Study on Embryogenesis of Rainbow trout (Oncorhynchus mykiss ) in Nepal

Abstract

The study consisted of various stages of embryonic development of Raninbow trout (Oncorhynchus mykiss).Approximately, 2000 eggs were incubated in standard hatchery troughs supplied with spring water at Fisheries Research Divison, Godawari , Lalitpur. Accepted fish cultural methods were followed to insure normal developmental patterns. Samples of these eggs were taken for study of 24 hours interval up to 5th day after fertilization ; then once every 48 hours until they were 23 days old ; then at 72 hours intervals upto 39th day. The embryos were fixed in 5 percent neutral formalin containing 0.95 percent sodium chloride, stained in Harris' hematoxylin and photographed. Egg size affects both fecundity and offspring fitness and larger eggs result in larger fry. The size of trout was found larger, greenish in colour. The development of eggs of trout was found regulated by temperature, slightly alkaline pH and high dissolved oxygen. In present study, water temperature ranged from 10.4 – 11.70 C, dissolved oxygen 7.21 – 7.37 mg/l and pH from 7.27 -8.0 respectively; these water quality parameters was recorded suitable for larval development of fishes. Rainbow trout embryonic and larval development required 1012 0 C but trout culture required an optimum temperature of 15-18°C. The hatching period, in present study, varied from 39-41 days at the water temperature of 10.4 – 11.70C. The alevins weighed about 40-50 mg and measured 1.3-1.8 cm in length. 5060% of body weight was yolk sac. Large size of alevins of rainbow trout might be due to large egg sized. The yolk sac of the pro-larvae were recorded to absorb within 7-18 days and started to swim slowly on water surface. Slow growth of embryonic and post hatched stage could be attributed to very low water temperature required for the development and rearing of trout. The feeding started after 15-20% of larvae appeared swimming on water surface. The average weight of first swimming fry was 0.07- 0.1 gm and they were fed Starter Feed containing 45-50% protein at the rate of 5-10% of body weight at the interval of every 3/4hours