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 shrimp trawls were investigated in the field and in the laboratory. Buried as well as emerged shrimps (Crangon crangon) exhibit tailflips 5–10 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 video recordings in aquaria under artificial light. Hence, the signal effective in triggering escape must be attenuated strongly with increasing distance. Sediment vibration, commonly assumed to be an important signal in triggering escape of shrimps, was found to decrease by a factor 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 an 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 the surface of a tidal sand flat during low tide, produced only one sharp signal of 100 Hz with an acceleration of 24 m·sec⁻², when a shrimp gear swept them on the submerged tidal flats. However, in aquaria short sinusoidal signals (<5 m·sec⁻²; 20 to 300 Hz) made buried shrimps and flatfish (Pleuronectes platessa, Solea solea, Microstomus kitt) hide rather than flee. The vibrations recorded directly at the rollers and the underlying jolting movements of the rollers induce corresponding pulses in the water surrounding the rollers in a layer of approximately 10–15 cm. Similar water displacement of high acceleration was experimentally produced by a spring loaded transparent lucite piston (7 cm in diameter) fitted to an accelerometer. Accelerating this piston (12–116 m·sec⁻², 50–200 Hz range) from 5 cm above towards the shrimp produced escape responses in up to 94% of the tests. Arthropods are known to perceive medium displacement rather than pressure. Hence, strong and rapidly rising water currents caused by the rollers rather than sediment vibration are assumed to mainly trigger the escape reaction, which makesCrangon accessible to the gear.