P.K. Hershberger
R. M. Kocan
2001
<p>When king salmon enter the Yukon River on their spawning migration in mid June, over 25% of the population are infected with <i>Ichthyophonus</i>. The percent of infected fish remains relatively constant until the fish pass river mile 1,319 at Dawson, Y.T., then it drops to 13% when they reach river mile 1,745 at Whitehorse, Y.T. When the sexes are examined separately, slightly more females are infected than males (29% vs 22%). The percent of fish exhibiting clinical signs (diseased) is 2-3% when they enter the river, but increases to over 20% at river mile 715 near Tanana, AK. Disease prevalence within the population remains constant at >20% until fish pass Dawson, then the percent of diseased fish drops to <9% at Whitehorse. When the sexes are examined separately, male disease prevalence is highest at Tanana (22.6%) then gradually drops to just 12.9% at Whitehorse. Females however, continue to show an increase in disease prevalence peaking at river mile 1,081 near Circle, AK, at 36.4%, then dropping to just 5.3% at Whitehorse. Data on infection and disease collected from kings at Nenana on the Tanana River more closely resembles that seen at Whitehorse than the lower and middle Yukon River.</p>
<p>When data collected in 1999 and 2000 are compared the prevalence of infection in males remains the same while a 15% drop in infection prevalence occurs in females. There is also a drop in the percent of infected fish showing <i>Ichthyophonus</i> infection of the muscle. This difference may be related to a 2°C lower river temperature in 2000 compared with 1999.</p>
<p>Significant egg resorption was seen in 25% of females but no correlation with <i>Ichthyophonus</i> infection could be made. The cause of resorption and the extent to which it affects fecundity has yet to be determined.</p>
<p>Attempts to experimentally infect Chinook salmon and rainbow trout with Yukon River <i>Ichthyophonus</i> isolates were essentially unsuccessful by both feeding of infected tissues and injection of cultured spores. However, other unrelated fish species were infected without difficulty.</p>
<p>A method for non-lethal sampling of adult spawning Chinook salmon for <i>Ichthyophonus</i> was developed using known infected fish and live returning spawners. The method consisted of taking punch biopsies of skin and muscle and culturing the biopsy tissue in vitro. A 100% correlation was made between known infected fish and cultured biopsy tissue. </p>
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U.S. Fish and Wildlife Service
Yukon River King Salmon - <i>Ichthyophonus</i> Pilot Study
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