Bycatch of non-target fish species in commercial fisheries causes significant welfare harm through injury, suffocation, and pressure trauma, with billions of fish discarded dead or dying annually.
Bycatch finfish experience multiple welfare insults during trawl fishing. Net contact causes scale loss and skin abrasion. As trawls are hauled in, crowding causes crushing injuries. Rapid pressure change during haul-up causes barotrauma including swim bladder expansion and organ damage. Fish reaching the surface are often asphyxiated or crushed before discard. Many are still alive when returned to the sea but die from injuries. Pelagic species are particularly susceptible to pressure trauma during rapid ascent. Gear modifications including turtle excluder devices, square mesh panels, and larger mesh sizes in trawl cod ends reduce the capture of non-target species. Circle hooks in longlines reduce bycatch of species including sea turtles and billfish.