Fishing and Gear Technology

Unit 11 - Trawl fishing gear and operation

11.2.Trawl Diagram

11.2.5.Scope ratio
It is the ratio between length of warp line released to the depth of water in which a trawl is operated. It varies with towing speed, type of ground, depth and type of species to be caught.
For bottom trawling the towing speed ranges from 1.5 to 3.5 knots depending on size of the vessel. The bottom towing speed generally ranges from 1.5-2.0 knots for shrimps, 2-3 knots for fishes. For mid water and pelagic trawls it is 3.5-6.0 knots.
Towing a particular trawl too slowly may cause the otter boards to close together, resulting in insufficient spreading of the net which may sag on the bottom. On the other hand, towing too fast could result in the net lifting off the bottom and floating, which may lead to its turning over and becoming foul gear. Scope ratio is around 1:5 in shallow waters less than 20 m with smooth ground. The warp length could be 5-10 times the depth in deep water for operating a heavy gear and in rough grounds length of warp could be 2.5-4 times the depth. Scope ratio is mainly depending on the nature of the fishing ground for e.g.

• For smooth fishing ground : slightly reduce the scope ratio
• For sandy bottom: slightly increase the scope ratio
• For rough ground: greatly reduce the scope ratio

Swimming speed of the fishes to a large extent determines the required trawling speed. As the trawl drag depends on the towing speed, maximum dimensions of the trawl will be determined by the available towing force of the vessel and the towing speed requirements for the target species. Swimming ability of fish depends largely on their size and varies with different species. If the trawling speed is above the cruising speed, the fish in front of the footrope would eventually tire and drop back to be caught in trawl codend.

• Typical towing speed for beam trawls 1.0 - 2.5 knots
• Typical towing speed for bottom trawls 2.0 - 4.5 knots
• Typical towing speed for mid water trawls 3.0- 8.0 knots