Muddiness

Freshwater fish sometimes carry an unpleasant muddy aroma. It’s most often encountered in bottom-feeding fish, especially catfish and carp that are raised in ponds dug directly in the earth. The chemical culprits are two compounds that are produced by blue-green algae, especially in warm weather (geosmin and methylisoborneol). These chemicals appear to concentrate in the skin and the dark muscle tissue, which can be cut away to make the fish more palatable. Geosmin breaks down in acid conditions, so there is a good chemical reason for traditional recipes that include vinegar and other acidic ingredients.