The Atlantic halibut (Hippoglossus hippoglossus) is the largest flatfish in the world, capable of reaching 300kg and 4.5m length in the wild. Wild populations have been severely depleted by overfishing and are now commercially rare. Farmed halibut represents both a potential relief for wild stocks and a significant welfare challenge.
Halibut are bottom-dwelling predators in the wild, with complex visual systems, strong stress responses, and relatively slow growth rates (3–5 years to market size of 5–7kg). These characteristics make them sensitive to poor husbandry.
Halibut larvae undergo a dramatic metamorphosis during which one eye migrates from the left side of the head to join the right eye on the dorsal (upper) surface. Failure of this metamorphosis — occurring at certain temperatures or in poor conditions — produces "reversed" fish with eyes on the wrong side. Reversed fish cannot feed properly lying on their correct side and suffer significantly. Water temperature control during the larval and juvenile phases is critical to welfare.
Farmed halibut frequently develop abnormal skin pigmentation — dark patches on the underside (normally white) or pale patches on the upper side. This is not directly a welfare problem but indicates disrupted development, possibly linked to diet deficiencies or abnormal lighting. Severely pigmentation-abnormal fish may have associated physiological problems.
Halibut are highly sensitive to handling stress. Acute cortisol responses to catching, crowding, or grading are more severe and longer-lasting than in salmon. Each handling event represents a significant welfare impact. Minimising handling frequency and duration, and using appropriate water temperatures during procedures, is essential.
Wild halibut rest on the seafloor. In tank systems, they show a strong preference for lying flat on surfaces. Welfare-conscious tank design provides:
Juvenile halibut show strong cannibalistic tendencies — larger juveniles readily eat smaller ones. Frequent grading to maintain size uniformity within tanks is essential for welfare but itself involves repeated stressful handling. Better early growth uniformity through controlled feeding and environmental management reduces grading frequency.
Vibriosis, Francisellosis, and various parasite infections affect farmed halibut. Vaccination programmes have improved health outcomes. Disease outbreaks cause significant suffering and mortality — biosecurity and water quality management are the primary preventive tools.
Halibut slaughter welfare requires specific consideration given their size and physiology. Percussive stunning (spike to the brain) followed by pithing is the recommended approach for large fish. Smaller halibut may be stunned electrically. Ice slurry without prior stunning is not acceptable welfare practice for halibut given the evidence for fish sentience.