The effects of pulse stimulation on biota – Research in relation to ICES advice – Effects on dogfish

In response to questions asked by ICES on the effects of pulse stimulation in commercial beam trawling on components of the marine ecosystem a number of preliminary studies were undertaken in the period between 1 November 2008 and 01 June 2009.

The study that we report on here involved the exposure of lesser spotted dogfish to a simulated electric pulse under laboratory conditions, and monitoring of mortality, injuries and behavioural responses, in particular feeding response. The electric pulse simulator was made available by Verburg-Holland Ltd. with pulse characteristics similar to the commercial Verburg pulse system.

On 4 December 2009, three groups of 16 fishes with similar lengths (0.3 – 0.65 m) were exposed to the electric stimulus, with each group in one of three distance ranges:

  1. A “far field” range with the fish exposed at 0.4 m side ways of a conductor element.
  2. A “above field” range with the fish exposed at 0.1-0.3 m above the center of a conductor pair;
  3. A “near field” range with the fish exposed at 0.1 m from the conductor element;

Furthermore, in order to be able to monitor the effects of transfer and other unknown influences a control group of 16 fish was confined in the same way, but not exposed to the electric stimulus. Each fish was exposed four times in a row. All fish were examined for injuries directly after the end of the last stimulus. Feeding response was monitored for 14 days after. Other behavioural responses (in particular contractions, swimming patterns) were monitored during stimulus and in the 14 days period following stimulation. Finally, fish were kept in husbandry for another 9 months. Additionally long term mortality and other behaviour such as egg production were monitored.

No evidence was found of differences in feeding response or likelihood of injury or death between the exposure groups. There was no evidence that fish sustained injuries as a result of the exposures. Respectively 8 and 9 months after the experiment a single specimen of the “above field” category and “near field” category died. In the 14 days observation period after the exposures no aberrant feeding behaviour could be distinguished. Fish in all tested groups started feeding normally the same day directly after the exposures. In a period of 7 months after the exposures all exposed groups produced eggs in numbers varying between 5-39 per group. Surprisingly the control group did not produce eggs.

Regarding the other behavioural responses (mainly reflexes and muscle contractions, and post-reactions, such as a rapid body reverse, short-curled body rotations and acceleration towards the water surface), there were some clear differences between exposure groups. The responses of the fish exposed in the “far field” range, representing the fish just aside the fished area of the trawl, were minor and ignorable. However, the responses of the fish exposed in the “above field” range were more pronounced with contractions, rapid body reverses, shortcurled body rotations and acceleration towards the water surface occurring. The responses of the fish exposed in the shortest possible range, the “near field” range, were the strongest with increased incidence of contractions and rapid body reverses, short-curled body rotations and acceleration towards the water surface. Although this experiment has not been set up, or designed, to investigate differences between exposure groups in terms of behavioural responses other than feeding responses, we note that a common behavioural response in the “near field” group was to ‘accelerate upward’. Since, in field situations this behaviour has been observed to lead to dogfish becoming entangled in the meshes of the top panel of the full-scale trawl this merits further investigation.