The opah or moonfish is the only known fully warm-blooded fish, making it physiologically unique and raising novel welfare questions about its experience in bycatch and captivity.
As fully warm-blooded fish, opah may have more capacity for sustained, active responses to painful stimuli than cold-blooded species. Their physiology makes them physiologically closer to mammals and birds in some respects, which has implications for how they experience bycatch stress. Longline capture involves being hooked, dragged at depth, and slowly retrieved over hours — a highly stressful process. Opah caught as bycatch are typically suffocated on deck. The welfare implications of warm-bloodedness in fish are an emerging research area.