Trawl fishing is one of the most widely used methods in commercial and industrial fisheries around the world. The effectiveness of this technique largely depends on the proper rigging of trawl gear. Rigging refers to how the different components of the trawl system are assembled and configured to achieve optimal performance, durability, and catch efficiency. From the headline to the codend, every detail matters in ensuring that the gear performs well in varying marine environments. Whether you’re using bottom trawls or midwater trawls, understanding the principles and techniques of trawl rigging can significantly enhance fishing productivity and reduce unintended bycatch.
Understanding Trawl Gear Components
Basic Structure of a Trawl Net
A trawl net is a conical-shaped net that is towed through the water to capture fish. It consists of several parts:
- Headline: The top edge of the net, which usually contains floats to keep it elevated.
- Footrope: The bottom edge that may carry weights or rollers to keep it close to the seabed.
- Wings: The side panels that guide fish toward the net opening.
- Body: The middle section leading to the codend.
- Codend: The closed end of the net where the fish are collected.
Towing System
To tow the net, a vessel uses a towing system that includes:
- Warp wires: Steel cables that connect the trawl doors to the vessel’s winch.
- Trawl doors (otter boards): Hydrodynamic boards that spread the net horizontally.
- Bridles and sweeps: These connect the doors to the wings of the net and help guide fish into the net opening.
Rigging Bottom Trawl Gear
Purpose and Usage
Bottom trawls are designed to operate along the seabed and are effective for species like cod, haddock, shrimp, and flatfish. Proper rigging ensures the gear can navigate uneven terrain while maintaining catch efficiency and minimizing damage.
Ground Gear Configuration
The ground gear consists of components like rollers, rockhoppers, or bobbins that help the net move over rough seabeds without snagging. Choosing the right ground gear depends on bottom type:
- Rubber discs: Suitable for smooth seabeds.
- Rockhoppers: Ideal for rocky or uneven bottoms, as they allow the net to hopĀ over obstacles.
- Bobbins: Help in maintaining contact with the bottom while protecting the footrope.
Floatation and Spacing
Floats are attached to the headline to help maintain the net’s shape and ensure that the mouth of the net stays open. The number and size of floats will vary depending on net size, depth, and the weight of the footrope. Balancing floatation and weight is key to keeping the net aligned correctly while towing.
Rigging Midwater Trawl Gear
Target Species and Depths
Midwater trawls are used for pelagic species like mackerel, herring, and sardines. These nets operate in the water column, and their rigging must ensure that the net remains suspended at the desired depth.
Depth Control Mechanisms
Unlike bottom trawls, midwater gear requires sophisticated systems to control net depth:
- Depth sensors: Monitor the position of the gear in real time.
- Autotrawl systems: Automatically adjust warp length and towing speed.
- Buoyancy materials: Used in the net to fine-tune floatation characteristics.
Light and Sound Attraction
For midwater trawling, artificial lights or sound-emitting devices can be used to attract target species. These are strategically rigged near the mouth or body of the net to increase fish aggregation before entering the codend.
Bridle and Door Rigging Techniques
Door Spread and Angle
One of the most critical factors in effective trawl gear rigging is the angle and spread of the doors. These must be positioned to maximize the horizontal opening of the net without causing drag that could collapse the net structure.
- Short bridles: Create a wider door spread but reduce net opening height.
- Long bridles: Increase vertical lift but can narrow the door spread.
- Adjustable doors: Modern trawls use doors that can change angle based on water pressure and speed.
Equalizing Load Distribution
Bridles must be rigged to distribute load evenly across the net. Improper load balance can cause twisting, damage to the netting, or inefficient towing. Using symmetry and proper measurements helps maintain structural integrity under tension.
Codend and Selectivity Devices
Codend Mesh Size
The codend must be rigged with appropriate mesh size to retain target species while allowing undersized or non-target fish to escape. Regulations in many countries mandate specific mesh dimensions and types to promote sustainable fishing practices.
Bycatch Reduction Tools
Selective rigging can reduce environmental impact and improve efficiency. Some common additions include:
- Bycatch reduction devices (BRDs): Openings or grids that allow non-target species to escape.
- Turtle excluder devices (TEDs): Used primarily in shrimp trawls to prevent sea turtle capture.
- Square mesh panels: These sections allow more juvenile fish to escape compared to diamond mesh.
Maintenance and Safety Practices
Inspection Before Deployment
Always inspect netting, ropes, and towing gear before launching. Worn or damaged materials can lead to gear loss or failure. Ensure that all connections are secure and that safety pins or quick-release mechanisms are functioning properly.
Regular Gear Servicing
Trawl gear should be serviced regularly to ensure peak performance. This includes checking for tears in the mesh, corrosion in metal components, and adjusting floats or weights as necessary. Maintenance not only prolongs the lifespan of the gear but also improves efficiency on the water.
Crew Safety
Rigging heavy trawl gear can pose serious risks. It is crucial that all crew members are trained in handling winches, cables, and towing equipment. Personal protective gear, communication devices, and emergency protocols should always be in place during rigging operations.
Environmental Considerations
Reducing Seabed Impact
Bottom trawls have been criticized for their potential to damage seabed ecosystems. To mitigate this, use lighter gear, reduced tow speeds, and selective rigging techniques that minimize contact with sensitive areas. The use of semi-pelagic doors and lighter footropes is becoming more common in sustainable trawling operations.
Responsible Catch Practices
Proper rigging also aids in reducing catch of non-target species. High-efficiency, low-impact designs help ensure that only the desired species are retained, supporting both fishery health and long-term economic benefits for fishing communities.
Rigging of trawl gear is a complex but essential process that directly affects the success of fishing operations. Each component, from the towing system to the codend, must be carefully considered and correctly configured based on target species, fishing environment, and regulatory requirements. Whether using bottom or midwater trawls, optimal rigging can increase catch rates, reduce fuel consumption, and support sustainable practices. As technology evolves, new materials and smart rigging systems continue to improve performance, making modern trawling more effective and environmentally responsible than ever before.