Abstract
The objective of the current study was to test the behavioural effect of exposure to the FG-FM sound series in more natural sound field conditions with fish in floating pen that are less restricted in their swimming patterns to allow exploration of a more natural spatial response. We conducted this study with a replicated set of 16 groups of four individual seabass (Dicentrarchus labrax), swimming around in a large floating net pen (Ø 11.5-12.5 m) in a sheltered harbour of at least 3m depth during the experimental trials. The origin of the fish was a hatchery and they were tracked in 3D by telemetry. In previous experiments, using the very same set-up, exposure tools and fish species, typical response patterns were diving down the water column, changing swimming speed and group coherence. The first of these parameters has been the most consistent in both indoor and outdoor conditions. In addition to these three parameters, we also measured the 3D-distance towards the speaker to test for spatial avoidance (taking distance in the horizontal plane into account as well as depth). We assessed the four behavioural parameters before, during and after the sound exposure and explored the detailed tracking for behavioural response correlations with sound level and individual sound stimuli. Application of the FG-FM will induce a sound level gradient with the highest levels close to the device (and presumable close to the danger area) and fading sound levels with distance depending on local propagation properties. In order to explore the effect of the FG-FM sounds at the range of sound levels encountered in the field, and with a dose-response curve as explicit target, we tested behavioural responses to the maximum output level and a step-wise series of lower amplitude. We also tested broad-band white noise sounds in the same temporal pattern as the FG-FM sounds at a slightly lower but similar amplitude level as a control.