Viable saprolegniaceae spores on the epidermis of the Salmonid fish Salmo trutta and Salvelinus alpinus

Abstract

Techniques are described for monitoring Saprolegniaceae in hatchery pond water and on the epidermis of hatchery-reared brown trout Salmo trutta L. and char Salvelinus alpinus L. The water supply to the hatchery occasionally includes spores of Saprolegnia type I (taxonomically in or near the S. diclina — S. parasitica complex), which are responsible for the outbreaks of Saprolegnia infection that occur in the hatchery. The saprolegniaceae spora in the pond water increased from 200 spores/1 or less to a maximum of 22 200 spores/1 when infected fish were present in the water. This increase was solely due to Saprolegnia type I spores. Experimental applications of high concentrations of secondary zoospores of Saprolegnia type I to trout and char indicate that although the majority of spores are removed or inactivated during the first 24 h of subsequent quarantine in clean water, a small but significant number of viable spores remain on the body surface. These findings are discussed in the light of our knowledge of Saprolegnia infections of fish and in relation to the possible defence mechanisms of fish to pathogenic fungi.