Effects on invertebrates

Pulse effects on various benthic invertebrates

Smaal and Brummelhuis (2005) exposed a variety of benthic invertebrates to a Delmeco sole pulse for 10 seconds. Some species showed a response to the electrical stimulus by closing their shells (bivalves), withdrawing themselves in their shell (whelk, hermit crab) or showing a tail flip response (decapod shrimps), while other species (polychaetes, Echinodermata) did not show a visible response. The experiments did not suggest that electrical stimulation affected the filtration rate of bivalves or the mortality as compared to the control group. Because the details of the pulse characteristics were not enclosed in the dataset, the results are only indicative for the possible effects.

Additional research (Boute et al., under review) has been carried out to measure the responses of starfish and serpent star, sea mouse, whelk, hermit crab and flying crab to pulse stimulation. The effects varied from no effect (starfish and serpent star) to moderate squirming (sea mouse) and fast retractions (whelk, hermit crab, flying crab). Within 30 s after stimulation, all animals resumed normal behavioural patterns, without signs of lasting immobilization. About two-thirds of the whelk (63%) ejected a white substance during or immediately after stimulation, presumably related to reproduction. No indications were found for compromising changes in righting reflexes and locomotor activity, except for significantly increased righting reflex duration after electrical stimulation in hermit crab due to increased retraction times. Animal survival was not negatively affected. These findings suggest that electrical pulses as used in pulse trawling are unlikely to substantially affect the behaviour and survival of the investigated species.

Pulse effects on shrimps and ragworms

To detect the safe range of pulse parameters, Soetaert et al. (2014) exposed brown shrimps and ragworms to a homogeneous electric field for up to 5 seconds and studied their behaviour, 14-d mortality rate, gross and histology. Pulse setting included the commercially applied frequency and field strengths. No adverse effects were detected except for an increase in a virus infection (IBV) in the hepatopancreas in shrimps exposed to the maximum field strength (200 V/m).

In a follow up experiment studying the effects of repetitive exposure in shrimps, however, this result could not be corroborated. In this experiment, brown shrimps were exposed 20 times during 4 days to either the sole pulse or the shrimp pulse. The survival, egg loss, moulting and the degree of IBV infection was compared shrimps exposed to electrical pulses, shrimps exposed to mechanical disturbance mimicking the conventional shrimp trawling and a control group. The sole pulse treatment gave a significant lower 14-day survival as compared to the control group, while moulting was reduced by mechanical disturbance.

Because pulse trawls used in the fishery for sole are towed at a slower speed (5 knots or less) than the traditional tickler chain beam trawls (towing speed between 6 and 7 knots), pulse trawls have a smaller annual footprint in terms of the area fished.

Pulse effects on burrowing organisms

During laboratory studies on burrowing organisms, no mortalities were observed from direct electrical stimuli. This indicates that electrical impacts on non-target species are non-lethal. Claims of burrowing organisms coming out of the sediment in response to electrical exposure are not supported by these studies though some evidence of increased burrowing behaviour was observed with A. marina. The results suggest that non-lethal effects and possible biogeochemical consequences (i.e. declines in sediment oxygen levels) due to changing behaviour are temporary. Compared to trawl-induced mechanical impacts, the effects of electrical exposure to macrofaunal functioning seem to be minor