The short-term behavioural effects of two types of boat noise were tested on Gobius cruentatus and Chromis chromis, i.e. one permanently and one temporarily benthic vocal fish species living inside the WWF-Natural Miramare Marine Reserve (Northern Adriatic Sea, Italy). The underwater noises produced by a 26-m tourist ferry and a 5-m fiberglass boat were recorded inside the core zone of the reserve. Each type of boat noise was subsequently played back in situ to 10 animals per species (C. chromis males caring their nests or G. cruentatus in their shelters). The 1/3 octave spectra of recorded sound pressure levels were compared to the underwater ambient noise level and to sound pressure level measured at the hearing threshold of the two species. The boat noise levels have been calculated in terms of particle acceleration for both field measurements and in situ playback projections and subsequently compared to the available measured values of particle acceleration at the hearing threshold. The animals were free to move in all directions during the whole experimental session. The behaviour of each fish was videotaped by an underwater camera for a total of 10 min (5 min before and 5 min during the noise playback). No short-term behavioural reaction (aversion) was observed in any of the specimen of the two species during the playback of the recorded noises, therefore suggesting no impact. However a time-budget analysis revealed a significant change in the total time spent in caring their nests (C. chromis) or inside their shelters (G. cruentatus). This result highlighted how analyzing fish reaction on a short-term might underestimate the effects of noise disturbance and indicated that the overall fish behaviour should be considered to assess noise impact.
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