Free-ranging Atlantic cod did not change their behaviour in response to a sparker seismic sound source

Sparkers are seismic sources commonly used to survey the seabed in areas planned for offshore wind farms. The almost continuous sounds produced by sparkers, with most energy in the frequency range 50Hz - 4 kHz, are audible to both fish and marine mammals and may cause disturbance and behavioural reactions. Here, we conducted a sparker exposure experiment with wild, free-ranging, tagged Atlantic cod (Gadus morhua). The fish were exposed to sound from a real sparker, experiencing varying sound exposure levels up to 130 dB re 1μPa²s over 10 s. During a 40 h exposure period, we used a randomised block design with three source levels, where the highest was as used in normal sparker operations with this source (800 J). The movement behaviours of cod were monitored using acoustic telemetry. Only one tagged cod left the test site during the exposure. The remaining tagged cod (n = 11) neither changed their location within the test site, nor did we detect any changes in swimming depth, activity levels or displacement attributable to the exposure. Although the sample size and number of replicates in this study were limited, these results indicate that the relatively low sound levels produced by a sparker source do not greatly disturb cod during the feeding season.