An evaluation of treatments to reduce mortality from coagulated yolk disease in hatchery-produced Chinook salmon

Three treatment, were evaluated for their effectiveness in reducing the mor tality of hatchery-produced fry of chinook salmon Oncorhynchus tshawytscha from coagulated yolk disease (CYD). The treatments consisted of (1) incubat ion of eggs and alevins in untreated (unfiltered, ambient temperature) wate r, (2) incubation of eggs and alevins in water supplemented with CaCl to in crease total hardness, and (3) incubation of eyed eggs and rearing of alevi ns on plastic substrates. Mortality was significantly greater in the untrea ted water treatment than in the control (filtered and chilled water) overal l (43.1% versus 24.7%) and for all life stages: green egg (24.0% versus 19. 3%), eyed egg (8.9% versus 4.5%), alevin (2.5% versus 0.3%), and fry (15.7% versus 2.0%). The pattern of mortality in untreated water during the fry l ife stage was similar to that reported in the literature for CYD-infected f ish. Mortality during the green egg life stage of fish treated with CaCl wa s significantly greater than in the control (22.2% versus 19.3%), and losse s appeared to be unrelated to CYD. There was no significant difference in m ortality associated with the use of plastic substrates or CaCl supplementat ion when the water supply was filtered and maintained below 14.0 degreesC. The use of CaCl, artificial substrates, and chillers during incubation did not prevent coagulated yolk disease from occurring in the Mokelumne River F ish Hatchery, but filtering and maintaining water temperatures below 14.0 d egreesC during incubation did reduce losses of chinook salmon that contract ed CYD.