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11.2.5.Scope ratio
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.
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
- 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
Last modified: Wednesday, 28 March 2012, 9:01 AM