Susceptibility of Different Fry Stages of Representative Salmonid Species to Oncorhynchus masou virus (OMV)

Abstract

Susceptibility in fry of several salmonid to Oncorhynchus masou virus (OMV) was studied. Each 100 fry of chum salmon (Oncorhynchus keta; 0, 1, 2, 3, 4, 5, 6 and 7-month-old), masu salmon (O. masou; 0, 3 and 5-month-old), kokanee salmon (O. nerka; 0 and 1-month-old), coho salmon (O. kisutch; 0 and 1-month-old) and rainbow trout (Salmo gairdneri; 1.6-month-old) were used. The fry were immersed for one hour in 10°C water containing 100 TCID₅₀/ml of culture grown OMV, and then held in running water at 10°-15°C.
The cumulative mortality of just hatching chum salmon, observed in ensuing 4 months was 35%, but between 1-month-old and 5-month-old fry, the cumulative mortality was more than 80%, in particular, at 3-month-old, the fry exhibited 98%mortality, the greatest sensitivity, At 6-and 7-month-old, the frys'susceptibility was reduced and only 7 and 2% fish had succumbed. On the other hand, masu salmon fry, at 1-month-old, was the most sensitive and the cumulative mortality reached 87%. In 3-month to 5-month-old fry, cumulative mortality decreased from 65% to 24%.
Comparing the five different salmonid fry, at the age of 1-month-old, for relative sensitivity to OMV, kokanee salmon exhibited the greatest sensitivity, and 100% of them died. Masu salmon and chum salmon exhibited high sensitivity at 87% and 83% mortality respectively. Coho salmon and rainbow trout were shown less sensitive to OMV infection at 39% and 29% mortality respectively.