In the context of fisheries, bycatch could be simply defined as the total catch of non-target animals; however, several definitions have been proposed: ‘‘the portion of the catch that is discarded at sea dead or injured to an extent that death is the result’’ or "that portion of the catch returned to the sea as a result of economic, legal or personal considerations plus the retained catch of non-targeted species".
Bycatch includes individuals of species that are targeted in other fisheries, juveniles of species that would be valuable if caught as adults, and species that have no economic value, but that may be important to the functioning of marine ecosystems.
Catch is used to refer to the “gross catch” and includes all living biological material retained or captured by the fishing gear, including fishes, marine mammals, birds, sea turtles, corals, jellyfish, tunicates, sponges and other commercial or non-commercial organisms, whether brought on board the vessel or not.
Bycatch occurs because different species overlap at least temporarily in time and space and because fishing gear is not very selective in terms of species. Trawl fisheries are particularly prone to bycatch, because, in a single tow, they capture individuals of many species.
Shrimp and demersal finfish trawl fisheries account for over 50 percent of total estimated discards while representing approximately 22 percent of total recorded landings. Tropical shrimp trawl fisheries have the highest discard rate and alone account for over 27 percent of total estimated discards. Small-scale fisheries generally have lower discard rates than industrial fisheries. Purse-seine, handline, jig, trap and pot fisheries have low discard rates. In geographical terms, the highest discards are in the Northeast Atlantic and Northwest Pacific, which jointly account for 40 percent of discards (FAO areas 27 and 61, respectively).
Some animals that interact with the gear escape, but may nevertheless die, contributing to the overall mortality caused by fishing; this kind of fisheries mortality is rarely considered in the assessment and evaluation of fish stocks.
The immediate effect of removing individuals from a population is to reduce its size. In the long term in an ecosystem, the effects of bycatch, in conjunction with the direct effect on target species, will tend to:
- reduce the abundance of large individuals, particularly predators and of long-lived, susceptible species such as sea turtles;
- increase the relative abundance of smaller, early maturing species with high reproductive rates;
- favor the increased abundance of scavengers, that feed on the discards; and
- add to the net loss of biodiversity and massive changes in the structure of marine communities.
There are many strategies to manage commercial marine fisheries bycatch. These include formal constraints through laws, regulations, and policies; multilateral accords; marine protected areas, including area and seasonal closures; best practices for handling and release of bycatch species; changes in fishing gear and methods; ecolabeling; industry self-policing; industry awareness-raising and capacity-building; and fleet communication programs. One of the more successful approaches is the use of turtle excluder devices (TEDs) on shrimp nets, which allow turtles to escape out of the net without significantly reducing the catch of shrimp. Ongoing studies are aimed at developing other effective bycatch-reduction technologies for fishing gear.
Hall states that the management objectives of a bycatch program should try to: avoid extinction of species, retain the basic structure and functioning of the ecosystems, reduce waste in fisheries, reduce interactions between fisheries, maintain fisheries open, attain marketing goals, rebuild depleted populations and control increasing populations.
- ^ Hall, M.A., D.L. Alverson, and K.I. Metuzals. 2000. By-catch: problems and solutions. Marine Pollution Bulletin 41(1–6):204–219.
- ^ McCaughran, D.A. 1992. Standardized nomenclature and methods of defining bycatch levels and implications. In: Schoning, R.W., R.W. Jacobson, D.L. Alverson, T.H. Gentle, and J. Auyong. Eds. Proceedings of the National Industry Bycatch Workshop. Newport, Oregon, February 66, 1992. Seattle, WA: Natural Resources Consultants. pp. 200-201.
- Gilman, E., P. Dalzell, and S. Martin. 2006. Fleet communication to abate fisheries bycatch. Marine Policy 30:360–366.
- Hall, M.A. 1996. On bycatches. Review of Fish Biology and Fisheries 6:319–352.
- Kelleher, K. 2005. Discards in the world’s marine fisheries: An update. FAO Fisheries Technical Paper 470:1-131.
- Rochet, M.J. and V.M. Trenkel. 2005. Factors for the variability of discards: assumptions and field evidence. Canadian Journal of Fisheries and Aquatic Sciences 62(1): 224-235.
- Zeller, D. and D. Pauly. 2005. Good news, bad news: global fisheries discards are declining, but so are total catches. Fish and Fisheries 6(2): 156-159.