Comparative susceptibility of turbot, halibut, and cod yolk-sac larvae to challenge with Vibrio spp.

In intensive aquaculture systems, high mortalities are frequently observed during the early life stages of marine fish. The aim of this study was to investigate differences in the susceptibility of turbot Scophthalmus maximus, halibut Hippoglossus hippoglossus and cod Gadus morhua to various strains of Vibrio anguillarum (serotypes O1, O2 alpha and O2 beta), V salmonicida and V. splendidus. The bath challenge experiments were performed using a multidish system, with 1 egg well(-1). Unchallenged eggs and larvae were used as controls. Larvae in challenged groups that suffered high mortality rates were examined by immunohistochemistry. The overall results with respect to mortality showed that the O2 alpha serotype was pathogenic to all 3 species, while the O1 serotype was pathogenic to halibut and cod. The immunohistochemical examinations revealed differences in histopathology. The O1 serotype produced more severe and highly developed infections than the O2 alpha serotype. In larvae exposed to the O1 serotype, necrosis and bacterial cells were seen in the dermis, gastrointestinal tract, brain and eye area, while in larvae exposed to the O2 alpha serotype, bacteria were usually limited to the gastrointestinal tract. These results suggest either that there are undetermined species differences in host immunity or that these pathogens are host-specific even in the early life stages of fish. The O2 beta strain did not cause an increased mortality to halibut and turbot.