Selectivity of gear
A combination turtle excluder device/bycatch reduction device manufactured by Saunders Marine Machine Shop. Fish escape by swimming forward and out of the large holes in the net. Shrimp are swept into the bag at the end of the net and cannot swim out
Courtesy of NOAA/W.B.Folsom
A key definition of selective fishing refers to a fishing method's ability to target and capture organisms by size and species during the fishing operation allowing non-targets to be avoided or released unharmed.
In a fishing area, a range of fish and other species and sizes normally occur together. 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 but it is used on specific schools that are actively sought and identified before being surrounded, limiting the problem to that school and its co-schooling species. No gear is known to be one hundred percent selective for a given species or size range. 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 target and non-target species. 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 accidental capture of any species, of any size or sex that is not specifically aimed at during fishing activity. Fishermen may aim at a mix of species. Bycatch, as a choice of the fisher, may be retained or discarded, and is mostly dead when discarded.
When bycatch is landed, the issue, as for any other landing, is the capability to collect information, assess the stock, and manage the multispecies mix with all the inherent difficulties. When bycatch is discarded, the implications are of biological, ecological, socio-economic and ethical nature:
The adoption of the ecosystem and precautionary approach, and the Convention on Biological Diversity have increased the necessity of regulators to pay attention to these factors that, most frequently, form the basis for their regulatory actions.
The total global discard is difficult to estimate. An early FAO estimate indicated an average annual discard of about 27 million tonnes in 1980-1990, a figure revised later on to about 20 million tonnes. The more recent FAO estimate for 1990-2000 is about 7 million tonnes. Even if comparisons are not completely warranted, the difference indicates a substantial decrease in discards, due in part to improved selectivity but also in part to a more intensive complete use of the catch.
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 it. 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 and technological developments will increase the success of such developments.