The use of wrasse (Labridae) as cleanerfish in salmonid aquaculture is the only method for removal of salmon lice that does not have any negative impact on the environment. The use of wild-caught wrasse has increased during the past few years due to increasing problems with salmon lice developing resistance to chemotherapeutica. The use of wrasse is not ethically acceptable, due to a continuous loss of wrasse in the salmon farms, resulting in an unacceptably high use of wrasse. One of the species used is the corkwing wrasse (Symphodus melops). Previous studies indicate that this species is susceptible to disease caused by Vibrio bacteria. Detection of Vibrio tapetis (LP2) in Norwegian corkwing wrasse was the first detection of this bacterium in fish and the first detection in Norway. In this Master thesis, V. tapetis LP2 was used in a series of challenge experiments with wild-caught corkwing wrasse. In addition to LP2, two other V. tapetis strains from clams were used; the reference strain CECT 4600 from Manila clams (Ruditapes philippinarum) in France and NRP45, from Manila clams introduced in Norway. Three experiments were performed: Corkwing wrasse were challenged with three doses of LP2 by bath challenge and intraperitonal injections in order to choose the better challenge method. The results were not consistent, but bath challenge with >106 CFU x ml-1was considered a reliable method, and applied in the following experiments. The next experiment was performed in order to study the effect of increasing temperature on LP2 challenge. Fish were challenged with 107 CFU x ml-1 at 6, 9, 12 and 15 °C respectively. The results revealed a trend of increased mortality at increasing temperatures, and a significantly higher mortality was measured at 15 °C than at 6 °C. In the third experiment, fish was challenged with LP2, CECT 4600 and NRP 45. LP2 was re-isolated from dead fish, whereas the other two strains were not. V. tapetis was also isolated from fish that were challenged with CECT 4600 and NRP 45. The results show that V. tapetis may be pathogenic to corkwing wrasse, but mortality was however not higher in the LP2 challenged groups than in the other groups. Other fish that died during the experiment died due to infections with other Vibrio spp, and revealed the same pathology as LP2 infected fish. The results indicate that corkwing wrasse carry Vibrios, and that in particular V. tapetis and V. splendidus strains may cause disease when corkwing wrasse are kept in captivity. Experiments were carried out from June until September. Comparing the results from the different experiments, we found a high mortality of females caught during the spawning season, and the catch of corkwing wrasse should thus be avoided in this period. The results presented in this thesis may be used as a background for further studies of bacterial diseases of wild caught wrasse, improve wrasse welfare and move us a step further on the way to obtain a sustainable use of wrasse as cleaner fish in the growing salmonid aquaculture
