Water quality management in net pen aquaculture is a primary welfare determinant for farmed fish, affecting every aspect of fish physiology and behavior.
Water quality is the invisible determinant of farmed fish welfare - poor water quality causes chronic physiological stress, suppressed immunity, and in extremis, mass mortality events. Fish in net pens with inadequate dissolved oxygen show accelerated breathing, reduced feeding, and crowding near the water surface - all welfare-significant stress responses. Sea lice cause welfare harm through tissue damage at attachment sites, blood loss, and secondary infection, in addition to the systemic stress of chronic parasitism. The challenge of maintaining adequate water quality in marine net pens is compounded by environmental variability - algal blooms, temperature fluctuations, and storm events - that require responsive management. Certification schemes that require water quality monitoring and response protocols provide some welfare assurance.