Introduction
The impact of anthropogenic noise on marine ecosystems has emerged as a critical area of research in recent years. Increased shipping traffic, marine construction, underwater mining, and recreational boating contribute significantly to noise pollution, altering the natural acoustic landscape. Fish are particularly sensitive to these changes, as they rely on sound for communication, navigation, and predator avoidance.
Noise acts as a potential stressor, inducing both physiological responses (increased cortisol secretion, changes in plasma metabolites, and expression of heat shock proteins ) and behavioral changes in fish (startle, avoidance, altered foraging, and disrupted collective dynamics) (Wysocky et al., 2006; Cox et al., 2018).
This study adopts a dose-response approach to explore the relationship between s ound intensity and both physiological and behavioral variables in European seabass exposed to boat noise, determining how these parameters mirror their respective variations and at which intensity noise may potentially cause harm.
Materials and Methods
We exposed 160 European seabass to sound stimuli realistically reproducing fishing boat engine noise. Fish were housed in experimental tanks (350 l) in groups of five, with each tank exposed to one of six distinct sound levels (95-154 dB , re 1 μPa) for one hour.
After exposure, we collected fish plasma following anaesthetic overdose and stored samples at -20°C for cortisol and glucose/lactate analysis via radioimmunoassay (RIA) and an automatic analyser (BIONSEN, EKF Diagnostics). Physiological indicators were compared across treatment Levels through ANOVA.
Behavioral data were collected on videos during three non-consecutive 10-min intervals before exposure and other three 10-min intervals during exposure. A focal animal, continuous sampling method was used to collect data on swimming behaviour (forward swimming, hovering ). DeepLabCut, a machine learning video analysis software, was trained to track fish and collect data about swimming speed and distance from other fish. Behavioural data were analysed separately for each Level of exposure via Generalized Estimating Equations, with sampling interval as a fixed factor and fish identity as a random factor.
Results and Discussion
The percentage of time spent swimming forward increased significantly with exposure to sound for all Levels between 0 and 3 (p < 0.001 for all models ) in the first 10 min of exposure, but remained increased for longe r with increasing sound intensity, to the point that no return to baseline was observed for exposure to Level 3 ; a delayed increase in forward swimming was observed in Level 4 (p < 0.001) and no increment was observed in Level 5 (p = 0.644). Hovering showed a mirroring pattern, decreasing significantly upon exposure to noise (p < 0.001), and taking longer to revert to baseline (if at all) with increasing sound intensity up to Level 3. As already observed for swimming , such response was delayed in Level 4 and absent in Level 5. Average swimming speed significantly increased in the first 10 min of exposure compared to all other sampling intervals, in Levels 0 to 3 (p < 0.05), while no change was observed in Levels 4 and 5.
Overall, these results indicate that noise exposure increases swimming activity, in agreement with previous studies (Buscaino et al., 2020) . However, the i ncrement in swimming speed was short- lived and independent of sound pressure, representing a transitory response scarcely indicative of the intensity of the effect of noise exposure . Conversely, the increment in forward swimming/decreased hovering and the clear relationship between noise intensity and the duration of such response likely represents the results of stress/arousal due to exposure and of subsequent habituation . The delayed and reduced effect observed in Levels 4 and 5 might reflect a disruptive effect of too intense noise, impeding the expression of an active behavioural response.
Overall, these results indicate that quantification of forward swimming and/or hovering is better suited than swimming speed to measure the impact of noise exposure. These results also suggest that the greatest effects of noise exposure on seabass behaviour occur for noise intensities around or above 130 dB ( re 1 μPa).
No differences were found in cortisol , glucose and lactate concentrations across treatments, indicating a lack of conventional physiological stress response despite significant behavioral alterations. This suggests a disconnection between immediate behavioral reactions and typical physiological stress markers in the context of noise exposure requiring further investigation.
Conclusions
In conclusion, this study identifies behavioural patterns which seem useful to characterise different aspects of European seabass response to noise exposure. The study indicates 130 dB as the intensity threshold around which seabass can no longer produce adaptive responses. The lack of significant changes in acute stress physiology indicators such as cortisol and glucose levels suggests that current models of stress measurement may require re-evaluation in the context of acute noise exposure. Further research is needed to explore these findings across different environmental contexts and stressors, for instance to assess the long-term impacts of noise pollution on fish survival, foraging, and reproductive success. Understanding the nuanced relationship between stressors and biological responses is vital for formulating effective conservation strategies and ensuring the welfare of marine organisms in increasingly noisy oceans.
Bibliography
Buscaino, G., Filiciotto, F., Buffa, G., Bellante, A., Stefano, V.D., Assenza, A., Fazio, F., Caola, G., Mazzola, S., (2010). Impact of an acoustic stimulus on the motility and blood parameters of European seabass (Dicentrarchus labrax L.) and gilthead sea bream (Sparus aurata L.). Mar. Environ. Res. 69, 136–142.
Cox, K., Brennan, L. P., Gerwing, T. G., Dudas, S. E., & Juanes, F. (2018). Sound the alarm: A meta‐analysis on the effect of aquatic noise on fish behavior and physiology. Global Change Biology, 24(7), 3105-3116.
Wysocki, L. E., Dittami, J. P., & Ladich, F. (2006). Ship noise and cortisol secretion in European freshwater fishes. Biological conservation, 128(4), 501-508.