Effects of salinity on infection with Lerneaocera branchialis (L.) (Copepoda: Pennelidae).

Aquarium cod infected with Lerneaocera branchialis were exposed to gradually diluted sea water from salinity of 34.5 to 9.2 per thousand (S = 34.5-9.2) at a temperature of 8.5 C +/- 0.5 C. Osmolarity measurements of cod plasma, parasite hemolymph, and surrounding water showed that, unlike the cod, L. branchialis does not osmoregulate. Parasite survival is probably salinity-restricted below 16-20 S, which also is supported by its absence from Baltic waters. In waters of widely varying salinity, the parasite is not suitable as a biological tag to separate different cod populations. Hematocrit values were significantly lower in infected cod compared to noninfected fish. The pathogenic effect can be measured by the hematocrit reduction, which is correlated with the number of parasites on the host as well as the body size of the host.