The trawl door (also known as the net plate) is a crucial diversion component in trawl fishing. Its primary function is to open the trawl net opening, allowing the net to cover a wider fishing area. The size of the trawl door is not a fixed value; it is flexibly set according to the specifications of the fishing vessel, the target fish species for fishing, and the environmental conditions of the fishing area.
During daily operations, the size of the trawl door is usually measured by area, height, and width. The specifications for small coastal fishing vessels and large ocean-going fishing vessels differ greatly, and the overall design must be matched with the power of the fishing vessel and the trawl specifications to ensure fishing efficiency and operational safety.
From practical applications, the trawl doors for small coastal fishing vessels (with a tonnage of less than 50 tons) typically have an area of 1-3 square meters, a height of about 1.2-2.5 meters, and a width of 0.8-1.5 meters. They are mostly made of steel plates or aluminum alloys and are suitable for fishing in nearshore shallow sea areas for shrimp, crabs, and small bottom fish. Medium-sized fishing vessels (with a tonnage of 50-200 tons) have trawl doors with an area of 4-8 square meters, a height of 2.5-4 meters, and a width of 1.5-2.2 meters.
The structure will have additional strengthening ribs to enhance stability, and they are suitable for fishing in the middle and far seas for species such as cod and yellowtail. Large ocean-going trawl fishing vessels (with a tonnage of more than 200 tons) use trawl doors with larger specifications, with an area of 10-20 square meters, a height of 4-6 meters, and a width of 2.2-3.5 meters. Some will use composite materials to reduce weight while enhancing strength, and they are suitable for deep-sea large-scale fishing operations such as catching cod and squid.
Apart from the tonnage of the fishing vessel, the fishing method also affects the size of the trawl net door. The trawl net door for single-towing operation is relatively smaller, while for double-towing operation, as it requires the two sides to open the fishing net simultaneously, the size will be slightly larger.
The trawl net door for bottom trawling needs to cope with the complex seabed terrain, and its size design pays more attention to stability. The trawl net for the surface layer can be appropriately optimized to increase the expansion effect.
Additionally, the water flow and wind wave conditions in different sea areas will also adjust the size. For example, in areas with rapid water flow, the area of the trawl net door will be appropriately reduced to lower resistance and ensure operational safety.