conclude that future research and management plans should consider the local differences in Lake Tanganyika’s pelagic food web.