CONSERVATION LIMNOGEOLOGY AT LAKE TANGANYIKA: NEW RESULTS FROM THE TUUNGANE PROJECT CO-MANAGEMENT AREA AT NORTHERN MAHALE (TANZANIA)

The Tuungane Project, a simultaneous population, health, and environmental issues initiative, has been active in the northern Mahale region (Kungwe Bay, Tanzania) of Lake Tanganyika since 2011. An important objective of the Tuungane Project has been co-management of coastal fisheries, which are threatened by climate change, sediment pollution from nearshore agriculture, and overfishing. Stabilization of the littoral fishery has focused on establishing small scale protected areas that are administrated by local beach management units, who are empowered to pursue and enforce sustainable fishing practices. Limnogeological research can feed valuable datasets to decision makers charged with conservation of fisheries. Side scan sonar (SSS) and bathymetric mapping in the northern Mahale region provides a new perspective on the distribution of benthic habitats, to include the spatial footprint of lake floor defined by ridges, boulders, or cobbles of metamorphic basement, which serve as a key fish breeding and nursery substrate. Other littoral substrates that are distinguishable from SSS data include sands, shell beds, and beach rocks; evidence for deltaic channels, mass wasting deposits, wave action, vegetation, and fish nests were also encountered. The controls on the distribution of substrate types, as well as isobaths, are interpreted to be normal fault distribution, patterns of deltaic sedimentation, and recent lake level changes. Across the sub-littoral zone of Kungwe Bay, a network of sediment cores dated using ²¹⁰Pb and ¹⁴C sheds new light on the spatial impacts of onshore land use change. Notably, onshore deforestation has been linked to sediment pollution and elevated sedimentation rates offshore, which can lead to fouling of benthic habitats. All sites, regardless of distance to deltas, show elevated mean linear sedimentation rates and sediment mass accumulation rates relative to control sites situated within a large protected area. Rates were laterally variable however, with the highest mean values recorded at sites in close proximity to more than one delta. The results of limnogeological research in the northern Mahale region can be used to inform fisheries conservation planning elsewhere in Lake Tanganyika and other tropical rift lakes.