[snorvich]

Hyposalinity refers to exposure of fish to a salt concentration that is lower than that in which they normally live (typical tropical marine systems range between 30–35 g/L (g/L=ppt)). Lower salinities are less easily tolerated by many common marine tropical species, which prefer a tighter range of salinity (stenohaline). Therefore, for many species, the lower the salinity, the shorter the time period the fish can tolerate. Freshwater or lower salinity dips (duration in minutes) or short or prolonged immersion baths (duration in hours or days) for tolerant fish species are commonly used to kill or reduce the numbers of external parasites on marine species. Since Cryptocaryon is deeply embedded, fresh water dips often do not reach and therefore do not affect the parasite.

However, Cryptocaryon has proven to be more challenging to treat using salinity changes. It is inherently difficult for most aquarists to properly maintain the appropriate level of hyposalinity due to evaporation, auto top offs, etc. If proper hyposalinity is not maintained, the clock restarts.

Because trophonts and tomonts are more protected, longer dips and baths will be required than for many other species of parasites. Exposure to freshwater for up to 18 hours did not seem to affect Cryptocaryon trophonts on the host (Colorni 1985). Prolonged exposure to 15–16 g/L salinity or less (Cheung et al. 1979; Colorni 1985) appeared to affect some life stages. Tomonts of one strain of Cryptocaryon were effectively killed after 48 hours of exposure to 15 g/L or less (Colorni 1985). Temperature will also determine whether hyposalinity will control the parasite, with temperatures outside the optimal range (23–30°C) causing greater breakdown of tomonts (Cheung et al. 1979).

More recently, studies have demonstrated different salinity tolerances among strains of Cryptocaryon. Yambot (2003) described one Taiwanese outbreak occurring in sea bream Sparus sarba at a salinity of 5 g/L, and another outbreak in sea perch Lates calcarifer occurring at a salinity of 10 g/L. These two strains were successfully propagated in the laboratory at 7 and 10 g/L, respectively, and are well below previously documented preferred salinities.

One suggested protocol that may have some effectiveness, depending upon temperature and the strain's salinity tolerance, is to maintain water at 15 g/L for 21–30 days (Noga 1996; Kinsler, pers. comm.). Salinity should be reduced gradually by 5 to 10 g/L per day until 15 g/L is reached.