Sharks and rays possess a specialised sense organ: the ampullae of Lorenzini. They use these to detect electrical signals, which they utilise, amongst other things, to locate prey and navigate. Electromagnetic fields from power cables may potentially influence this detection. As part of the six-year ElasmoPower research project, Annemiek Hermans, a PhD candidate at Wageningen University & Research and employed by Witteveen+Bos, investigated the potential effects of these fields on sharks and rays in the North Sea.

Hermans investigated the animals’ exposure and responses both at sea and in the laboratory. For the eDNA study, she analysed 436 water samples from four operational wind farms over a two-year period. DNA from five species of sharks and rays was found in these samples. This suggests that these species are present in operational wind farms with power cables and do not, at any rate, avoid these areas. However, eDNA cannot be used to determine how individual animals react to a power cable, how long they remain in its vicinity, or whether they alter their swimming behaviour when they cross a cable during their migration.

In a specially constructed 15-metre-long tank, Hermans also investigated the behaviour of fourteen adult lesser-spotted dogfish and eighteen adult thornback rays. The animals were exposed to alternating current fields, direct current fields and a control situation without an artificial electromagnetic field. No clear indications of attraction or avoidance were found. Nor did the animals display any startle response or clear change in their use of space.

'Our results show that the electromagnetic fields around power cables do not deter sharks and rays', says Annemiek Hermans. 'They continue to be found in wind farms after construction and do not hesitate to cross a cable. What we do not yet know is what long-term exposure means for their interactions with conspecifics and whether they are still able to locate their prey effectively.'

However, the electromagnetic fields were not entirely without effect. Dogfish were active for approximately 25 percentage points less time only when exposed to a direct current field, whilst their swimming speed was higher during active movement. In the case of spiny dogfish, the responses differed between the sexes: females became approximately 15 percentage points more active when exposed to an alternating current field, whilst males became approximately 14 percentage points less active in the presence of both alternating and direct current fields.

For the study on thornback ray embryos, eighteen fertilised egg capsules were divided between an exposed group and a control group. The exposed embryos developed under electromagnetic fields whose strength varied daily between levels measured at different wind speeds around an offshore electricity cable. They exhibited 33 percent more tail movements and 150 percent more whole-body movements. Remaining still is, in fact, an important response to potential danger for embryos. As there were no predators present in the experiment, it cannot be determined whether the increased movement makes them more likely to be detected by predators. No effects were observed on their development time, the successful hatching of the embryos, their weight or their general health after hatching.

In a separate follow-up study, Hermans compared seventeen young spiny dogfish and nineteen young dogfish that had or had not been exposed to electromagnetic fields during their embryonic development. Compared with the control group, the tails of exposed juvenile spiny dogfish were approximately 5 percent shorter one and three months after hatching. In exposed juvenile dogfish, the pectoral fins were approximately 11 percent larger at hatching; after one month, this difference was no longer visible. The ecological significance of these differences is not yet clear.

The results for dogfish and thornback rays cannot simply be extrapolated to all sharks and rays. Their sensitivity may be linked to factors such as their life stage, mode of reproduction, habitat and migratory behaviour. In particular, embryos of oviparous species may be exposed to electromagnetic fields for prolonged periods, as their egg capsules are located on or near the seabed during development.

Furthermore, based on measurements and modelling, Hermans demonstrates that exposure varies significantly. The intensity and range of an electromagnetic field depend, amongst other things, on the type of cable, the amount of electricity transmitted and the way in which the cable is laid.

The experiments provide no evidence of significant acute effects on the animals studied. However, the implications of repeated or prolonged exposure in the natural environment cannot yet be determined. Nor is it known whether animals become accustomed to electromagnetic fields, or whether minor behavioural changes ultimately affect, for example, foraging, migration, energy expenditure, reproduction or survival.

Furthermore, with the continued construction of wind farms and interconnections between countries, sharks and rays will come into contact with subsea power cables in an increasing number of locations. Further research is therefore needed into the exposure and behaviour of marine animals, with particular attention to vulnerable species and life stages.

'Sharks and rays in the North Sea are already under pressure from various stress factors, such as fishing and the degradation or loss of habitat', says Annemiek. 'Electromagnetic fields represent a new, long-term source of stress in this context. Model calculations in my research show that by 2030, an estimated more than five percent of the Dutch section of the North Sea will be affected by these fields. For that reason alone, it is important to investigate the subtle and long-term effects further.'

Annemiek Hermans recommends that electromagnetic fields be included in environmental impact assessments and assessments of cumulative effects on the North Sea. In her thesis, she also discusses a number of possible precautionary measures. For example, cables could be routed around key spawning and nursery areas wherever possible. Short, straight cable routes and the bundling of cables may also reduce the area in which animals are exposed to electromagnetic fields. The effectiveness of such measures requires further investigation.

The research forms part of ElasmoPower, a six-year collaborative project involving Wageningen University, Wageningen Marine Research, Naturalis Biodiversity Centre, TenneT, Witteveen+Bos and Stichting De Noordzee. Rijkswaterstaat is involved through the Offshore Wind Ecological Programme (Wozep).

Annemiek Hermans has defended her thesis, 'Charged Encounters: Effects of electromagnetic fields on sharks and rays', on 28 August at Wageningen University & Research.