Shark preferences: Only subtle effects of electromagnetic field exposure and substrate type on small-spotted catshark...

Offshore wind energy plays a crucial role in the transition to low-carbon power generation. The expansion of offshore wind farms (OWFs) introduces anthropogenic electromagnetic fields (EMFs) from subsea power cables. Elasmobranchs, including sharks and rays, are sensitive to natural electric and magnetic fields used for navigation and prey detection, making them potentially vulnerable to anthropogenic EMFs as they may alter orientation, foraging or habitat use. Coastal shelf seas, where most OWFs are constructed, often overlap with essential elasmobranch habitats, including complex biogenic structures such as mussel beds that provide foraging and refuge opportunities. We aimed to determine whether exposure to an AC-EMF representative of subsea power cables and/or complex mussel substrate over non-complex substrate influences behaviour and substrate preferences in juvenile small-spotted catsharks (Scyliorhinus canicula). This study investigated the behavioural responses of 14 juvenile S. canicula exposed to a gradient of alternating current (AC) EMF (max. RMS 2 μT) and complex biogenic mussel substrate in a 2² factorial design in a circular preference chamber. Across 28 trials, time spent and distance travelled in each quadrant did not differ with EMF exposure and/or the presence of complex biogenic substrate. Hidden Markov models revealed subtle but significant changes in behavioural states (inactive, local activity, transit), where sharks were more inactive (+6%pt.) and less locally active (-5%pt.) in the EMF + mussel quadrant compared to the bare and EMF-only quadrants, and increased transiting behaviour (+6%pt.) in the mussel-only quadrant relative to the EMF + mussel quadrant. These findings show that S. canicula juveniles show no preference in substrate or alternating-current EMF exposure, but they did subtly change their behaviour. Future work should examine direct-current systems and field-based tracking and assess the extent to which these observations generalise to field settings and other elasmobranchs.