The salt myth revealed: Treatment of gyrodactylid infections on ornamental guppies, Poecilia reticulata

Salt is commonly recommended as an inexpensive treatment against many fish parasites in freshwater fish culture; however, few studies have scientifically evaluated its efficacy. Amongst the monogeneans, salt has only been previously tested against Gyrodactylus salaris infecting Atlantic salmon (Salmo salar) and G. derjavini infecting rainbow trout (Oncorhynchus mykiss). Here we tested the efficacy of salt treatments against G. bullatarudis and G. turnbulli on guppies (Poecilia reticulata), both commercially important pathogens in the ornamental fish industry. In vitro survival of both parasites was negatively correlated with increasing salinities of 3, 5, 7 and 33 gL− 1. Parasite establishment on guppies maintained at 0, 3 and 7 gL− 1 salinity decreased drastically for G. turnbulli from 94% in the control to 78 and 0% on fish in 3 and 7 gL− 1 salinity, respectively. G. bullatarudis establishment was still 100% at 3 gL− 1 salinity but was reduced to 73% in 7 gL− 1. Throughout an infection, parasite populations of both species increased faster on guppies in 3 gL− 1 salinity compared to dechlorinated water, whereas population growth was severely affected at 7 gL− 1 salinity. Overall a short duration, high concentration salt bath was most effective at treating gyrodactylid infections: 15 min exposure to 25 gL− 1 salinity for adults or 5 min for juvenile fish removed 100% of G. turnbulli or 72% of G. bullatarudis. The results reflect the generalist characteristics of the more tolerant G. bullatarudis compared to G. turnbulli, but have wider implications for treatment application: clearly one treatment regime does not suit all even within a genus.