Fishing and Gear Technology

Unit 11 - Trawl fishing gear and operation

11.2.Trawl Diagram
11.2.3.Selecting Netting Materials
The material most widely used in demersal trawls is Polyethylene. This is usually blue, orange or green in colour, and may be constructed in a twisted or braided form. The codend section of the trawl is most commonly constructed from braided twine, and is often made in the form of a double netting.
Polyamide or Nylon netting is widely used in the construction of Beam trawls, Mid water trawls and Shrimp trawls.
All synthetic materials are subject to shrinkage, which may have several causes, and could lead to a net contravening a fishing gear regulation.
Polyethylene is subject to shrinkage caused by prolonged periods of storage in direct sunlight.
Polyamide may be subject to shrinkage by submersion in water unless adequate heat setting processes have been undertaken during the twine and netting manufacture.
Contact with chemicals, hot water pipes etc. may also cause problems. Another form of shrinkage is caused by dimensional change. This is particularly common with braided twines that are used in close contact with a sandy sea bed. The sand particles find their way into the lumen of the twine causing an increase in diameter and reduction in length of the twine bars.
Rigging of otter boards:-Otter boards are rigid sheer devices which are used to keep; the trawl mouth, bridles and warps horizontally open. They keep the bottom trawl in contact with the sea bed and help to maintain the fishing depth of the mid water trawl .Otter boards were first used trawling in 1894 in Scotish waters. By the end of the century otter trawler had become popular. Originally otter boards were attached directly to the wings of the net. By around 1920, Vigneron-Dahl system was introduced. In this system otter boards were attached to wing by means of sweep lines and bridles. (Fig.page 69 Modern F.G.). There was an increase in the catch rate, the effective swept area and through the herding effect of sweep lines, bridles and otter boards on fin fishes. Otter boards contributes about 25% of the total drag of trawl system and is responsible for about 16% of total fuel consumption in trawling operation.