A comparative study of spinal injuries in fishes caught by pulse trawling and traditional beam trawling

Pulse trawling is an experimental bottom fishing method used to target common sole in the southern North Sea. The electrical pulse stimulus invokes a cramp response which immobilises the fish in front of the net. This pulse fishing technique has several advantages over conventional beam trawling with tickler chains, including reduced fuel consumption, fish and benthic discards, and seabed damage, whilst survival of undersized flatfishes and revenues are higher. However, spinal injuries have been reported in non-target species such as Atlantic cod and whiting, which have raised concerns regarding possible adverse effects on marine organisms. Here, we present results of a comparative, quantitative assessment of spinal injuries in a range of fish species caught by commercial bottom trawlers. To pinpoint the cause for observed injuries, we extensively sampled fishes from pulse trawlers with the electrical pulses on and off, and from ticker chain beam trawlers. All specimens were analysed by radiography to visualise internal spinal injuries. X-ray images were analysed using custom-made software to highlight the type, location, and severity of the injuries, if present, in the fishes. Our data show that fish species vary in their susceptibility to spinal injuries observed for the different bottom trawling techniques. In addition, we explore whether susceptibility to spinal injuries depends on fish size. These results will be discussed in relation to the use of electrical pulses and differences in mechanical stress. Subsequent studies may point out the cause for the observed injury susceptibility differences in various fish species.