VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

ENVIRONMENTAL HYPOXIA

Definition

• Environmental hypoxia refers to the condition with  low concentration of dissolved oxygen (DO) that exists in water. Oxygen is the most important water quality factor for proper fish health, but it is poorly soluble in water.

Sources of oxygen in the culture system

• Oxygen can enter water through        
  • photosynthesis
  • diffusion of atmospheric oxygen.
• In a pond without mechanical aerators, photosynthesis is the most important source of oxygen. A certain amount of algae in a pond is desirable, since it increases the oxygen production and thus allows a greater number of fish to be stocked. 
• Oxygen concentration is highest near sunset because net oxygen production occurs during the day. 

Causes of environmental hypoxia

• Low oxygen is common in ponds especially in summer, when warm temperature both decrease the oxygen solubility and increase the pond organism’s metabolism and subsequent oxygen demand.
• Cloudy weather decrease photosynthesis and thus reduces net oxygen production.
• A crash or massive death of algae, a common but usually unpredictable event, can cause severe oxygen depletion.

Fish kills due to Hypoxia

Source: www.greenwichtime.com 

Clinical signs of environmental hypoxia

• Acute environmental hypoxia is defined as a rapid drop in dissolved oxygen (DO) to lethal or near-lethal level. It is often accompanied by acute and frequently catastrophic mortalities.
• Common behavioural signs include lethargy and the congregating of fish near the air-water interface, where oxygen levels are the highest.
• Hypoxic fish are often anorexic. A classical sign of asphyxiation is an agonal response, with the mouth open and the opercula flared.

Diagnosis of environmental hypoxia

• Definitive diagnosis of low DO can only be done by measuring the DO in water at the site or by immediately preserving the water sample.

Treatment of environmental hypoxia

• Acute environmental hypoxia is an emergency situation and immediate step must be taken to provide fish with oxygenated water. For large culture system holding high fish densities, this means having adequate emergency aerators and power sources to handle hypoxic event.
• Supplemental aeration should begin if DO drops below 3-4 mg/l. Aeration equipment includes both pneumatic and mechanical devices. Water may also be transferred from an adjacent and to the hypoxic pond. Oxygenated well water may also be used.
• If pond fish are raised at high densities, keep constant vigilances for environmental hypoxia. An oxygen meter is essential for a commercial aquaculture enterprise.

Aerator (Paddle wheel) in a pond

Source: www.aquaculture-product.com