Development of a grid for separation of squilla from shrimp catches– preliminary results

Abstract

Shrimp trawling contributes significantly to the total marine fish landings along Indian coast. But, the generation of bycatch is often high and about one third of the fauna caught, are discarded during trawling for shrimps. Changes are made to the shape/size of the codend and mechanical structures (grids) are introduced, to exclude non-targeted catch. Along Kerala coast, the abundance of shrimp and squilla coincides and it becomes difficult to segregate the shrimp from squilla which forms a discard. An oval shaped grid made of stainless steel was developed based on the premise that squilla would swim up and enter the upper codend whereas shrimps would enter the lower codend through the grid spaces. The lower part (65% of the total height) of the grid had horizontal rods of 4 mm spaced at 25 mm distance and upper part (35% of total height) formed a free space for the squilla to swim and enter the upper codend. The upper and lower parts of the grid ended into separate codends. A 27 m shrimp trawl was rigged with the grid and field trials were carried out off Cochin. A method was also developed for deriving the selection properties of the grid by in-situ method by simulating the free fall of individuals of the respective species used for study. A total of 13 hauls were tried using the gear and CPUE was 5.75 kilogram per hour. The upper codend recorded a CPUE of 3.88 kilogram per hour. while it was only 1.76 kilogram per hour in the lower codend. Anchovies (26.47 per cent ) , pomfrets (24.8 per cent ), shrimps (13.58 per cent), squids (11.03 per cent), croakers (8.11 per cent), carangids (5.2 per cent), leiognathids (1.9 per cent) and squilla (8.73 per cent) formed the major catch. The L50 values for P. stylifera were 95 mm and 105 mm respectively for the 22 and 25 mm spaced grids. The L50 values for M. monoceros were 113 mm, 128 mm and 129 mm respectively for the grid spacings of 22, 25 and 30 mm. The L50 values for squilla was lower than the shrimp species viz., 82 mm in 22 mm grids and 96 mm in 25 mm spaced grids. It shows that these spacing would not be enough for the separation. Grids with spacing of 25 mm and individuals of squilla with L50 more than 13 cm would be required for effectuating the separation of species. On-field experimental trials using grids with different spacing are required before the results can be up scaled for experimental trials in commercial trawls.