2013 Volume 48 Issue 3 Pages 75-80
The occurrences of Caligus sclerotinosus, a parasitic copepod on cultured red sea bream Pagrus major, were surveyed in fish farms and the phototaxis of adult caligid was experimentally examined. The parasite infection occurred from April to December, in which up to approximately 500 caligids infected the external surface of each diseased fish, covering the whole body surface, eyes and fins. The main disease signs were scale exfoliation, abrasion and ulcerative lesions on the body surface, and chipped fins. Listless swimming near the water surface was also observed. Daily mortalities were usually below 0.1%. When illuminated at different light intensities, most adult caligids showed positive phototaxis and those illuminated at 50 μmol/m2/s swam significantly faster than those illuminated at 200 and 800 μmol/m2/s. When infected fish were illuminated from one direction in a tank, 2.5% of caligids moved from fish to the illuminated wall of tank. When an infected fish and an uninfected fish were separated with a screen partition in each tank and illuminated from one side, 25% of caligids moved from fish in the shaded side to fish in the illuminated side, but never vice versa, showing that fish-to-fish transfer of C. sclerotinosus adults was affected by light conditions.