Oxygen is constantly entering and leaving water, but there is a certain
amount of oxygen in water at all times. This is because water has a natural
attraction to oxygen. When oxygen comes in contact with the surface of
water, the oxygen enters the water, becoming dissolved oxygen.
A concentration of five parts per million (ppm) (=
5/1,000,000)
or higher is recommended for optimum fish health.
Most species of fish are distressed when DO falls to 2-4 ppm.
Mortality usually occurs at concentrations less than 2 ppm.
The number of fish that die during an oxygen depletion event is
determined
by how low the DO gets and how long it stays down.
Usually larger fish are affected by low DO before smaller fish are.
The amount of attraction between oxygen and water depends on the
amount of oxygen already in the water. If there is very little oxygen
in water, then the water is very attractive to oxygen. But when water
has a high concentration of DO, then the water is saturated,
meaning that the water contains as much oxygen as it can hold.
Warm water is less capable of holding oxygen in solution than cool
water. For example, water that is 90° F can only hold 7.4 ppm DO at
saturation, whereas water that is 45° F can hold 11.9 ppm DO at
saturation.
Warm water puts the fish in double jeopardy because at high water
temperatures their metabolic rates increase, hence their physiologic
demand for oxygen increases.
A fish's metabolism also helps determine the
amount of oxygen which a fish needs to survive. Fish with a high
metabolism are fast-moving but also require a great deal of oxygen
to survive. Fish with a slow metabolism are more sluggish and
require less oxygen.
Carp for an example which is a warm water fish can survive in
water with as little as 3 ppm of oxygen, while largemouth bass
require 5 to 8 ppm.
