Water quality in aquaculture systems is the most fundamental determinant of fish welfare, with dissolved oxygen, temperature, pH, ammonia, and pathogens all affecting fish health and behaviour.
Water quality management is the most direct welfare intervention in aquaculture because it affects every physiological process of every fish in the system simultaneously. Poor water quality - low oxygen, high ammonia, inappropriate temperature - causes chronic physiological stress that impairs immune function, reduces growth, causes pain-like experiences, and ultimately leads to mortality. The welfare and production case for good water quality management is identical, which creates a strong business case for investment. However, economic pressure leads many producers to run systems at the edge of acceptable parameters, accepting some welfare cost for reduced operational cost. Welfare certification schemes that specify water quality parameters provide an external check on this tendency.