Researchers have found that decreasing fish populations in Lake Tanganyika, one of the most biodiverse freshwater environments, can be attributed to global warming. The study, just published in the journal Proceedings of the National Academy of Sciences, shows that overfishing alone isn’t to blame for crashing populations.
Lake Tanganyika is one of the oldest lakes in the world and houses a diversity of plant and animal species. It’s the deepest lake in Africa and the tropical conditions allow for a huge amount of biodiversity compared to other freshwater systems. Fisheries set up in the lake have become less and less productive since fishing operations started in the 1950s. This led researchers to assume that overfishing was the cause of reduced animal populations.
A research team analyzed the paleoecological record of Lake Tanganyika by examining layers of sediment at the bottom of the lake. They found that as the lake began to heat up 150 years ago, the diversity and quantity of animal species began to drop. This process began before the start of commercial fishing. The authors concluded that climate change, not overfishing, was the major cause of the lake’s reduced productivity.
As the water temperature of Lake Tanganyika increases, the bottom of the lake begins to lose oxygen. This harms animals that live on the lake’s floor, including mollusks such as snails. The increased water temperatures also affect seasonal mixing. Normally, as water temperatures change throughout the seasons, the top layer of water mixes with the bottom layer due to differences in density. This mixing allows oxygen to reach the bottom of the lake while nutrients from the bottom rise, providing food for algae. These algal blooms feed many of the lake’s residents, including fish. If global warming heats the lake, mixing won’t occur and less food will enter the ecosystem.
Climate change may be mostly to blame for the lake’s problems but the authors caution against ignoring the problem of overfishing. Increased fishing still contributes to the crashing fish populations and needs to be considered when planning for the future of the lake’s fisheries. The study highlights the importance of studying climate change in freshwater ecosystems.
Andrew S. Cohen et al. Climate warming reduces fish production and benthic habitat in Lake Tanganyika, one of the most biodiverse freshwater ecosystems. Proceedings of the National Academy of Sciences (2016).