Fish fuelled by methane

Image above: In the deep waters of eutrophic reservoirs and lakes, the decomposition of organic matter produces methane, which serves as a source of carbon and energy for specialised groups of microorganisms, primarily methanotrophic bacteria. Photo: Jiří Peterka

Methane is produced at the bottom of freshwater reservoirs during the decomposition of organic matter. Before it escapes into the atmosphere, a substantial portion of it is consumed by specialized microorganisms known as methanotrophic bacteria. These microbes incorporate methane-derived carbon into their biomass. They are then eaten by small invertebrates. When fish feed on these invertebrates, carbon originally derived from methane reaches the top of the food chain. Scientists now recognise that this pathway can support a substantial portion of invertebrate biomass in some freshwater ecosystems, however the transfer of methane-derived carbon to higher consumers such as fish has rarely been explored.

In the deep, nutrient-rich Římov Reservoir in South Bohemia, researchers investigated the origin of carbon in fish bodies. “In our study, we used naturally occurring stable isotopes to test the importance of methane as a potential source of carbon and energy for common fish species,” says hydrobiologist Mojmír Vašek from the Biology Centre of the Czech Academy of Sciences. “We found that although fish rely mainly on primary production based on photosynthesis, a small yet significant part of their biomass originates from methane. A key role is played by midge larvae living in the dark, deep parts of the reservoir. By consuming methanotrophic bacteria, they can obtain more than half of their carbon from methane,” he explains.

Methanotrophic bacteria serve as a valuable food source for midge larvae of the genus Chironomus. These larvae can obtain more than 50% of their carbon biomass from methane by consuming these bacteria. Photo: Yevdokiia Stepanyshyna

Midge larvae thus act as a crucial link transferring methane-derived carbon to fish. They are primarily eaten by bottom-feeding species such as common bream, whose protrusible mouth allows it to suck larvae from soft sediments, and ruffe. In contrast, species feeding mainly in open water or near the surface, such as roach or bleak, showed only a minimal contribution of methane-derived carbon.

The results suggest that methane is not only a gas escaping into the atmosphere, but in reservoirs and lakes it can also serve as an energy source for aquatic organisms. This process, known as microbial chemosynthesis, contributes to fish production in nutrient-rich waters. Because methane is a potent greenhouse gas, a better understanding how it is recycled through aquatic food webs may help refine our view of its role in the global carbon cycle.

Midge larvae play an important role in transferring methane-derived carbon to higher levels of the food chain, including fish. The common bream (pictured) has a protrusible mouth that allows it to suck midge larvae from the muddy bottom. Photo: Jiří Peterka

The ruffe also feeds on midge larvae. In our study, we found that up to 30% of the carbon in the biomass of some individuals originated from methane. Photo: Jiří Peterka

Publication:

Vašek M., Blabolil P., Šmejkal M., Tušer M., Bartoň D., Draštík V., Kočvara L., Kubečka J., Říha M., Jůza T. (2026). Methane-derived carbon contributes to fish biomass in a deep, eutrophic reservoir. Freshwater Biology 71: e70200. https://doi.org/10.1111/fwb.70200