Fish can suffer lethal damage to their swim bladder or other organs due to loud impulse sounds such as pile driving noise. However, detailed dose-response studies are still scarce, especially for the early life stages. In view of the rapid extension of offshore wind farms in the North Sea, there is an urgent need to acquire more knowledge on the effects of noise caused by pile driving. This study focussed on the effect of piling noise on the survival of fish larvae.
The first goal of this study, to develop a laboratory set-up in which impulse sounds representative of pile driving noise can be generated, was achieved successfully. The device consists of a rigid-walled cylindrical chamber (110 mm diameter, 160 mm high), driven by an electro dynamical sound projector. Samples of up to 100 larvae can be exposed simultaneously to a homogeneously distributed sound pressure and particle velocity field, at a controllable static pressure up to 3 bar. Two configurations are available with either a dominant sound pressure or a dominant particle velocity exposure. Recorded piling noise can be reproduced in a controlled way, in the frequency range between 50 and 1000 Hz, at peak pressure levels up to 212 dB re 1 μPa2 and single pulse Sound Exposure Levels up to 187 dB re 1 μPa2s, or peak particle velocity levels up to 147 dB re 1 (nm/s)2 and particle velocity exposure levels up to 124 dB re 1 (nm/s)2s.
The laboratory set-up was used in a pilot study, which aimed at determining the sound threshold for larval mortality. The study was limited to lethal effects on the larvae of one fish species: common sole (Solea solea). Experiments were carried in which different developmental stages were exposed to various levels and durations of piling noise. The initial series of experiments indicated that an effect of sound pressure exposure may occur, but the differences were not statistically significant, possibly due to sample size. The results were used for a power analysis to determine the batch size and number of replicates required in next experiments. The project was elaborated with a second series of experiments, which consisted of three treatments: two sound pressure exposures and one control group. Each treatment was repeated 15 times (with 25 larvae per batch), for each of three larval stages. The highest exposure level (cumulative SEL=206 dB re 1 μPa2s) represented 100 pulses at a distance of 100 m from a ‘typical’ North Sea piling site. No significant effects were observed in any of the three larval stages.
The fact that we didn’t find significant effects at a cumulative SEL of 206 dB was remarkable, given the US interim criterion for non-auditory tissue damage in fish <2 gram at a cumulative SEL of 183 dB. Also, the assumption of 100% mortality within a radius of 1000 m around a piling site used in the Appropriate Assessment of Dutch offshore wind farms, appears to be too conservative in the case of common sole larvae. The results of this study cannot be extrapolated to fish larvae in general, as interspecific differences in vulnerability to sound exposure may occur. However, this study does indicate that the previous assumptions and criteria may need to be revised.