Response of methane gas fluxes to cage aquaculture in African lakes-Kivu and Muhazi, Rwanda

Aquaculture in floating cages has rapidly expanded in African lakes, driven by economic development and the need to increase fish production. However, this development has raised concerns about environmental impacts, including potential alterations of the benthic processes that could contribute to increased greenhouse gas emissions from cage culture sites which have not been measured so far. This study examined methane concentrations and fluxes from tilapia cage farming in Lakes Muhazi and Kivu, currently some of the major sites for cage aquaculture development in African lakes. In Lake Kivu, methane fluxes (FCH₄-C) varied between 0 - 45.6 mg/m²/h in sites with cages, and 0 - 5.1 mg/m²/h in sites without cages. In Lake Muhazi, the FCH₄-C varied between 0 - 67.6 mg/m²/h in sites with cages, and 0 - 7.7 mg/m²/h in sites without cages. Methane fluxes generally were higher in the shallow Lake Muhazi than in the deep Lake Kivu. Cage farms in bays exhibited higher fluxes than the cages located on open water. These findings suggest a contribution of cage aquaculture to methane fluxes in lakes, especially in shallow and enclosed areas. Understanding the dynamics of methane fluxes in response to aquaculture practices is essential for developing strategies for sustainable aquaculture that balance fish production in cages with environmental conservation.