Short-term effect of pulsed direct current on various species of adult fish and its implication in pulse trawling for brown shrimp in the North Sea

- Marine fish species were exposed to an electrical field used to catch brown shrimp.
- Twenty-four hours following exposure, no mortality nor spinal injury were observed.
- Behavioral reactions were species dependant, 15% of sole swam upward upon exposure.
- Microscopic bleeding was noted in the dorsal muscle of 2 out of 25 exposed plaice.
- Increase of the number melano-macrophagecenters in the spleen of exposed cod.

Electric pulses in fishing gear are increasingly used in the North Sea and are considered a promising alternative to ameliorate the sustainability of demersal trawl fisheries. The electrotrawl for brown shrimp employing low frequency pulsed direct current (PDC) selectively induces a startle response in shrimp engendering decreased environmental impact and reduced by-catch. Prior to commercially introducing this fishing technique, data on its impact on marine organisms are crucial. The aim of this study was to evaluate the short-term effects of this pulse used for electrotrawling for brown shrimp on five marine fish species inhabiting shrimp fishery areas. For this purpose, 25 European plaice (Pleuronectes platessa), 30 Dover sole (Solea solea), 20 Atlantic cod (Gadus morhua), 19 bull-rout (Myoxocephalus scorpius) and 20 armed bullhead (Agonus cataphractus) were exposed to the shrimp pulse for 5 s. Before, during and till 20 min following exposure, the behaviour of the fish was monitored. Twenty-four hours post-exposure, all fish were sacrificed, inspected and samples for histological analysis were taken from the gills, dorsal muscle and internal organs. To investigate possible spinal injuries radiographs were taken. Behavioural responses were variable and species dependant. Roundfish species, cod in particular, were displaying more active and agitated fast swimming activity during exposure. The majority of flatfish showed only minor reactions and remained close to the bottom throughout the observation period. However, 15% of the exposed sole actively swam upwards during exposure. Mild multifocal petechial haemorrhages and suffusion, encountered mainly in plaice and sole, were not significantly different between exposed and control groups. Upon histological examination, in two exposed plaice, a focal small haemorrhage between muscle fibres was found, which was not encountered in control animals. In addition, the number of melanomacrophage centres in the spleen of exposed cod was significantly higher than in the non-exposed animals. In conclusion, under the circumstances as adopted in this study, the electrical field seemed to have only limited immediate impact on the exposed animals.