a measure of how acidic or alkaline a solution is, i.e. the concentration of hydrogen ions in a solution (log to base 10 of the reciprocal of the hydrogen ion concentration). pH 7.0 is neutral, lower values are acidic and higher values are alkaline. Acid rain decreases the natural pH of rivers and lakes. Some fish require a specific pH, others will live at wide range of values: the general range for fish is 6-9. The internal pH of a fish falls after death but rises as amines are formed during spoilage
Dictionary of ichthyology. 2009.