A note on the behaviour of adult Atlantic halibut, Hippoglossus hippoglossus (L.), in cages

Abstract. Observations of adult Atlantic halibut, Hippoglossus hippoglossus (L.), behaviour in cages were made with underwater camera and video recording equipment. Significantly more fish (P < 0·01) were found on the cage bottom than in the water column, fish tending to congregate around the outside rather than in the centre of the cage. During rough weather, the cage bottom was observed to heave violently and proportionately fewer fish remained on the bottom, those which did adopting an arched body posture with heads and tails not in contact with the cage bottom. Around 25% of the caged fish were active at any time, most of the active fish remaining close to the cage surface or bottom, with little use being made of the remaining cage volume. Fish were observed to swim in a circular pattern close to the cage walls (mean velocity 0·53 m/s), maintaining such behaviour for periods of up to 5min with little change in swimming speed. Whilst all food types were ingested, items delivered directly lo the cage bottom were taken only if detected within the first minute or so. The implications for rearing Atlantic halibut in conventional cages are discussed.