Effects on fish
Electro-fishing in freshwater has been well studied and there is ample evidence for vertebral fractures and associated haemorrhages (review in Soetaert et al., 2015). Electrofishing in the marine environment has been studied in several (commercial) fish species that were exposed to a flatfish or shrimp pulse stimulus. The following paragraphs provide a synthesis of the main results for the effects on sole, cod, seabass, dab and other fish species.
Detection threshold of a pulse stimulus by fish
Tank experiments of Boute (in Rijnsdorp et al., 2020) showed that two electrosensitive species and one non-electrosensitive species can detect pulse stimuli when the field strength exceeds a level of about 5 V/m. Although electroreceptive fish like elasmobranchs are highly sensitive to low frequency electrical stimuli generated by their prey organisms, they are not specifically sensitive to high frequency pulses used in the pulse trawl fishery for sole.
The detection threshold found in the experiments corresponds to the field strength observed within a pulse trawl. Outside the pulse trawl, the electric field the field strength quickly dissipates with increasing distance to the electrodes. The tank experiments did not show any attraction or avoidance response of fish to pulse stimuli that occur outside the trawl.
Pulse effects on sole
Sole exposed to a 5Hz pulse showed a flight response and muscle contractions similar to the normal fin fluttering. Pulse frequencies of 47 Hz or gigher invokes a cramp response during which the fish bended in a U-shape. After exposure, all soles showed normal behaviour (Soetaert et al., 2016).
Pulse effects on cod
In terms of behaviour, cod showed a flight response to pulse frequency of 5 Hz. A cramp response was induced in cod exposed to pulse frequencies of 40Hz or higher (Soetaert et al. 2016) and in cod exposed to a field strength of 37 V/m and higher (de Haan et al., 2016). High field strength invoked an epileptic response. Within 10 minutes after exposure, most of the fish were breathing normally but showed little swimming activity and weak reactivity to tactile stimuli. All fish survived and showed normal behaviour 24h post exposure (Soetaert et al., 2016). Cod resumed feeding after exposure although their appetite was related to the field strength. Cod exposed to a field strength that invoked vertebral fractures (82 V/m) were passive and did not resume feeding (de Haan et al., 2016).
Laboratory experiments show that the occurrence of spinal injuries in cod increase with the field strength (de Haan et al., 2016). Cod exposed to a field strength of less than 37 V/m, which is typical for the maximum field strength that is measured outside of the array of electrodes (outside of the pulse gear), will unlikely develop a vertebral fracture. Cod, that are small enough to escape through the 80mm meshes of the cod-end, do not develop fractures when exposed to a pulse stimulus (de Haan et al., 2016). The sensitivity for spinal injuries may depend on the body condition of cod (Soetaert et al., 2016). Cod exposed to a shrimp pulse did not develop fractures but showed a significant increase in melanomacrophage centres in the spleen (Desender et al., 2016).
Pulse effects on seabass
Seabass showed a cramp response when exposed to a pulsed bipolar current of 80 Hz, pulse width of 250 μs, duty cycle of 2% and exposure duration of 2 seconds of wire-shaped electrode. Directly after exposure, the fish showed a strong flight response swimming away from the point of exposure. When removed to their housing tank, all fish resumed normal swimming behaviour. During the 2 week observation period after exposure, all fish showed normal feeding behaviour. In lab experiments, none of the small and large seabass exposed to a sole pulse stimulus developed a vertebral fracture or any other lesion and survived the 14 days after exposure, although the number of fish tested (31 tested, 13 control) was relatively small (Soetaert, 2015).
Pulse effects on dabb
In response to reports on an increase in the incidence rate of ulcers in dab off the Belgium coast coinciding with the start of the pulse trawling, a laboratory experiment was conducted in which 100 wild caught dab were exposed close to the conductor generating a commercial pulse trawl stimulus and 50 dab were used as control. The fish were kept for 2 weeks in the lab and euthanized for post-mortem analysis. After exposure, all fish showed normal behaviour and resumed feeding. One dab died on day 13 without any visible injury and likely unrelated to the pulse stimulus. No difference in the incidence rate of lesions of the exposed dab with the control fish was observed (de Haan et al., 2015).
Pulse effects on sandeel
Three experiments were carried out with 253 lesser sandeel and 49 greater sandeel exposed to a commercial sole pulse. In two of the 230 sandeel exposed to a pulse stimulus a spinal injury was recorded against none in the 211 sandeel that were handled but not exposed. Given the low injury rate of exposed sandeel in our experiment, we conclude that the high injury rate observed sandeel sampled from commercial pulse and tickler chain beam trawlers is likely due to mechanical damage inflicted during the catch process and subsequent processing of the catch on deck (Schram et al., in Rijnsdorp et al., 2020).
Pulse effects on other fish species
Desender et al (2016) exposed plaice, bull-rout, armed bullhead to a shrimp pulse and could not detect any lesions, except for a small haemorrhage in 2 of the 25 exposed plaice.
Occurence of spinal injuries on commercial catches
The occurrence of spinal injuries was studied by sampling the catch of commercial pulse trawlers and compared to samples from commercial tickler chain trawlers and pulse trawlers where the pulse stimulus was switched off (Boute in Rijnsdorp et al., 2020).
It was found that 36% of the sampled cod showed spinal injuries similar to those found in the exposure experiments to a pulse stimulus. The injury probability was highest in medium sized cod of around 40 cm and decreased in smaller and larger cod. In all other fish species relatively low spinal injury probabilities were found (<2%), except for cod. No clear difference was found in the injury probability between pulse-on and pulse-off caught fish for dab, plaice, grey gurnard and whiting. For tub gurnard a spinal injury was observed in 3 out of 249 tub gurnards caught with the pulse-on, but none in 67 tub gurnards caught without the electrical pulse stimulus.
The probability of spinal injuries observed in conventional beam trawl catches was at the same level as observed in pulse trawl caught fish, or slightly higher. In lesser sandeel (Ammodytes tobianus) and greater sandeel (Hyperoplus lanceolatus), however, injury probability in both the pulses on and tickler chain catches are elevated. Since injury probability in the tickler chain catches are highest, these injuries are likely caused by mechanical stimulation. As these species are relatively slender and elongated, a potential selection bias of injured specimens in the 80 mm meshes of the cod-end could result in an overestimation of the injury probability.
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