Methane (CH4), marsh gas is formed during decomposition (fermentation product) of organic matter in bottom of marches, ponds and lakes. Concentrations as high as 40 ml/l have been recorded in certain bottom waters. A major effect in such cases is also physical in that methane would effectively displace other gases including oxygen in upper layers of water when it bubbles up. Due to the rarity of such occurrence it does not deserve further consideration with reference to aquaculture.
