Reduction of fish by-catch in shrimp trawl using a rigid separator grid in the aft belly

Abstract

Abstract In 1989 and 1990, a new concept to avoid the by-catch of fish in a shrimp trawl was developed in Norway. The system consists of a rectangular aluminium grid with longitudinal bars. The grid is installed in the extension piece, just in front of the codend, angled 48°, with a fish outlet on the top. In front of the grid a guiding funnel or flapper is mounted. Investigations into different grid sizes, bar distances, size of fish outlets, and guiding funnel or flapper constructions during practical tests, have led to several recommended measures. Smaller coastal shrimpers should use grids at least 0.7 m wide and 1.40 m long, while the bigger offshore trawlers should use a 1.0 m wide and 1.5 m long grid, both with a recommended bar separation of 19 mm, which gives an acceptable shrimp loss below 5%. RCTV-observations of the sorting grid system during fishing have shown that most of the shrimps (Pandalus borealis) passed in a straight line through the grid, while fish were guided upwards towards the fish outlet, either by swimming in front of the grid, or sliding along the bars. Some of the smaller fish passed through the grid and ended up in the codend. A combined speed and angle sensor, mounted just behind the grid, measured the horizontal speed of water through the grid to be 0.7 of the towing speed, and the angle of the grid to be 3–5° less than the theoretical mounting angle of 48°. The separator grid allowed an increasing escape of cod (Gadus morhua) and haddock (Melanogrammus aeglefinus) up to a length of 20 cm, whereby all fish escaped. The 100% escape lengths for redfish (Sebates sp.), long rough dab (Hippoglossoides platessoides), and polar cod (Boreogadus saida) were 18, 24 and 24 cm, respectively. The Greenland halibut (Reinhardtius hippoglossoides) gave a rather high 100% escape length of 30–32 cm, probably due to the behaviour of this flatfish which swims on its side. The handling of the separator system has so far caused only minor problems, and the pre-sorting of fish by the separator grid results in less sorting work on deck. The separator grid was made compulsory in the northern coastal shrimp fishery from 1 March 1990, from 1 January 1992 in the offshore fisheries and will be compulsory in the Barents Sea and Spitsbergen waters from 1 January 1993.