Trophic Niche Differentiation and Mercury Levels in Cyphotilapia frontosa and C. gibberosa Along the East Coast of Lake Tanganyika

Diet, trophic niche, and mercury were examined for Cyphotilapia frontosa and C. gibberosa along the east coast of Lake Tanganyika, from northern Burundi to the southern region near the Zambian border. Mercury levels were generally low across populations, indicating that Lake Tanganyika is a low-mercury environment. However, fish from Burundi had significantly higher mercury concentrations (0.1 mg−kg) compared to all other sites, which showed mean mercury values less than 0.04 mg−kg. Diet analysis based on δ13C and δ15N revealed variation in both diet composition and trophic niche width along the lake. Four sites exhibited isotopic niche overlap with their adjacent location, forming two clusters of sites with similar trophic structure. In contrast, several other sites showed little to no overlap, indicating greater isotopic differentiation. Because detailed dietary information for Cyphotilapia is lacking, we selected nine prey groups and compiled their isotopic signatures from the literature for use in MixSIAR dietary analysis. The model indicated clear dietary differentiation among sites. Mussels, jellyfish, and fish were important dietary contributors, while zooplankton, snails, and crustaceans contributed moderately. Insects formed only a very small contribution to the diet. The relative importance of each prey group varied among sites, demonstrating differences in resource use along the lake.