The hydrodynamic performance of one full-scale T0 (mesh size 90 mm) and three T90 (mesh size 90, 100, and 110 mm) codends was investigated and compared using flume tank testing, with and without a small-mesh cover. We evaluated how flow velocity, mesh circularity, and drag changed in each codend at five different towing speeds (0.5–0.9 m/s). The results demonstrated that flow velocity decreased along the length of a codend, and this effect was pronounced in the T0 codend. Increasing the mesh size of T90 codends from 90 to 110 mm did not significantly affect flow velocity. A novel parameter, termed mesh circularity, was developed and introduced to describe mesh opening. Mesh circularity in the T0 codend decreased along the length of the codend, which contrasted with the T90 codends. Results showed that the T90 codends maintained relatively open meshes (circularity ranged from ~0.8 to 1.0 along the length of the codend) compared to the T0 (circularity ranged from ~0.6 to 0.4). Each T90 codend had a significantly (p < 0.05) higher drag than the T0 codend when using the same simulated catch. For the covered codend comparisons, the flow velocity in the area between codend and cover did not change for the T0 codend (p > 0.05), but was significantly different for the T90 codend (p < 0.05). The results of this research provide fundamental knowledge useful for understanding and improving selectivity of trawls in marine fisheries, especially for revealing the masking effects of the cover net on the codend.

A multi-mesh trammel net has been developed and used for sampling freshwater fishes. However, little is known about the catch efficiency of the net. This research investigated the catch efficiency of a multi-mesh trammel net (nominal mesh size; 10.0, 30.0, 50.0, and 70.0 mm) for fish sampling in the Yangtze River of China. Catch composition and factors affecting catch per unit effort based on fish number (CPUE_N), weight (CPUE_W), and species number (CPUE_S) were analyzed. The results showed that the net was capable of capturing a wide range of fish sizes (total length, 5.5 to 121.0 cm) and species (n = 50). Increasing soak time from 9.4 to 24.0 h resulted in a decrease in CPUE_N and CPUE_W while a longer soak time increased CPUE_S. CPUE_N, CPUE_W, and CPUE_S varied significantly with fishing locations. The net provides a potential complement to the current fish sampling techniques used in freshwater ecosystems (e.g., large rivers, lakes, and reservoirs). The findings of this research help to improve our understanding of the catch efficiency of the multi-mesh trammel net and provides insight into better designs for gears and methods for sampling diverse fish sizes and species.

Traps with various designs have been developed for improving catch efficiency. Traditionally, traps with two or three entrances have been used in recreational crayfish (Procambarus clarkii) fisheries in China. More recently, traps with more than three entrances have been used in fisheries; however, it is unclear whether increasing the number of entrances on a trap increases crayfish capture efficiency. In this research, we evaluated and compared the capture efficiency of crayfish with traps varying in numbers of entrances (2–7). We assessed the catch per unit effort (CPUE) of crayfish in traps through 10 fishing trials. Our results suggested that CPUE did not differ significantly (p = 0.070) among the tested traps; however, CPUE by weight did vary significantly (p = 0.029) among the traps. The tested six-entrance traps caught more crayfish by weight than the five-entrance traps (p = 0.019). The results also revealed that the entrance number did not significantly (p = 0.29) affect the size of crayfish captured. The results are of practical significance for stakeholders to choose traps with efficient designs, and they are important for understanding and improving the catch efficiency of traps.

The Japanese mantis shrimp (Oratosquilla oratoria) is a commercially important crustacean species distributed around the coastal waters of China, which is harvested with both active and passive fishing gears. Among these, traps are one of the primary passive gears used in the Bohai Sea, which is one of the most important fishing grounds for Japanese mantis shrimp in northern China. However, the use of traps with a small mesh size challenges the sustainability of this fishery. Therefore, this study quantified the size selectivity and harvesting pattern for traps with different mesh sizes for harvesting Japanese mantis shrimp in the Bohai Sea. Results showed that the 50% retention length increased from 6.39 cm to 12.44 cm as mesh sizes increased from 20 (T20) to 60 mm (T60). T20 retained 86.8% and 100% of individuals, respectively, below (np-) and above (np+) the minimum landing size. Increasing mesh sizes decreased both np- and np+. Compared with T20, T30 reduced undersized catch by 67.8% while decreasing legal-sized catch by 8.2%. Increasing mesh sizes beyond T40 provided no significant reduction in np- but caused further reduction in np+. Therefore, the T30 mesh trap was recommended to replace the currently applied T20 in the Bohai Sea.