Ichthyophthirius multifiliis (Ich) Epizootics in Spawning Sockeye Salmon in British Columbia, Canada

Abstract Epizootics of Ichthyophthirius multifiliis occurred in adult prespawning and spawning sockeye salmon Oncorhynchus nerka during the 1994 and 1995 spawning seasons in the Skeena River watershed in northern British Columbia, Canada. Exceedingly high mortalities occurred at Fulton River and in three spawning channels adjacent to Babine Lake. During these 2 years, the infection and subsequent death of prespawning adults resulted in an estimated 153.6 million fewer sockeye salmon fry produced from Fulton River and the three spawning channels adjacent to Babine Lake than the historical average. Resident fish were the most likely source of the parasite in the watershed because several species were found with light infections of I. multifiliis. Transmission of the parasite to anadromous sockeye salmon was enhanced by the high density of fish held below the spawning grounds for days or weeks prior to moving into the spawning channel. This is the first report of an epizootic of ichthyophthiriasis in wild sp...