Mercury in fish from three rift valley lakes (Turkana, Naivasha and Baringo), Kenya, East Africa

Abstract

Total mercury (THg) concentrations were measured for various fish species from Lakes Turkana, Naivasha and Baringo in the rift valley of Kenya. The highest THg concentration (636 ng g −1 wet weight) was measured for a piscivorous tigerfish Hydrocynus forskahlii from Lake Turkana. THg concentrations for the Perciformes species, the Nile perch Lates niloticus from Lake Turkana and the largemouth bass Micropterus salmoides from Lake Naivasha ranged between 4 and 95 ng g −1. The tilapiine species in all lakes, including the Nile tilapia Oreochromis niloticus, had consistently low THg concentrations ranging between 2 and 25 ng g −1. In Lake Naivasha, the crayfish species, Procambrus clarkii, had THg concentrations similar to those for the tilapiine species from the same lake, which is consistent with their shared detritivore diet. THg concentrations in all fish species were usually consistent with their known trophic position, with highest concentrations in piscivores and declining in omnivores, insectivores and detritivores. One exception is the detritivore Labeo cylindricus from Lake Baringo, which had surprisingly elevated THg concentrations (mean=75 ng g −1), which was similar to those for the top trophic species ( Clarias and Protopterus) in the same lake. Except for two Hydrocynus forskahlii individuals from Lake Turkana, which had THg concentrations near or above the international marketing limit of 500 ng g −1, THg concentrations in the fish were generally below those of World Health Organization's recommended limit of 200 ng g −1 for at-risk groups.