Animal welfare impact on captured fish
Fish caught by trawling are chased to exhaustion by a bag-shaped net towed through the water. Once exhausted, the fish become swallowed by the net and start to move into the much narrower cone-shaped part of it. Here they become confined and will start to panic. As they thrash their tails in attempts to escape, they will incur scale damage from bumping against the net and each other. Eventually they pass to the end of the net, called the cod-end, which is yet narrower. As the number of fish in the cod-end increases, the fish will experience compression under the crush. This may stop some of them being able to move their gills in order to breathe, resulting in suffocation. It may also stop the blood supply, resulting in death from circulatory failure.
Trawl-caught fish showing signs of decompression. Credit: Alaska Fisheries Science Center, Marine Observer Program
The trawl tow may last for many hours. For species that have a closed swim bladder, the sudden change in pressure caused by raising them from some depth, results in rapid decompression. Parts of the gut may be forced out of the mouth and anus, eyes may be forced from their orbits and the swim bladder may burst.
Environmental impact
Trawl nets catch everything in their path which is not small enough to escape through the holes in the mesh, resulting in bycatch. Trawling and tropical shrimp trawling account for 55% and 27% of all global discarded bycatch (i.e. catch thrown overboard) respectively.
Most of the fish and invertebrates landed in this trawl were bycatch. Greenpeace, supported by more than a thousand scientists, has argued for a moratorium on bottom trawling in international waters to reduce damage to marine life caused by this fishing method
Credit: © Greenpeace / Roger Grace
Trawls towed along the sea bottom (bottom trawls) can also be highly damaging to the seabed, destroying fish habitat. Greenpeace have film footage of this:
Fish that are thrown back into the sea after landing because they have been identified as bycatch, often die as a result of capture. So, too, do some fish that are caught in trawl nets and subsequently escape. Escapee and discarded fish may die from injury, or from being too exhausted or stressed to adequately evade predators or from infection following scale damage.
Trawl gear may theoretically be made “selective” by modifications called bycatch reduction devices (BRDs). BRDs work by allowing unwanted species to escape though holes in the net while retaining the target species. For gear to be truly selective, the fish escaping through the BRD must be sufficiently unharmed to survive. Another method of bycatch reduction is to reduce fishing effort, e.g. by closing a fishery at a particular time and place when bycatch levels are particularly high.
A bycatch reduction device for shrimp trawls to allow turtles and fish to escape
Credits: National Oceanic and Atmospheric Administration/Department of
Commerce.
For some species, the survival chances of discarded bycatch are likely to be increased by better handling of the fish on deck (especially reducing time spent out of water). Survival chances are also likely to be increased with shorter time spans between putting the net out and landing the fish. Other factors, such as tow speed, temperature and depth, can also affect the survival chances for escapees and/or discards.
Overfishing is a major environmental problem and is discussed in reducing numbers.
Measures to reduce suffering in trawling
The following measures, combined with humane slaughter immediately the fish is landed, would improve the welfare of fish captured by trawls:
Reducing the suffering in trawling:Reduce the duration of capture
Reduce the numbers of bycatch animals
Reduce stress and injury during landing
Reduce harm to other non-target animals
|
Latest updates