The effect of pulse stimulation on biota – Research in relation to ICES advice – Progress report on the effects on cod

These activities involved the exposure of cod to a simulated electric pulse under laboratory conditions and observation of behaviour, including the foraging response, and monitoring mortality and possible internal injuries such as vertebral damage by X-ray photography. The research was conducted in cooperation with the Institute of Marine Research Bergen, IMR research station, Austevoll. This research station facilitated the research, target fish which were derived from their own aquaculture research stock and video observation equipment. The electric pulse simulator was made available by Verburg-Holland Ltd. with pulse characteristics similar to the commercial Verburg pulse system.

Groups of 20 fishes with similar lengths (0.41 – 0.55 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 0.2-0.3 m above the centre of a conductor pair;
3. A “near field” range with the fish exposed at 0.1 m from the conductor element;

To exclude the effects of transfer and other unknown influences a control group of 20 fish was confined in the same way, but not exposed to the electric stimulus.

The fish exposed in the “far field” range, representing the fish just outside the working range of the trawl, showed hardly a reaction to the exposures and responded normally to the feeding cycles. The fish exposed in the “above field” range showed a moderate contraction of the muscles, but all recovered well and responded normally to the feeding cycles. The effects on the fish exposed in the “near field” range were more pronounced, 4 fishes died shortly after the exposure, and another 2 died in the observation period thereafter. In the observed period of 14 days after the exposures the surviving fish packed together outside the feeding zone and hardly responded to the feeding cycles.

The fish of the control group, exposed to a similar treatment as the exposed groups except receiving the pulse stimulation, showed a decrease in appetite compared to the fish exposed in the “far field” and “above field” ranges. This could have been related to the fact that this group was treated towards the end of the experimental period and thus stayed the longest time in the transfer tank.

Post mortal analysis using X-ray scans revealed that 5 out of 16 remaining fishes exposed in the “near field” range had hemorrhages close to the vertebral column, and of these five, 4 had vertebral bone fractures. No injuries were found on the fish exposed in the “above field” range, that showed weaker reactions to the electric exposure.