Effects of Ichthyobodo necator infections on seawater survival of juvenile chum salmon ( Oncorhynchus keta)

Abstract

Effects of the ectoparasitic flagellate Ichthyobodo necator on growth, survival and seawater adaptation in juvenile chum salmon ( Oncorhynchus keta) held in fresh water were evaluated by an experimental infection. The parasite density on the skin increased 2 weeks after infection, peaked at week 6, and then decreased to near zero by week 10. During this 10-week rearing period in fresh water, the mortality in the infected fish accumulated to 12.4% compared with only 1.9% in the uninfected controls. The heavy infection had significant effects on the feed efficiency, but not on the growth of host fish. High mortalities (63–70%) occurred among infected fish in seawater challenge tests (salinity 33‰ for 48 h) conducted between weeks 4 and 6 when heavy infections caused intensive erosions and hemorrhages in the epidermis, whereas mortalities were rare (0–3.3%) among the controls in the test. Serum Cl − concentration of infected fish was significantly lower in fresh water and higher in seawater, compared with that of the controls. A previous acclimation in one-third seawater did not improve the seawater survival of infected fish. These results indicate that Ichthyobodo infection has a marked effect on osmoregulation in juvenile salmon due to destruction of the skin and will consequently reduce marine survival of anadromous hosts.