Locally produced fish food pellets yield little evidence as a dietary substrate for net pen-reared Nile tilapia (Oreochromis niloticus) in Lake Victoria, Kenya

Net pen-reared Nile tilapia ( Oreochromis niloticus ) and samples of fish food provisioned to fish cages were collected from two aquaculture facilities operated in Winam Gulf, Lake Victoria, and another open lake site on Lake Victoria, Kenya. Samples of fish food and net pen-reared fish were analyzed for stable isotopes and total mercury (THg) concentrations. Values of δ 13 C in net-pen fish were 9.4–12.7 ‰ higher compared to values in provisioned food. Values of δ 15 N in fish were 5.36–5.66 ‰ above those measured in the provisioned food at two fish cage facilities in Winam Gulf but not significantly different from one another at the open lake facility. Stable isotopes and THg in aquaculture fish from Winam Gulf were compared against wild-caught lower food web samples from the same region. Residues of THg and δ 15 N in net-pen fish were similar to wild conspecifics from Winam Gulf. Values of δ 13 C tended to be elevated in net pen-reared fish compared to wild conspecifics but found to be within the range of δ 13 C values measured in lower trophic level freshwater shrimp ( Caridina niloticus ) collected from diverse locations throughout Winam Gulf. Overall, isotope signatures of net pen-reared fish and food provisioned to cages were dissimilar, whereas stable isotope signatures and THg in net-pen fish resembled patterns observed in wild-caught Nile tilapia and other lower food web items from Winam Gulf. The results imply net pen-reared fish are not benefiting from food provisioning and that aquaculture operators could realize economic benefits and lower their environmental impacts by reducing food provisioning rates.