Given the increasing amount of anthropogenically induced underwater sound into the marine environment, a better understanding of the impact of impulsive underwater sound on marine life is needed. This study tackles the impact of impulsive sound, related to pile-driving activities for offshore wind energy development, on the mortality, stress and condition of post-larval and juvenile European sea bass Dicentrarchus labrax.
A ‘worst-case scenario’ field experiment was carried out on board of a piling vessel, exposing 68 and 115 days post hatching (dph) (<2 g wet weight) to the sound generated during 1.5 hours of pile-driving. The number of strikes ranged from 1740 to 3070, with a single strike sound exposure level between 181 and 188 dB re 1µPa².s, resulting in cumulative sound exposure levels ranging from 215 to 222dB re 1µPa².s. Immediate and long-term survival of the exposed fish was high and comparable to the control groups. Although not significant, control fish had elevated whole-body cortisol levels (31.7± 12 ng/g fish) compared to unhandled fish (5±3.7 ng/g fish) which reflects the impact of handling, transport and reduced appetite on board the piling vessel. Fish exposed to pile driving had high whole-body cortisol levels (87±50 ng/g fish), thereby confirming that pile driving is perceived as an acoustic stressor. Furthermore a 50 % reduction in the oxygen consumption rates is observed in the exposed fish compared to the control fish. Lactate level remained unchanged between exposed, control and unhandled fish. Together these results might suggest a reduction in motility, whether this is involuntary immobility or strategic freezing is unclear. Back in the lab, the fish were held under optimal lab conditions over a 30-day period with no differences in length, weight and condition. Overall, a strong stress response is observed during the pile driving exposure but has no long term detrimental effects, at least not under laboratory conditions. During the sound exposure, the lower energy levels might negatively affect the general alertness, anti-predator response and activity levels, making the fish vulnerable.
Laboratory experiments with the larvaebrator and a SIG sparker, operated in seismic research were used to compare stress responses with the in situ experiment.