Abstract
Saprolegnia infections cause severe economic losses among freshwater fish and their eggs. The banning of malachite green increased the demand for finding effective alternative treatments to control the disease. In the present study, we investigated the ability of boric acid to control saprolegniosis in salmon eggs and yolk sac fry. Under in vitro conditions, boric acid was able to decrease Saprolegnia spore activity and mycelial growth in all tested concentrations above 0.2 g/L, while complete inhibition of germination and growth was observed at a concentration of 0.8 g/L. In in vivo experiments using Atlantic salmon eyed eggs, saprolegniosis was controlled by boric acid at concentrations ranging from 0.2–1.4 g/L during continuous exposure, and at 1.0–4.0 g/L during intermittent exposure. The same effect was observed on salmon yolk sac fry exposed continuously to 0.5 g/L boric acid during the natural outbreak of saprolegniosis. During the experiments no negative impact with regard to hatchability and viability was observed in either eggs or fry, which indicate safety of use at all tested concentrations. The high hatchability and survival rates recorded following the in vivo testing suggest that boric acid is a candidate for prophylaxis and control of saprolegniosis.
Highlights
Aquatic diseases caused by species in genus Saprolegnia in farmed freshwater fish and their eggs constitute a major economic problem especially in salmonid hatcheries [1,2,3]
Formalin is banned in some countries [10], and several countries are expected to follow, European Union (EU) included
boric acid (BA) was tested for control of Saprolegnia infection of eyed salmon eggs and yolk sac fry and its safety in vivo was assessed
Summary
Aquatic diseases caused by species in genus Saprolegnia in farmed freshwater fish and their eggs constitute a major economic problem especially in salmonid hatcheries [1,2,3]. Several Saprolegnia species that cause more than 90% mortality in fish and eggs have been isolated [4,5,6]. In the past this problem was solved with the extremely effective fungicide malachite green (MG) [7], but due to suspected teratogenicity [8] it was banned for use in fish intended for human consumption [9]. Bronopol is an alternative treatment available in the market [7], but is of inferior effect compared to malachite green. Attempts have been made to find alternatives to MG [5,11,12,13], but still no comparably effective alternatives have been found
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