At what concentration of dissolved oxygen (DO) in water do fish typically begin to die?

Fish typically begin to experience significant stress and eventually die when dissolved oxygen levels drop to around 2 parts per million (ppm). Although some fish may tolerate slightly lower levels for brief periods, anything below this threshold can lead to hypoxia, a condition where there is insufficient oxygen for their survival.

In particularly stressful scenarios, such as during a hot summer or overgrowth of algae, the capacity of water to hold dissolved oxygen decreases. Fish species have varying tolerances, but generally, at levels around 2 ppm, you are entering a critical zone where mortality rates increase.

At levels such as 1 ppm and lower, the situation becomes increasingly dire for aquatic life, leading to rapid deterioration in health and eventual death. In fact, levels at 0.5 ppm are often considered lethal for most fish, indicating that the survival threshold is very narrow and dependent on the overall health and species of the fish involved.

Consequently, while lower levels of dissolved oxygen can negatively impact fish, the level where they typically start to die is correctly identified around 2 ppm.