The susceptibility of 4 salmonids [rainbow trout Oncorhynchus rnykiss, brown trout Salmo trutta, a Baltic (Iijoki) and an Atlantic (Conon) strain of Salmo salar] to infect~on with a Danish isolate of Gyrodactylus derjavinj was tested in the laboratory (12 to 13°C). Initial parasite attachment numbers did not differ between species, but parasite populations increased significantly faster and to a higher level in rainbow trout compared to the other salmonids, with S. trutta exhibiting an intermediate position. The 2 strains of S. salar became only slightly infected during the following weeks despite a heavy infection pressure. The parasites' selection of microhabitat on the hosts also differed between species and changed over time on the same host species. The tail fin was the preferred site in all salmonids although the pectoral, pelvic and anal fins were important sites in the initial period of the 6 wk investigation. It is notable that the cornea1 surface, especially in the Baltic salmon, became increasingly more heavily infected in the later stages of the infection period. Explanations for the differing degrees of susceptibility are suggested and some of these were investigated. Lysozyme activity in the fin epithelia and the density of mucous cells in the fins from the hosts were compared. Marked lysozyme activity was detected in tail fins from the 4 salmonids, but the activity was not associated with host resistance to infection. In contrast, the density of superficial mucous cells, which showed regional variation on the tail fins, was negatively correlated with susceptibility to infection. Thus, fish with high density of epithelia1 mucous cells showed the lowest susceptibility to infection, and this finding is discussed in relation to host resistance.

The intestinal diplomonadid flagellate Hexamita salmonis was isolated from the intestinal contents of freshwater reared ra~nbow trout Oncorhynchus mykiss in Denmark and maintained in axenic cultures in the laboratory. A cell culture medium (MEM) or MEM supplemented with a bile solution (Keister's modified TYI-S-33) and penicillin, gentamicin and nystatin (mycostatin) was used as culture substrate. Temperature, pH and bile requirements for laboratory maintenance were investigated. Optimum temperature for H. salmonis population increase was determined to be 10°C. Satisfactory although decreased growth was found at 5OC. At 15°C and 20°C flagellate cells survived but did not exhibit significant population growth. In the pH range from 4 to 10 parasite survival was seen from 5.5 to 9.0, although optimum pH conditions for population Increase were pH 7.5 to 8.0. Bile In low concentrations (30 to 960 mg ]-') enhanced flagellate propagation sl~ghtly. This study on the basic requirements explains the occurrence of the flagellate in rainbow trout in less warm seasons and biotopes as well as particular microhabitats in the host.

ABSTRACT:The relationship between host size and infection, the site selection in the host and the association between health status and infection were studied in Hexamita salmonis (Moore, 1923) infected rainbow trout from commercial trout farms. During a 1 yr survey it was shown that the flagellates occur primarily In the smallest fry. The site occupied by the parasite is preferentially the pyloric region and the anterior intestine but in heavily infected fish the parasites occur throughout the entire length of the gut. Infected fish exhibited significant anorexia and significant body weight reduction.

, J. L. (1997). Vaccination improves survival of Baltic salmon ( Salmo salar ) smolts in delayed release sea ranching (net-pen period). Aquaculture,, 335-348. DOI: 10.1016/S0044-8486(97) AbstractBaltic salmon (Salmo s&r) of the Finnish Iijoki stock were hatched and reared in freshwater in a salmon hatchery on the Danish island of Bornholm in the Baltic sea. Salmon parr were divided in three groups each comprising 22 000 fish. One group was vaccinated by intraperitoneal injection with a non-mineral oil-adjuvanted vaccine consisting of formalin killed Aeromonas salmonicida, Yersinia ruckeri and Vibrio anguillarum (serotype 0 1 and 02). A second group was vaccinated by 1 h bathing in a corresponding vaccine without adjuvant. A third group was left as untreated control. Subsequently, presmolt groups were transferred to three identical net-pens located next to each other in the Baltic Sea (salinity 8 ppt), 500 m from the north-eastern coast of Bornholm, where they were studied for four months until tagging and release for restocking purposes. Mortality during this period in the ip vaccinated group was minimal (0.02%, RPS (relative per cent of survival) 99.80) and significantly lower compared to 10.13% mortality in the control group and 2.51% mortality (RPS 75.2) in the bath vaccinated group. Specific disease outbreaks were not observed during the four months. Growth was significantly enhanced in the injection vaccinated group compared to both the unvaccinated control and the bath vaccinated group. The humoral antibody response to the various bacteria was significantly elevated in the injection vaccinated group showing 4-5 fold titre increases three and four months after immunization. In contrast, no increase of titres was seen in the bath vaccinated and untreated groups. Buchmann et al./Aquaculture 156 (1997) 335-348 Marked cellular reactions in the abdominal cavity of injected fish were registered. A total of 3000 fish have been tagged and released to evaluate the effect of vaccination on the recapture rate. The implications of immunoprophylactic measures in the restocking programme with Baltic salmon are discussed. 0 1997 Elsevier Science B.V.

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