Fishing and Gear Technology

Unit 10-Maintenance and storage of gears

10.7.Factors affecting fishing gear design
Important factors which influence the design of fishing gears are discussed below:

10.7.1.Biology, behaviour and distribution of target species
Choice and design of fishing gear is greatly influenced by biological characteristics such as body size and shape, feeding habits and swimming speed; behaviour in the vicinity of fishing gear and during capture process; spatial distribution and aggregation behaviour of the target species.
Body size and shape determine the mesh size required to enmesh and hold the fish in gill nets and the mesh size to retain the target size groups of the species with out gilling in the trawls, seines and traps. This is also related to the tensile strength requirements for the netting twine in gill nets and hook size and lines in hook and line. Body size is again directly proportional to the swimming speed. Swimming speed is directly proportional to the body size which is a significant attribute to be considered in the fishing success of a dragged gear. Feeding habit of the target species is more important in passive fishing methods like hook and line and traps where the fish is attracted by the bait, and in the active fishing methods like troll line used for catching predatory fishes.
Swimming speed of the target species is important particularly in the active fishing methods like trawling, seining and trolling. Fishes are known to sustain a cruising speed of 3-4 body lengths per second for long periods without fatigue and burst speeds of 10 body lengths per second for short duration. During burst speeds reserve energy supplies in the fish muscle is used up. Fish in front of the trawl mouth will be eventually caught if the trawling speed is greater than the cruising speed of the fish. Behaviour of different species might vary when they turn back into the trawl. It is reported that flat fish and cod turn back in the horizontal plane close to the bottom; whiting turn back at a level higher than this and haddock rise and turn at a still higher level. Such differential behaviour makes it possible to separate the different species using separator panels inside the trawl. Selective capture of the slow moving crustaceans providing opportunity for the fast swimming non-target fin fishes to escape, could be possible by controlling the towing speed and minimizing the longitudinal length of the trawl net.