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 less well studied. Therefore, several experimental studies have been carried out into the effects of pulse on fish in saltwater, in which several (commercial) fish species 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.

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 40 Hz or higher invoked 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

Laboratory experiments suggest that only those cod that are located within the trawl track run the risk of being exposed to a field strength that may invoke a vertebral fracture. In particular the cod that are located in close range to the electrodes are prone to develop a vertebral fracture. 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. Because the occurrence of vertebral fractures appears to be restricted to the cod that are retained in the net, it is unlikely that pulse trawling cause additional mortality to the population. Another experiment indicates that small cod, that are small enough to escape through the 80mm meshes of the cod-end, do not develop fractures.

De Haan et al (2016) analysed the effects on cod of exposure to pulse in lab experiments. It was concluded that:

  • None of the cod of a size class that can escape through the 80 mm meshes of the sole fishery, that were exposed to the highest field strength close to the conductor did develop fractures.
  • 70% of the marketable sized cod exposed to the highest field strength close to the conductor developed a fracture in the spine, haemal and/or neural arches.
  • Vertebral fractures were associated with a haemorrhage and a discoloration of the body. The probability to develop a fracture (or haemorrhage) increased with field strength and decreases with frequency.
  • In the marketable sized cod, the fracture probability decreased with body size.

In another experiments with similar pulse settings and similar location of the cod next to the conductor, much fewer fractures were observed, suggesting that body condition may influence the sensitivity for injuries (Soetaert et al., 2016a).

When examined histologically, cod exposed to a homogeneous electric field with a range of pulse settings, including those of the commercial fisheries, did not show any abnormalities except for one cod showing a spinal fracture (Soetaert et al., 2015). 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).

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).

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 dab

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 other fish species

Desender et al (2016) exposed plaice, bull-rout, armed bullhead to a shrimp pulse and could not detect any lessions, except for a small haemorrhage in 2 of the 25 exposed plaice.

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