The size selectivity of four codends were compared in the Gulf of St. Lawrence, Canada redfish fishery, including the regulated diamond mesh codend with a mesh opening of 90 mm (T0) and three experimental codends of different mesh openings (90, 100, 110 mm) in which the netting is turned 90° to the direction of tow (T90). Results for the regulated codend showed that there was little size selection, catching greater than 97% of redfish over all of the length classes observed. Considering the fished population, the smallest T90 codend would catch 30% fewer redfish under the Minimum Landing Size (MLS) of 22 cm compared with the T0 codend, but would also lose 16% of catch above 22 cm. The T90 codend with 100 mm mesh opening had the same size selectivity as the smallest T90 codend. The 110 mm T90 codend would catch 50% less redfish below MLS but lose 40% of redfish above. Overall, results show that T90 codends improve size selectivity in which large proportions of undersized fish are successfully released.

The size selectivity and usability of two diamond mesh codends, a traditional two-panel codend versus an experimental four-panel ultra-cross knotless mesh codend, were compared using the covered codend method in the Iceland redfish (Sebastes norvegicus and S. viviparous) fishery. Results showed that there was no significant difference in size selectivity between the codends at lengths greater than 29 cm for S. norvegicus and 19 cm for S. viviparous. At smaller lengths, size selectivity was undetermined due to small catches at those sizes. For S. norvegicus, both codends demonstrated a high retention ratio (93.4 and 92.9%, respectively) above the minimum reference length (MRL; 33 cm), but also had a high retention below MRL (90.9 and 83.4%, respectively). However, the actual proportion of catch below MRL was low due to few small fish on fishing grounds. Since these fish are difficult to tell apart and have similar morphologies, we investigated the size selectivity of the two codends for both species combined, resulting in similar results of no difference in size selectivity, but a large increase in actual catches below MRL, which were primarily S.viviparous. This study concludes that the experimental codend does not improve the size selectivity or usability in the Iceland redfish fishery and both codends will retain large proportions of undersized fish if present on fishing grounds; however, few undersized fish were present in the study area.

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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.

To reduce the occurrence of ship collisions, immediate danger, and close-quarters situations in narrow inland waterways, a step-by-step early warning system for ship collision-avoidance actions was developed, along with an early warning method and model of collision risk based on coordinated collision-avoidance actions. This study first analyzed the importance of coordinated collision-avoidance actions in inland waterways, and the process and key components of coordinated collision-avoidance actions were studied. Then, the early warning method of inland ship collision risk based on coordinated collision-avoidance actions was introduced; the effectiveness of the early warning method was comparatively analyzed via experimental observations. A framework of early warning model of inland ship collision risk was created based on the early warning method; a collision risk early warning model for inland ships based on coordinated collision-avoidance actions was proposed according to the relationship between the distance/time to the closest point of approach (DCPA, TCPA), coordination degree of collision-avoidance actions of the two considered ships and collision risk; moreover, the early warning model of inland ship collision risk was further considered for quantitative calculation. Finally, the application of the early warning method and model was demonstrated using a case study. The results indicate that the early warning method of inland ship collision risk based on coordinated collision-avoidance actions could effectively reduce the emergence of close-quarters situations and immediate danger, and the early warning model could quantitatively show the evolution of collision risk of two ships along with the process of coordinated collision-avoidance actions.

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