Although gillnet fisheries are increasingly common in Korea, few studies have conducted sea trials using tie-down gillnets. Here, we analyzed the mesh size selectivity using tie-down gillnets with four different mesh sizes (84, 90, 105, and 120 mm) to catch blackfin flounders. A total of 10 sea trials were conducted at depths of 100–140 m in the waters of Yangyang County, Gangwon Province, Korea, and the catching efficiency and mesh size selectivity were comparatively analyzed. The net with a mesh size of 84 mm showed the highest number of catches (373), followed by the 90-mm (363) and 105-mm (307) meshes. The results of the master curve estimation showed that the larger the mesh size, the higher the total length representing the same retention probability. Therefore, the catch rate of small fish decreased with the increasing mesh size. For the 84-mm net, the 50% selection length for small fish (total length ≤ 170 mm) was estimated at 186.9 mm, which was approximately 17 mm larger than the length limit. The results show that when using tie-down gillnets, the size of the fish caught changed according to the changes in the mesh size.

1. Ghost fishing is caused by derelict synthetic-fibre nets that have been lost at sea. Thus, biodegradable nets have been developed with the aim of protecting marine ecosystems. 2. The performance of biodegradable nets and their ability to replace conventional gillnets was assessed. Two types of biodegradable nets were used in the experiment: one made from a blended resin containing 90 wt% polybutylene succinate (PBS) and 10 wt% polybutylene adipate-co-terephthalate (PBAT) (Bio-1), and the other made from a polybutylene succinate co-adipate-co-terephthalate (PBSAT) resin (Bio-2) with improved flexibility and physical homogeneity. 3. For No. 3 monofilaments (standard diameter, Ø, 0.284 mm), Bio-1 and Bio-2 both showed a wet straight-line breaking strength of around 99% that of polyamide (PA). For No. 2 monofilaments (Ø 0.200 mm), the wet straight-line breaking strengths of Bio-1 and Bio-2 were 84.9 and 93.0% that of PA, respectively, indicating worse physical properties for Bio-1 at No. 2 thickness. 4. The wet knotted breaking strengths of No. 3 thickness Bio-1 and Bio-2 were 92.9 and 82.7% that of PA, respectively. The straight-line breaking strength of Bio-2 was 1.1 times that of Bio-1. The wet knotted breaking strengths of Bio-1 and Bio-2 were 80.2 and 93.4% that of PA, respectively, and the knotted breaking strength of Bio-2 was around 1.2 times that of Bio-1. 5. Regarding flexibility, for No. 3 thickness monofilaments, the mean wet stiffness values of PA, Bio-1, and Bio-2 were 9.50, 15.23, and 9.68 g, respectively, indicating that Bio-2 was similar to PA and around 1.6 times softer than Bio-1. 6. The fishing performance, by weight, was PA > Bio-2 > Bio-1; however, the difference between PA and Bio-2 was not statistically significant. Biodegradable nets may not be a perfect solution, but we believe that they can contribute greatly to reducing ghost fishing and protecting marine ecosystems.

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