Changes in Behaviour of Atlantic Halibut (Hippoglossus hippoglossus) and Turbot (Scophthalmus maximus) Yolk-Sac Larvae Induced by Bacterial Infections

Eggs of Atlantic halibut (Hippoglossus hippoglossus) and turbot (Scophthalmus maximus) were exposed to Flexibacter ovolyticus and pathogenic Vibrio sp. strains prior to, and during hatching. Activity, buoyancy and mortality of the yolk sac larvae were monitored from hatching until time of first feeding. The halibut larvae showed reduced activity and increased mortality in response to the challenge of bacteria, compared to uninfected control groups. In addition, the infected halibut larvae showed increased specific density compared to the uninfected larvae. These responses were not found for turbot. However, turbot larvae infected with Vibrio anguillarum had lower activity than larvae infected with F. ovolyticus. The reduced activity of halibut larvae occurred 1–2 weeks prior to the increased mortality, allowing infections to be detected at an early stage. The results suggest that the behaviour of fish larvae is influenced by bacterial infection.