Seasonal differences in the sound detection ability of Arctic cod, Boreogadus saida.

Human activities are rapidly altering marine ecosystems, with increasing vessel traffic elevating underwater sound levels worldwide. Polar regions are particularly vulnerable, as declining sea ice has expanded access to the Arctic Ocean. Arctic cod (Boreogadus saida) is a key species that spawns under sea ice, yet its hearing abilities remain entirely uncharacterised. Understanding how this species detects sound is essential for assessing the potential impacts of rising anthropogenic sound. Here, the detection sensitivity of spawning and non-spawning Arctic cod were evaluated using auditory evoked potentials (AEPs). Both groups were most sensitive to frequencies below 200 Hz when assessed using sound pressure. However, particle motion sensitivity differed markedly: spawning fish showed peak sensitivity at 400 Hz, whereas non-spawning fish were most sensitive near 90 Hz. Overall, Arctic cod exhibited significantly enhanced hearing sensitivity and an expanded detectable frequency range during spawning compared with non-spawning individuals. These results provide the first evidence of seasonal differences in auditory plasticity in a broadcast-spawning fish and highlight the potential vulnerability of Arctic cod spawning behaviour to increasing anthropogenic sound in a rapidly changing Arctic environment.