The hot season in the Mekong Delta and the Southeast region starts from early March and lasts until the end of June. The outdoor temperature can be up to 38 – 40°C, the surface water temperature in the pond and in rivers can also go above 32°C. Fish during this time are often stressed by high water temperatures.
Fish are cold-blooded species – also known as thermogenic animals. They cannot control their own body temperature, instead, they rely on the temperature of the surrounding water to regulate their body’s metabolism. Therefore, periods with high temperatures that are far from the optimum for physical development will reduce fish growth as well as cause bacterial diseases.
Organisms in the water such as farmed fish, plankton, other animals, plants and bacteria need more oxygen for their respiration when the water temperature rises. However, when the water warms up, the amount of dissolved oxygen in the water decreases, become insufficient for farmed fish, causing them to become stressed.
Toxicity of ammonia increases as water temperature rises, while dissolved oxygen in the water decreases making fish more susceptible to disease and, in severe cases, the fish can suffer internal organ damage, respiratory arrest. When the water temperature is above 32°C, the beneficial bacteria that maintain the water quality balance gradually decrease, leading to a dangerous accumulation of ammonia and nitrite that is harmful to fish. The high water temperature in the hot season will also help the Aeromonas hydrophila bacteria, which cause hemorrhagic disease and Streptococcus agalactiae (protrusion of the eyes and abdomen), to increase in number in the water. Fish loss is mainly due to these two diseases during the hot season.
In order to maintain the efficiency and profit of the farm, fish farmers must have appropriate preventive approach to minimize the impact of rising water temperature in the hot season, especially for breeding systems such as ponds and outdoor rafts.
Below are some suggestions to help fish increase stamina and reduce loss in the hot season.
1.Increase dissolved oxygen in water
Water with a high temperature will have a lower concentration of dissolved oxygen than water with a low temperature. At the same time, the fish are more active and consume more oxygen as the water temperature rises. Water aeration or water fan (water pedal) is an effective solution to help supplement dissolved oxygen in water and stabilize water temperature.
To recognize whether the fish is deprived of oxygen or not, observe the fish’s activity. Anoxic fish will concentrate more at the surface of the water, sucking air bubbles continuously or gather close to where the water enters the pond.
2. Adjust the amount of fish feed
When the water temperature rises, fish tends to hunt for food more. The amount of fish feed consumed at 32°C will be higher than at 30°C. However, this characteristics does not mean fish will eat more when it is hot. It is recommended that fish feed amount used at 30°C water temperature be used as a standard amount for feeding fish when the water temperature is at 32°C. Feed the fish slowly according to the fish’s bait rate. Do not feed fish in raft when the water is standing still because at this time the dissolved oxygen in the water is lower than when the water is flowing. It is best to feed the fish when the tide is nearing its peak or starting to recede. In general, only 80-90% of the food the fish can catch should be fed. Cease the feeding when the level of fish focus on catching bait decreases.
3. Separate fish, reduce density
During the peak period of the hot season (April and May), there should be a plan to separate the fish. The idea of initially distributing fish at high density, then let the loss rate gradually decrease the fish number to achieve the desired density is quite popular among Catfish and Red tilapia farmers. High fish density will increase oxygen consumption in the water, which in turn will decrease dissolved oxygen levels. In addition, high density fish will also increase the amount of waste. These two factors will reduce the biological treatment capacity of the farming system itself.
4. Clean up waste in ponds and rafts
Excess food, fish carcass and mud accumulated at the bottom of the pond are not good for ponds and rafts environment, especially in the hot season. As these harmful components decompose, they create toxic ammonia gas and reduce dissolved oxygen in water. When the water temperature is higher than normal, besides the reduced amount of dissolved oxygen, a high ammonia content in the water will also be toxic to fish, affecting livestock productivity.
5. Increase water level in pond
In the hot season, the water level in the pond should be raised. High water level will help the water in the pond have a stable temperature, cooler than shallower ponds. Deep water also helps fish have more distance to swim, grow thicker and firmer than, reducing the percentage of belly fat in the body. It is recommended to change 20 – 25% of the water in the pond on hot days.
6. Create shades for the fish
Catfish, red tilapia can survive at 32 – 34°C, but this is not the optimal growth temperature for them. Create shades with nets in the raft. For breeding ponds, it is necessary to shade the fish in the pond.
For ponds and rafts that are not shaded, direct sunlight during the day can cause the water temperature to rise higher than the air temperature. On clear nights, the temperature can drop lower than the air temperature. This causes the ponds and rafts that are not shaded to have a higher temperature difference between day and night, affecting the growth of farmed fish.
7. Enhance nutrients to withstand stress
During hot weather, fish are subjected to a lot of stress factors. Therefore, append organic zinc or zinc acetate at 5-10 mg/kg feed and vitamin C at 50 mg/kg to the feed. Zinc and vitamin C also help skin wounds heal faster, reduce losses after transporting, transferring ponds, rafts or sizing fish.
8. Check water quality
If the fish swim in an unusual or slow manner, check the water quality. Measure surface water temperature with a thermometer, measure ammonia, nitrite and dissolved oxygen levels with chemical kits.
The GREENFEED livestock solutions set is researched and developed based on actual livestock farming conditions, thereby providing measures to minimize losses and improve productivity for farmers. Contact us for advice and guidance on scientific and effective livestock farming!