The Common Causes of Fish Death in Aquaculture

In aquaculture, good production is attained by maintaining good growth, high survival rate and good fish condition and appearance. This could be achieved with good aquaculture practices, good feeding regime and maintaining healthy stocks. The water in which the fish lives contributes significantly to the overall health and well-being of the fish. Furthermore, presence of pathogens including fungus, bacteria, virus and parasites could bring harm to fish stocks and disturb the system. Regular monitoring of water quality and daily assessment of fish condition could help fish growers prevent further imbalance in the system or possible infection if there is, which could lead to disease and even death and massive fish kills. Although prevention is always better than cure, fish farmers also must be knowledgeable about the specific treatment to a specific fish disease just in case.

The following practices must be observed to make sure fish stocks are healthy:

1. Use pathogen-free water
2. Buy stocks only from reputable source; if possible, with certificates proving that stocks are not carrying any diseases.
3. Strictly observe cleanliness and biosecurity all though-out the rearing system, sanitize when necessary and monitor water quality.
4. Make your culture system as natural as possible for the fish; create artificial habitats when necessary
5. Maintain proper stocking density to reduce stress
6. Feed fish with high quality balanced diets observing appropriate feeding rate and feeding frequency to lessen ammonia level in the water that could intoxicate fish stocks

In checking fish condition, which must be done daily even during feeding time, look for the following common signs of unhealthy fish:

1. Gasping of air at the water surface
2. Rubbing of fish on the sides of the tanks
3. Lethargy
4. Abnormal swimming behavior
5. Redness or discoloration of fins, gills, eyes and body
6. Death

Water quality problems that affect fish health:

It is most likely due to the environmental condition if significant number of fish are showing signs of stress or disease. Levels of ammonia, nitrite, pH, dissolved oxygen and temperature must be checked, immediately and respond accordingly.

Ammonia and Nitrite

Overfeeding and high fish density could lead to higher ammonia level if aeration system is not enough. However, in aquaponics ammonia are converted to nitrate and used up by plants to grow. Thus, if balanced number of fish and plants are maintained as well as correct feeding practice is observed in fish, ammonia and nitrite poisoning is not much of a problem.

Signs of ammonia and nitrite toxicity:

• fish gasping for air
• swimming near the water surface
• dark gills
• lethargy
• redness of eyes and fins

Remedies:

• Reduce the number of fish stocks
• Check pumps and measure dissolved oxygen
• Add number of plants when necessary
• Check the feeding ration

pH

In aquaponics, pH should be monitored strictly and must be maintained between 6.4 – 7.4 to be tolerable for fish, plants and the nitrifying bacteria.

Signs of pH stress in fish:

• Hyperactive swimming behavior
• Increased mucus production
• Difficulty in breathing
• Opacity in skin and gills

Remedies:

• Change water
• To increase pH, add potassium bicarbonate
• To lower pH, add phosphoric acid

Common Diseases

Here are some of the most common pathogens in aquaculture. Familiarize yourself with them as they become critical to growing healthy and efficient fish.

• Fungal Diseases
• Bacterial Diseases
• Parasitic Diseases
• Viral Diseases