Isotopic signatures induced by upwelling reveal regional fish populations in Lake Tanganyika

Abstract

Lake Tanganyikas pelagic fish sustain the second largest inland fishery in Africa and are under pressure from heavy fishing and global warming related increases in stratification. Only little is known about whether basin-scale hydrodynamics - including a more stratified north and an upwelling-driven south - induce ecological and genetic differences among populations of highly mobile, pelagic fish inhabiting these different areas. Here, we examine whether the basin-scale dynamics leave distinct isotopic imprints in the pelagic fish of Lake Tanganyika, which may reveal differences in habitat, diet, or lipid content. We conducted two lake-wide campaigns during different seasons and collected physical, nutrient, chlorophyll, phytoplankton and zooplankton data. Additionally, we analyzed the pelagic fish - the clupeids Stolothrissa tanganicae, Limnothrissa miodon and four Lates species - for their isotopic and elemental carbon (C) and nitrogen (N) compositions. The {delta}13C values were significantly higher in the productive south after the upwelling/mixing period across all trophic levels, implying that the fish have regional foraging grounds, and thus record these latitudinal isotope gradients. By combining our isotope data with genetics, we demonstrate that the fish form regional populations on a seasonal to multiannual time scale. Based on {delta}15N and C:N ratios, we found no strong evidence for varying diets or lipid contents between those regional populations. Additional analyses revealed that isotopic variations between specimens from the same location are not linked to genetic differences. We suggest that the development of basinscale ecological differences in response to the prevailing hydrodynamic regimes may be inhibited by lake-wide gene flow on the long term. Our findings show that the pelagic fish species are genetically adapted to the whole lake, but they form regional populations on short time scales. This implies that sustainable management strategies may adopt basin-scale fishing quotas.