Low concentrations of sodium hypochlorite affect population dynamics in Gyrodactylus salaris (Malmberg, 1957): practical guidelines for the treatment of the Atlantic salmon, Salmo salar L. parasite

Abstract

Atlantic salmon, Salmo salar L. parr (age 1+), infected by the monogenean ectoparasite Gyrodactylus salaris (Malmberg, 1957), were exposed to chlorine (Cl)-enriched water at three different concentrations: Cllow (0-5μgClL(-1) ), Clmedium (18μgClL(-1) ) and Clhigh (50μgClL(-1) ). There was a negative correlation between G.salaris infections and the hypochlorite concentrations added. The parasite infection was eliminated by day 6-8 and day 2-4 in the groups Clmedium and Clhigh , respectively, while inhibition of G.salaris population growth was observed in the Cllow group. An important note to this matter, however, is that the G.salaris specimens observed at day 6 in Clmedium and at day 2 in Clhigh were all considered dead by subjective judgement. No mortality in the salmon parr was observed during the first 8days of the experiment, demonstrating that Cl has a stronger effect on G.salaris than on the salmonid host. The differences in sensitivity between the parasite and the Atlantic salmon indicate that hypochlorite has a potential use as a parasiticide with a therapeutic margin. The low-dose sensitivity may imply that Cl pollution in urban areas may pose a greater risk towards biodiversity than previously assumed.