## 4 Discussion  
The gillnet technique is relatively simple and does not require special fishing equipment or facilities, compared with other types of fishing techniques for large fish. There are several gillnet fisheries actively operating in coastal waters. Gillnet fishing is characterized by the fish being caught by the gills or its body being held by the mesh. Therefore, the mesh size tends to be proportional to the size of the target fish [2; 5; 25; 30; 31]. Gillnets have a relatively high catching selectivity compared with the trawling and trap fishery techniques [25]. However, to increase the probability of catching fish in gillnet fisheries, the heads of the fish should be narrower, as in fusiform (spindle-shaped) fishes. Since flounders have wide bodies, they are sometimes caught by being held in the mesh, tangled in the net, or surrounded by the gillnet [32; 33; 34; 35; 36]. The number of fish species caught along the east coast of Korea is low, and flounder catches account for a large proportion of these, serving as an important source of income for fishermen. Flounders are mostly caught with gillnets in coastal waters and by trawling offshore. In the past, a single gillnet was primarily used in Korean gillnet fisheries, but target catches were frequently dropped during the hauling operations, and the catches were not landed [37; 38]. Trammel nets have been used more recently; however, trammel nets can catch fish of various sizes, and their use is strictly regulated due to the risk of overfishing. The catching efficiency of the trammel net has been reported to be 1.4-2 times higher than that of a single gillnet [18; 19; 23; 25; 29; 39]. Recently, tie-down gillnets have been widely used and adjusted to form wrinkles in the net by adjusting the height of the gillnet. This comprises a form of fishery intermediate between a trammel net and a single gillnet. However, the catching efficiency of this technique has not been quantitatively estimated due to the limited number of studies on the fishing performance and catch size selectivity of tie-down gillnets. In addition, tie-down gillnets have been suggested to pose a risk of overfishing similar to that posed by trammel nets, but this has not been established through research. Various types of support line installations have been developed recently, which increase the catching efficiency of gillnets. Therefore, a systematic study on the support lines of tie-down gillnets is needed. This study aimed to investigate the size selectivity of nets with different mesh sizes. In the sea trial results from Table 2, the smallest mesh size (84 mm) had the highest catch number, and the length of the fish caught was also relatively small. In addition, the number of catches tended to decrease with the increasing mesh size. The average weight of the blackfin flounder increased as the mesh sizes of the tie-down gillnet increased. The total length (represented by the mode) showed a slight increase with an increase in the mesh size. Recently, fisheries regulations for the protection of fish stock have specified the length limits for several species of flounders, including the blackfin flounder, the main target fish of coastal gillnet fisheries in the waters of Yangyang County, Gangwon Province. The length limit for the blackfin flounder is currently set at 170 mm and will gradually increase. The mesh sizes currently used by most fishermen are 84 mm and 90 mm. The 50% selection  
Figure 7: Relationship between mesh size and total length in each selection length. length (\(L_{50}\)) for the 84-mm net was 186.9 mm, which is approximately 17 mm longer than the length limit. We suggest that continuous monitoring of these factors is necessary in the future, as well as an increase in the length limit of the blackfin flounder. In addition, the standards and regulations for the shape or design of tie-down gillnets have not yet been established.