The selectivity of fishing gears is based on the relationship between the size of the target creature and the size of the area through which the target creature passes, such as the mesh size in the case of netting gears, or the size of drain hole in the case of tubular pot used to catch conger eel. In the case of gillnets, if fish body girth is smaller than the inner circumference of the mesh, it can pass through, but the matching individual will be held by the mesh and caught. And, if the body circumference is much larger than the inner circumference of the mesh, it will not be retained. Therefore, the selectivity of gillnet is an bell-shaped curve centered on a certain size. On the other hand, in the case of trawlnet, the body girth is larger than the inner circumference of the mesh is retained in the net, so the retention rate of individuals over a certain size is 100%, which is drawn as an S-shaped curve (sigmoid curve).
Gear selectivity is necessary information in determining the mesh size of commercial fishing gear for resource management and in determining the specifications of research gear to know the size composition of the target species in resource investigation. Since the 1990s, research on gear selectivity has been conducted extensively in terms of assessing the performance of Bycatch Reduction Devices (BRDs) in addition to mesh selectivity, with the goal of achieving selective fishing for species and sizes, primarily to reduce bycatch and discards. In particular, there have been many studies of trawl gears, which are used in many regions of the world and are prone to bycatch because of their fishing process. Therefore, the study of selectivity for species and size in trawl gears has progressed rapidly from the 1990s to the 2000s. These studies have greatly matured selective fishing techniques, but recently, the impact of thinning out large size individuals on the structure and ecology of the target stock has been suggested, and since the late 2000s, some studies have attempted to evaluate the impact of selective fishing.
In this section, the principle of selectivity will be explained, and the selectivity of recent bycatch prevention devices will also be explained with examples of research.