Effects of Seawater Acclimation on Juvenile Salmonids infected with Tetraonchus (Monogenea) and Ichthyophonus (Phycomycetes)

Abstract

Effects of seawater acclimation on juvenile salmonids infected with gill monogeneans, Tetraonchus awakurai and T. oncorhynchi, and systemic fungus, Ichthyophonus hoferi, were investigated.
In the acclimation test from freshwater to 50 % seawater, the serum sodium level of Tetraonchusparasitized amago salmon smolts (Oncorhynchus rhodurus) increased during the first 24 hours but decreased afterward and reached the same level as in marine fishes (ca. 150 mM) after 4 days, indicating the infection has no negative effect against the seawater adaptation. The two species of Tetraonchus died when exposed to 4.5% NaCl for 10 min. or 2.5% NaCl for 60 min. This result indicated that they are eradicated in seawater during the course of seawater acclimation.
In Ichthyophpnus-infected juvenile rainbow trout (Oncorhynchus mykiss), on the contrary, the serum sodium level increased rapidly and exceeded 200 mM with 95 % mortality after transfer to 100% seawater for 4 days. Apparently, Ichthyophouns was not affected by exposure of trout to seawater or the increment of the host NaCl level.