The effect of electric pulse stimulation to juvenile cod and cod of commercial landing size
Juvenile (0.12-0.16 m length) and large cod of 0.5 m length were exposed to simulated pulse stimuli representing commercially applied pulse systems with varying amplitude, frequency and pulse width at fixed positions in close range of a conductor. The juvenile fish were exposed with the highest possible amplitude in the nearest possible range of a conductor in a horizontal plane, along the conductor and perpendicular to the centre of a conductor. The field strength in the exposed position was in a range of 250-300 V/m. All juveniles (involving 12 trials and a total of 168 specimen) survived this harshest condition even after 4 exposures in a row. The post mortem research showed not a single vertebral injury or haemorrhage.
Larger cod of 0.5 m length were exposed to pulse patterns with a field strength of 40 to 100 V/m, depending on the pulse amplitude. Similar results as obtained in a pilot study of 2008 with similar length of cod with a series of 4 exposures were found in this present study using a single exposure.
Vertebral injuries occurred in 50-70 % of the fish under various pulse compositions (26 trials and a total of 262 specimen) and did not reduce when the shape of the burst was modulated according the "tooth-shape model". Injuries occurred in all three tested pulse concepts, although the Delmeco TX19 type had a lower score. This could have been related to the frequency (80 Hz), which also reduced the injuries when the HFK stimulus was tested at that frequency and probably also the shorter pulse duration and so the energy contents. We found a reduction of injuries with rising frequencies ≥80 Hz. At 180 Hz visible injuries reduced even to zero.
The post-mortem results did confirm the observations of injuries directly after the exposures and they show that the pulse stimuli can harm both the musculature and skeleton in cod, including fractures of the neural and hemal arch. In most cases the injuries are a combination of haemorrhages and vertebral ruptures. In a single case minor haemorrhages were found without bone fractures (Delmeco TX19, trial 19).
Vertebral injuries in cod of 0.5 m occurred when using the Delmeco TX68 stimulus in a field strength range of 40 to 100 V/m with 40 V/m as threshold value where injuries reduced from 60 to 30 % measur-ed in the closest possible range from a conductor. The exposures with the Delmeco TX68 under nominal amplitude setting (50 V) were an overdose of 54 %. Under this condition a field strength of 91 V/m was measured in the laboratory in the exposed reference position of the fish, while under real fishing practice this rating was 59 V/m. However, the field strength measured under fishing condition is still above the threshold field strength level (38 V/m) where injuries reduced to 30 % and the fished field strength (59 V/m) is well inside the range (Figure 14) where 60 % of the fish were injured in the laboratory (Appendix B, Table 4, trial 7). Field strength measurements on the HFK pulse concept showed that the conditions measured during the experiments were similar to the field strength measured with the gear on the seabed for both juvenile and larger cod. These observations showed the laboratory experiments were conducted in a relevant range of field strengths and that injuries observed are likely to occur under full-scale practice.
As the fish became shortly paralysed in most of the cases after the exposures or showed a disorientated behaviour it is likely that most of the fish will be caught when they move through the net in close prox-imity of the conductors. This injured fish is of lower market value than fish that is landed without injur-ies. However, next to the quality issue there is also an ethical motive to find solutions to reduce such injuries. Reduced injuries can be achieved by increasing pulse frequency. The research showed that electric pulse systems did not reach their optimum in terms of electrode and conductor efficiency and this research also showed that there is room for further technical improvements.
It can be considered to conduct full-scale electric field measurements on the TX19 as with the Delmeco TX19 pulse concept the effects to large cod were lower than with other pulse concepts and to include this vessel in future monitoring field tests.
The field strength measurements at sea showed that reference measurements with the gear hanging vertically aside of the ship do not produce the same figures as in the actual fished condition with the gear on the sea bed. Reasons are the differing geometry of the electrodes in the hanging condition, and the different electric field around the conductors due to sea bed presence and conductivity. Field strength measurements as a standard inspection routine with the vessel in the harbour are not accurate and could contain an error of 10%.