Dissolved oxygen is the most critical water quality parameter for farmed fish welfare. Low oxygen causes acute suffering and is a major preventable source of mass mortality in aquaculture systems.
Oxygen deprivation causes suffering in fish through a process similar to suffocation. Fish deprived of oxygen show escape behaviour — crowding at the surface or water inlets — before progressive weakening and death. The suffering during hypoxic events is acute and can be prolonged over hours before mortality. Oxygen depletion events, though sometimes natural in origin, are often exacerbated by management decisions — overstocking, inadequate monitoring, or failure to respond to early warning signs. Mortality events represent both welfare failures and significant economic losses, yet monitoring and aeration infrastructure is still inadequate in many operations.