In a fishing area a range of species and sizes normally occur together, both fish and non-fish. When encountered by a fishing gear, they will be captured at different rates, depending on the gear design and its mode of operation. Various gears are known to be more selective in this regard than others. A purse seine has a poor selective performance, as it will catch most of the surrounded individuals. No gear is known to be one hundred percent selective for a given species or size range of individuals. Most gear and methods fall into a group that has some selective performance, and their ability to select targets can be altered through modification of design and operation methods.
The catch in many fisheries thus consists of a mixture of targets and non-targets. What does or does not comprise targets depends to a large extent on the market and whether there are regulations in place prohibiting capture of species or certain sizes of organisms. Non-targets are often synonymous with bycatch, a concept defined differently by various people. A generic definition often used is that bycatch refers to the capture of any species, size of species, or sex of species that is not the primary target(s) of a fishing activity. Bycatch, as a choice of the fisher, may be retained or discarded, alive or dead.
The significance of the discard depends on biological, ecological, or socio-economic factors. The nature of discards can possibly be classified into four categories: (1) population status, (2) socio-economic, (3) ecological and (4) public concerns. These factors most frequently form the basis or regulatory actions taken by management agencies.
The total global discard is difficult to estimate, but one assessment came up with a level of 27 million tonnes for the period 1988 through 1990. A 1996 FAO Technical Consultation in Tokyo concluded, however, that the 27 million tonnes might be an overestimate, elaborating different reasons.
Besides bycatch of fish and fishlike species, other animals might incidentally be captured with fishing gears. Such includes among others various species of whales, turtles and seabirds. Although bycatch of such organisms seldom is a threat to their population size, public concerns make it necessary to reduce such bycatch.
Improved selectivity can be achieved in different ways, by modifying the gear design and/or operation and by using alternative fishing gears. In trawls and gillnets mesh size is a well- known measure to regulate the size of captured organisms. For mobile gears, like trawls and seines, improved selectivity can also be achieved by using square meshes in the codends and by inserting filtering grids in front of the codend. Successful separation of targets and non-targets species can also be achieved by using grid devices. The principles for such selectivity are detailed in the referenced documents.
Successful technical solutions have also been found to reduce capture of non-fish species like mammals, turtles and seabirds. The capture of dolphins in the purse seine fishery for tuna has been reduced to an insignificant level by using a combination of technical changes, rescue techniques, education of fishers and management actions. Bycatch of turtles in the tropical shrimp fishery can be avoided by using a turtle excluder device (TED), which is a rigid or soft (netting) structure inserted in the aft part of a trawl, in front of the codend proper. Incidental capture of seabirds in longline fisheries can be significantly reduced by underwater setting of the line, night setting and scaring the seabirds away from the baited hooks.
At international level the most prominent action to reduce bycatch during fishing is the ban on large size driftnetting on the high seas (UN General Assembly resolution 1990). The FAO Code of Conduct for Responsible Fisheries of 1995 is a major international document addressing the problem with selective fishing. The most recent international contribution is the International Plan of Action for Reducing Incidental Catch of Seabirds in Longline Fisheries adopted by the FAO Committee on Fisheries in 1999. FAO has recently developed a project (with GEF funding), to enhance the capability of developing countries to reduce environmental impact of tropical shrimp trawling through the introduction of bycatch reduction technologies and change of management.
At national and regional levels regulations, minimizing unwanted bycatch and discards are being introduced. These include minimum mesh sizes, mandatory use of selective devices like TEDs and sorting grids, areas closed for fishing, etc. Many countries are putting a great deal of effort into research and development aimed at improving the selectivity of fishing gear and methods that presently capture much of the unwanted catch.
Concerns about bycatch are expected to increase in the future, and therefore focus will continue to be on the development of selective technologies. It is expected that better knowledge of organism behaviour (which should be treated separately) and technological developments will increase the success of such developments.