저층 트롤 끝자루의 어획 선택성 기반 소형어 혼획저감 연구

Abstract

Towed fishing gears such as trawl, seine were considered one of the most efficient fishing gears in commercial fisheries. In 2017, fisheries based on the towed gears in Korea produce 16.6% of the gross domestic sea water fisheries production. Considering their industrial importance in Korean fisheries, it is also required to insure the sustainability of the fishing methods and gears related issues. In towed gears, bycatch, discard and bottom effect were one of the major issues during their fishing operations and practices. In the light of resource utilization, both discard and bycatch are the matters needed to be addressed in fisheries. Concerning with the problems, researchers have been a great deal of experiments and trials for towed fishing conducting gears. The essential part to address the matters is to find out the behavioral properties in capture process. Based on the findings, it is possible to approach toward the phenomenon. In late 1980s, British scientists tried to conduct direct observations during trawling operations. It was based on underwater vehicle with a diver and also connected with a research vessel via acoustic communication tools. From their efforts, the fish behaviors were observed and also allowed to make the advancement on the matters of bycatch reduction. During trawl cruises at National Institute of Fisheries Science (NIFS), Korea, the direct observation trials were conducted for the last three years. However, there were obstacles such as mud cloud from sweep, trawldoors and low light intensity. As an alternative to approach fundamental matters on fish behavior and distribution proportion of towed fishing gears, tank experiments were conducted. the behavioral properties were categorized in accodance with zone concept; pre-trawl, between door and net mouth and inside trawl net. A model bottom trawl net and trawldoors were used to simulate the fishing operations. From the experimental results, the distribution ratio in pre-trawl zone was 21.3%. The ratio between trawldoors and net mouth was 35%. the rate inside the trawl net from net mouth was 43.7%. Total 30 trials in the experimental tank were conducted with 300 juvenile olive flounders (Paralichthys olivaceus). During the capture process of trawling, codend is the last part of the capture process. Most species that entered in a codend are considered to be exhausted and tried to escape the net. The phenomenon was also recorded from the NIFS trawl cruises. Therefore, structural approach to the codend is an essential part of the reduction bycatch matters. Fishing industries in towed gears have been requesting to allow the use of strengthening bags to prevent the breakage accidents from a codend. However, there has been no research for the strengthening codend in Korea. Four different types of strengthening bags and regular codend were used for the research from 2013 to 2015. From the experiments for the sea perch (Doederleinia berycoides), if the strengthened codend applied eight times size of regular codend mesh size, the selection curve will be approached to the regular single codend. Bycatch reduction devices are one of the most effective methods in towed fishing gears. On the other hand, there is consistent reluctance for fishermen to use the reduction devices in Korea. As an alternative, it is considered to change netting materials of codend. There has been research on square mesh applications into codends. In Korea, there has a comparison research between diamond mesh and square mesh. In early 2000s, Turned 90 degree named T90 mesh was introduced in fishing industry. it has been proved the effectiveness on bycatch reduction and reducing drag forces from the last research. Although there has not been research on tuned mesh codend in Korea, feasibility study was conducted based on mesh shape; diamond, T90, square and knotless mesh. From the field experiments by a research vessel, there was difference on selection length. In case of silver croaker (Agyrosomus argentatus), there was an increase of selection length. In a T90 mesh coded, 50% selection length was increased 14.4% compared to a diamond mesh codend. Square mesh codend was increased 21.6% in 50% selection length. On the other hand, there was a decrease in a knotless mesh codend. it was less –1.7% compared with a diamond mesh codend.