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

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 - lead to regional fish populations with varying ecological adaptations. 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. However, the degree of regional isolation is insufficient to suppress lake-wide gene flow, suggesting that the fish form regional populations only on a basin- wide and seasonal scale. Based on {delta}15N and C:N ratios, we found no strong evidence for varying diets or lipid contents between those populations. Additional analyses revealed that isotopic variations between specimens from the same location are not linked to genetic differences. Our findings provide fundamental insight on the connectivity and ecology of Lake Tanganyikas pelagic fish and imply that sustainable management strategies may adopt basin-scale fishing quotas. Statement of significanceOnly little is known about how hydrodynamic gradients, leading to nutrient-richer and nutrient-poorer areas, induce ecological and genetic differences among populations of highly mobile, pelagic fish inhabiting these different areas. Here, we investigate the pelagic fish species of Lake Tanganyika, which support the second largest inland fishery in Africa. We characterize the effect of the basin-scale hydrodynamics - including a heavily stratified north and an upwelling-driven south basin - on the isotope signatures of the food web and examine the connectivity and ecology of the northern and southern fish populations in the lake. By combining our isotope data with genetics, we demonstrate that the fish form regional populations on a seasonal to multiannual time scale. Based on their isotope and elemental ratios, we found no significant differences in diet or lipid content between the regional populations. We suggest that the development of basin-scale ecological differences in response to the prevailing hydrodynamic regimes may be inhibited by lake-wide gene flow on the long term.