Zooplankton mortality and distribution around a seismic survey.

Anthropogenic underwater noise from seismic airguns is increasing, posing a potential threat to marine life. Despite their ecological importance, knowledge of the impacts of seismic surveys on zooplankton remains limited. In the North Sea, we utilized various methods to assess potential effects on zooplankton around an ongoing seismic survey. A seismic airgun array (3060 inch3) performed a survey along parallel lines, while we sampled from a research vessel positioned at the end of three of these lines. The airgun-generated noise reached a maximum broadband sound exposure level of 182 dB re 1µPa2s at the closest proximity (~50 m). Zooplankton biomass exhibited a consistent distribution in line with hydrography and chlorophyll distribution, before and after airgun exposure. Immediate mortality in Calanus spp. was uniform across sound levels, never exceeding 35.9%. Cultured Calanus finmarchicus were exposed while submerged in bags. These displayed low immediate mortality (<10%), with an increasing trend (<30%) up to 7 days post-exposure. These findings highlight the interplay between sound exposure, environmental conditions, and the impact on zooplankton in areas of seismic activity, indicating that immediate impacts of seismic surveys on zooplankton may be limited. However, delayed impacts that could lead to population effects require further investigation.