This study established the conditions in which mooring load is minimized in a fish cage that includes a damping buoy in specific wave conditions. To derive these conditions, numerical simulations of various mooring contexts were conducted on a fish cage (1/15 scale) using a simplified mass-spring model and fifth-order Stokes wave theory. The simulation conditions were as follows: (1) bridle-line length of 0.8–3.2 m; (2) buoyancy of 2.894–20.513 N for the damping buoy; and (3) mooring-rope thickness of 0.002–0.004 m. The wave conditions were 0.333 m in height and 1.291–2.324 s of arrival period. Consequently, the mooring tensions tended to decrease with decreasing mooring line thickness and increasing bridle-line length and buoyancy of the buoy. Accordingly, it was assumed to be advantageous to minimize the mooring tension by designing a thin mooring line and long bridle line and for the buoyancy of the buoy to be as large as possible. This approach shows a valuable technique because it can contribute to the improvement of the mooring stability of the fish cage by establishing a method that can be used to minimize the load on the mooring line.

The National Institute of Fisheries Science in Korea is developing marine mammal bycatch reduction devices (BRDs) for midwater trawl gear. In this study, we tested two BRD-type guide nets (inclined net panel) with 30° and 45° tilt angles to prevent marine mammals from reaching the codend and facilitating their escape from the net. Fishing operations were conducted along the east and south coasts of South Korea, and cameras were installed in front of the BRDs to monitor their performance. The catch loss of herring with the 30° guide net was 13% and 11% in number and weight. The catch loss of hairtail was 53% and 51% in number and weight with the 30° guide net. Mackerel showed a 97% catch loss in number and weight with the 45° guide net. The 30° guide net resulted in lower catch loss for rudderfish and jack mackerel compared to the 45° guide net. The jellyfish discard rate of the BRD was 5% and 7% in number and weight with the 30° guide net and 12% and 11% with the 45° guide net, indicating that the 30° guide net was more effective at discarding jellyfish. Mesh selectivity was not strongly related to target species body length.

We sought to develop and evaluate an escape vent designed for undersized swimming crabs (Portunus trituberculatus) to reduce bycatch, contributing to the preservation of marine resources. To this end, we conducted aquarium experiments and selectivity analysis to determine the appropriate size of the escape vent that would allow only undersized crabs with a carapace length of 64 mm or less to escape. The optimal dimensions for the escape vent were approximately 34.1 mm in height and 69.1 mm in width. During the sea trial, the average bycatch rates for undersized crabs per pot were 57.2%, 15.0%, and 22.3% for the control, basic, and door types, respectively. Regarding legal-sized crab catch per pot, averages of 1.40, 1.72, and 1.62 individuals were obtained for the three pot types. To our knowledge, this study is the first to assess the optimal size for an escape vent capable of reducing the bycatch of undersized crabs while maintaining legal-sized swimming crabs capture.

Marine mammal Bycatch Reduction Devices (BRDs) can result in the escape of target fish species. In the present study, a modified single-layer codend configuration developed from previous studies was evaluated using BRDs with guide net angles of 30° and 45°. Catch was collected separately from the codend and the BRD cover to estimate catch loss rates and characteristics. For caught species, catch loss rates based on the number and weight of individuals were 9% and 13% for herring with the 30° guide net and 16% and 18% with the 45° guide net, respectively. For Korean pomfret, catch loss rates were 24% and 23% at 30° and 4% at 45°. These results suggest that further modification of BRD is necessary. In contrast, catch loss rates of mackerel with the 30° guide net reached 56%, whereas hairtail showed loss rates of 60% and 65% with the 45° guide net. Underwater observations indicated that catch loss was influenced by swimming behavior and entanglement with the guide net. Despite the modification of the codend to a single layer, the present study still observed high catch loss rates. However, no bycatch of marine mammals occurred during the experimental operations.