PHYSICAL AND PHYSIOLOGICAL-ASPECTS OF GEAR EFFICIENCY IN NORTH-SEA BROWN SHRIMP FISHERIES

In search of means to reduce the by-catch of juvenile flatfish in the shrimp fishery, vibrations and changes in current velocity caused by s hrimp trawls were investigated in the field and in the laboratory. Bur ied as well as emerged shrimps (Crangon crangon) exhibit tailflips 5-1 0 cm before being touched by the rollers of a shrimp gear approaching them at a speed of 0.5 m . sec(-1), as was revealed by slow motion vid eo recordings in aquaria under artificial Light. Hence, the signal eff ective in triggering escape must be attenuated strongly with increasin g distance. Sediment vibration, commonly assumed to be an important si gnal in triggering escape of shrimps, was found to decrease by a facto r 100 . m(-1). Signals from the rollers of a commercial shrimp gear in operation (towing speed 1 m . sec(-1)) were directly recorded with an accelerometer. Their frequency ranged from 50 to 500 Hz and reached a n acceleration of 40 m . sec(-2) on soft bottom or up to 100 m . sec(- 2) on hard substrate. Accelerometers, which had been buried right at t he surface of a tidal sand flat during low tide, produced only one sha rp signal of 100 Hz with an acceleration of 24 m . sec(-2), when a shr imp gear swept them on the submerged tidal flats. However, in aquaria short sinusoidal signals (<5 m . sec(-2); 20 to 300 Hz) made buried sh rimps and flatfish (Pleuronectes platessa, Solea solea, Microstomus ki tt) hide rather than flee. The vibrations recorded directly at the rol lers and the underlying jolting movements of the rollers induce corres ponding pulses in the water surrounding the rollers in a layer of appr oximately 10-15 cm. Similar water displacement of high acceleration wa s experimentally produced by a spring loaded transparent lucite piston (7 cm in diameter) fitted to an accelerometer. Accelerating this pist on (12-116 m . sec(-2), 50-200 Hz range) from 5 cm above sec towards t he shrimp produced escape responses in up to 94 % of the tests. Arthro pods are known to perceive medium displacement rather than pressure. H ence, strong and rapidly rising water currents caused by the rollers r ather than sediment vibration are assumed to mainly trigger the escape reaction, which makes Crangon accessible to the gear.