Atlantic Salmon Disease Fungus

Abstract

INITIAL reports of the salmon disease from Ireland suggested that the problem was purely bacteriological. This was despite the fact that the presence of a fungus, Saprolegnia, constituted the only clear outward symptom by which the disease was recognized. Stuart and Fuller have discussed the possible role of the fungus more realistically and have given details of their mycological researches¹. With the advent of the disease to the English Lake District, especially in the autumn of 1967, I have also had the opportunity to make observations, and these to some extent parallel those of Stuart and Fuller. Further details of the fungus have, however, become known. Saprolegnia isolates were made from diseased salmon, trout and char (Salvelinus willughbii) netted in Windermere, and from further specimens of the latter which succumbed in hatchery ponds immediately adjacent to our laboratory. The water supply for the hatchery ponds derives directly from Windermere and is untreated. Using GYS-tellurite agar, a medium previously employed for ecological studies on the Saprolegniaceae², the isolates were soon free of bacteria and sterilized hemp seeds were added. These were removed after 8 h and transferred to dishes of sterile lake water where they grew abundant fringes of the fungus. These growth fringes produced sporangia releasing zoospores, but not oogonia. In the absence of these any specific epithet which may be applied to the purely asexual condition (for example, Saprolegnia parasitica) can have little or no meaning or usefulness. Accordingly attempts at oogonial induction were set in hand, using oatmeal and cholesterol³ agars of different strengths with single or paired isolates. Isolates were also paired in simulated natural conditions in contact with sterile mud or soil, following the method of Sherwood⁴ for Dictyuchus. All these attempts were unsuccessful.