Effects of rearing conditions on growth and mortality of juvenile chum salmon (Oncorhynchus keta) infected with Ichthyobodo necator

Chum salmon (Oncorhynchus keta) juveniles, experimentally infected with the bodonid flagellate Ichthyobodo necator (Henneguy, 1883), were reared in 18-L tanks at three densities (300, 600, and 1200 fish) for 5 weeks. Water inflow rates were adjusted to give five combinations of crowding and flow rate. The parasite density on the fish body surface increased to 2700–4100 parasites-mm−2 over the course of the experiment and did not differ significantly among the groups. Total mortality was 76–90% in infected groups kept under the most unfavorable conditions (crowding or low water supply) but 11–15% in the other infected groups. The former groups also showed apparent growth reductions. In the uninfected controls, mortality was low (0.7–2.6%) and the degree of crowding or water supply had no significant effect on growth or mortality. Infected salmon were also much less resistant to seawater challenge; all fish kept at high stocking density or low inflow rate died in seawater. These results indicate that Ichthyobodo infections have marked effects on both host growth and survival when infections are combined with environmental stress induced by overcrowding and (or) inadequate water supply, although the parasite density is not affected by these factors.