Fishing and Gear Technology

Unit 10- Maintenance and storage of gears

10.7.Factors affecting fishing gear design
10.7.1.Biology, behaviour and distribution of target species
Behavioural differences between fish and crustaceans and size difference between them, could be used in the design of selective trawl designs. In such designs rigid grids are placed at an angle, before codend. Small sized prawns move through the grid into the codend while fish and other non-target species are deflected by the grid and are released through an escape chute. Such devices are sometimes called Trawl Efficiency Devices as the they reduce the sorting time and thus increases the efficiency of operations. Protected species like turtles are allowed to escape in a similar way using Turtle Excluder Devices(TEDs)
Large mesh trawls and rope trawls, in which front trawl sections are replaced with very large meshes or ropes in order to reduce drag, make use of the principle of repulsion of herding to guide the finfish into trawl codend. In the conventional trawling systems, herding effect by the otter boards, wires and sweeps and sand-mud cloud created by the boards on finfishes in between the boards, is made use of to improve the catch rate by increasing the effective sweep area. Long leader nets placed in the path of migratory fishes guide them into large set nets operated in Japan. Tendency of some fishes to aggregate towards light is used in squid jigging, light-assisted purse seining and dip net operations. Behaviour of fishes like tuna to aggregate around the floating objects, is utilized successfully in Fish Aggregating Devices (FAD)-assisted purse seining.
Catching efficiency is maximized when the vertical opening of the trawl mouth, vertical dimension in gill nets, and the catenary of the main line of the long line with branch lines and hooks, coincide with the vertical range of the layer of maximum fish abundance. Hence knowledge of the vertical distribution of the target species could be used to optimize the horizontal and vertical dimensions of the netting panels in gill nets, main line catenary in long line and mouth configuration in trawls. Some species of fish are sparsely distributed either singly or in small groups and thus exhibits a pronounced patchiness, while some others form dense schools. Sparsely distributed and scattered fish are more efficiently caught by passive fishing methods such as gill netting and long lining, where as schooling fishes are effectively caught by purse seining and aimed midwater trawling.