Fish Hatchery Management - fisheries & aquaculture

FISH HE,AL'I'H MANAGEMEN'Iisolation, positive identification requires neutralization of the virus by aspecific antiserum.Fish that survive an infection become carriers: both sexes shed the virusprimarily with sex products. Gonadal fluids are used in bioassays to detectcarrier populations. Naturaltransmission occurs from infected fish tononinfected fish through the water, or from the exposure of susceptible fryto sex products of carrier adult broodfish. The virus also can be transmittedwith eggs or by the feeding of infected fish products.Only rainbow trout and chinook and sockeye salmon have been shownto be susceptible to IHN. Coho salmon apparently are resistant to thevirus. Resistance increases with age and deaths are highest among youngfry and fingerlings. However, natural outbreaks have occurred in fish rangingfrom yolk-sac fry to 2 years of age. The incubation and course of thedisease are influenced strongly by water temperature. At 50"F, mortalitymay begin 4 to 6 days after exposure. Numbers of dead usually peakwithin 8 to 14 days, but mortality may continue for several weeks if thewater temperature remains near 50"F. Below 50'F, the disease becomes prolongedand chronic. Above 50'F, the incubation time is shorter and thedisease may be acute. Some epizootics have been reported at temperaturesabove 59'F.Outbreaks of IHNdisease have occurred along the Pacific Coast fromthe Sacramento River in California to Kodiak Island, Alaska. Although thevirus may not exist in all populations of sockeye salmon, the virus hasbeen detected in all major salmon production areas. Among chinook salmon,the disease is a particularly serious problem in the Sacramento Riverdrainage; it has been found also in fish of the Columbia River. Outbreaksof IFIN in rainbow trout have been much more restricted. Isolatedhatcheries where carriers and outbreaks were identified are known fromSouth Dakota, Minnesota, Montana, Idaho, Oregon, Washington, Colorado,and West Virginia. AII involved fish or eggs from a known carrierstock. However, there has been no recurrence of the disease at most ofthese hatcheries after the original outbreak. IHN also occurred in Japan insockeye salmon from eggs transported from Alaska.An effective method of control is to maintain the water temperatureabove 59"F while fish are being reared. This principle has been used successfullyto control IHN in chinook salmon along the Sacramento River.However, it is expensive to heat large volumes of water. Furthermore, rearinginfected fish at elevated temperatures does not eliminate the carrierstate.In rainbow trout, IHN virus is believed to be transmitted with eggs as acontaminate. Disinfection of eggs with iodophors usually will destroy thevirus.