Charged encounters: Effects of electromagnetic fields on sharks and rays

This thesis investigates whether electromagnetic fields (EMFs) generated by subsea power cables affect sharks and rays, with a focus on benthic elasmobranchs in the North Sea. Offshore wind farms are expanding rapidly, and this brings an increasing number of subsea power cables into marine habitats. These cables produce electric and magnetic fields that may be detected by elasmobranchs, because sharks, skates and rays naturally use electromagnetic cues for prey detection, navigation, orientation and interactions with conspecifics. The central question of the thesis is therefore whether EMFs from subsea power cables pose a relevant ecological risk to these species. The thesis begins with an ecological risk assessment for benthic elasmobranchs on the Dutch Continental Shelf. This assessment combines information on cable-generated EMF exposure, known sensory sensitivity and reported behavioural effects. It shows that the current evidence base is limited and uncertain, but that risk may differ strongly between species and life stages. Embryos, locally dwelling individuals and migrating animals may encounter EMFs in different ways, and therefore require separate consideration. This risk-based framework is then used to guide the experimental chapters. Two species were studied in detail: the small-spotted catshark (Scyliorhinus canicula) and the thornback ray/skate (Raja clavata). Experiments were conducted across life stages, including embryos, juveniles and adults. Chronic exposure of thornback ray embryos to alternating-current EMFs resulted in increased embryonic movement, suggesting that EMFs can affect behaviour even before hatching. However, exposure did not lead to strong negative effects on survival, development time or body condition. The thesis also examined whether embryonic exposure affected later growth and juvenile behaviour. Subtle differences in growth were observed: rays showed indications of reduced growth, while sharks showed indications of a transient increase in size. These effects were small and may be obscured by natural variation in the field. After hatching, juveniles did not show clear stress responses or major behavioural changes related to previous EMF exposure. Adult behavioural experiments tested whether sharks and skates avoided or were attracted to AC or DC EMF gradients comparable to those near subsea cables. No strong attraction, avoidance or startle responses were found. However, subtle changes in swimming behaviour occurred. Adult small-spotted catsharks reduced total swimming time and increased swimming speed during DC exposure. Thornback skates also showed subtle behavioural responses, but these were sex-dependent: males and females responded differently, and in some cases in opposite directions. This suggests that sex can be an important factor in EMF response and should not be ignored in future studies. Overall, the thesis shows that realistic EMF exposure does not appear to cause strong short-term avoidance, attraction or stress responses in the studied species. However, subtle effects on embryo movement, growth and swimming behaviour were detected. Because subsea cables operate for decades, such subtle effects cannot automatically be dismissed. The thesis therefore recommends a balanced approach: optimise cable and grid design where possible, improve monitoring, use realistic exposure scenarios and prioritise vulnerable species and life stages in future research.