How to prevent and treat saprolegniasis in freshwater pomfret during overwintering

1. Diseases commonly occurring in freshwater pomfret during overwintering

The most common fish diseases occurring in freshwater pomfret during overwintering are saprolegniasis, ichthyophthiriasis, trichodiniasis, chilodonelliasis, etc., especially ichthyophthiriasis, which can easily cause explosive death. The causes of various fish diseases are due to the low water temperature during overwintering, the fish not feeding, high physical energy consumption, and deteriorated constitution, or due to careless fishing and transportation in low-temperature weather causing injuries or low-temperature frostbite, which allows large numbers of various parasites and bacteria to take advantage and invade and harm the fish body, resulting in various diseases. Therefore, the epidemic prevention policy of “prevention first” should be adopted. While doing a good job in overwintering warmth preservation and strengthening feeding management, disease prevention and treatment work should also be done well. Before entering the pond, the fish body should be bathed in 4% to 5% salt water for 10 to 15 minutes. If diseased fish occur, timely symptomatic treatment should be applied. However, when using medication, never use trichlorfon and malachite green.

2. Saprolegniasis

Saprolegniasis is the most common and most harmful disease in freshwater pomfret during overwintering, and it is most likely to break out in fingerling ponds in early spring.

(1) Pathogen

Saprolegnia spp., Leptomitus spp.

(2) Symptoms

Many cotton-like white hairs can be seen with the naked eye growing on the local surface of the diseased fish body (in the early stage of infection, symptoms cannot be seen with the naked eye). The diseased fish is restless and mostly swims slowly on the water surface, has reduced appetite, and the fish body becomes emaciated, leading to death.

(3) Prevention and treatment methods

① Try to avoid catching and transporting fingerlings in low-temperature seasons to prevent mechanical injury and frostbite. Especially after early spring, fingerlings should only be caught and transported after the water temperature has stabilised above 22°C and the fish body has recovered through feeding.

② Before overwintering, the pond water should be deepened so that the water depth is above 2 m, or other insulation measures should be taken to keep the water temperature above 19°C to prevent frostbite of the fish body. At the same time, attention should be paid to the water body environment; never let the water quality be too fertile, and the water colour should be tea-coloured or light yellow-green.

③ When transporting fingerlings, about 1% salt should be added to the transport water for sterilisation and disinfection; when transferring fingerlings between ponds, they should be soaked in 1% to 2% salt water for 5 minutes before being put into the pond for culture.

④ For diseased ponds, 0.04% salt can be splashed over the whole pond; or 0.04% baking soda can also be added and splashed over the whole pond at the same time; or for each mu of water surface, 2.5 to 5 kg of mashed Acorus calamus leaves plus 0.5 to 1 kg of salt, added to 2 to 20 kg of human urine, soaked for 24 hours and then splashed over the whole pond, has a certain therapeutic effect.