Common Rainbow Trout Diseases
The main diseases observed are as follows:
Fin Rot: One of most predominant diseases. This occurred among fish of greater than 1kg body weight. Fish less than 1kg were less vulnerable but not free of this disease. Fin rot was most predominant in caudal fin though occasionally pectoral and pelvic fins were also affected although fish seldom died. In newly infected parts a white streak appears near the caudal fin16 broadening later on as the muscle and rays start to degenerate slowly.
Usually, such affected fins can heal and regenerate.
Hepatoma: This was generally seen in fish bigger than 1kg body weight. This is a cancerous disease found in the liver. About 30% of fish examined had the hepatoma disease. This has also been reported from Trishuli Fisheries Research Station. The infected liver becomes enlarged and pale to white. Seriously infected fish lose balance. This disease is caused by feed contamination with aflotoxin produced from mold, Aspergillus flavus. Hepatoma arises after 4-6 months of exposure18. A dose of 0.1 to 0.5 ppb of aflatoxin-B1 in feed can cause the liver cancer.
Fungus is mostly a problem in fertilized eggs during incubation, and in postspawning fish. This might be due to bruises caused by handling. In grown out fingerlings fungal diseases were hardly seen as they are not subjected to stressful handling. The most prominent fungus is Saprolegnia sp.
This appears as ‘cotton’ and rapidly spreads to cover the whole body, initially attacking open wounds in adult fish16. Severe cases can kill the infested fish.
Some intestinal disorders were seen during the study period. The most common occurrence was carcasses with a swollen stomach. Often in such specimens a hemorrhage and presence of watery fluid in the stomach were observed. In such cases the stomach was totally empty indicating that the fish had not consumed any feed for several days before death. The swollen stomach was also associated with redness of anal opening. Such specimens often discharged dark yellow fluid if pressed gently at proximate end of the opening.
• Blunt snout: Some individuals had blunt or asymmetrical snout. It seemed that the lower jaw grew normally but upper jaw remained undeveloped during the course of growth and development.
• Twisted alevins: In 1992 some abnormal twisting occurred in under-developed alevins. These were segregated from the healthy ones but did not survive long.
• Abnormal gills: Abnormal gills were fringed and not properly developed. Such gills lacked compactness and redness in appearance.
• Degenerated operculum: In some fry the rear part of operculum was missing and gills were not completely covered with the operculum. The reasons of the degenerated operculum were not known.
• Blindness: Sometimes one of the eyes was absent due to unknown reasons. In the beginning the lost eyes were opaque but ultimately led to complete degeneration. Such fish were found to lose their balance.
Trout are delicate and demand highly oxygenated waters 7,8.
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