Introduction
It is widely recognized that the marine environment is impacted by noise pollution generated by human activities. Maritime traffic is the primary source of diffuse broadband noise in the marine environment, including aquaculture systems, and can have negative effects on marine organisms. Exposure to ship noise pollution can elicit behavioral responses and increase physiological stress in fish and appears to mask acoustic communication with effects on foraging, navigation and reproduction (Popper & Hawkins , 2019). The boat is an essential tool for sea-cage rearing, but it can itself be a frequent and long-lasting stressful factor for the fish that cannot escape. The mode of noise exposure, i.e. gradual or immediate, is also an aspect to be considered in order to have the necessary information to design or apply noise mitigation measures in sea farming but there are few studies that provide information on those aspects.
By combining field monitoring and controlled exposure experiments, the present study aims at assessing the acute noise effects on European seabass stress response and behaviour and the potential effects related to the familiarity of the boat and the sound exposure pattern.
Materials and Methods
90 adult European seabass were exposed to boat noise in experimental tanks in groups of five, after elastomer tagging and acclimation. From the acoustic recordings collected at sea, two 30-minute boat noise tracks were selected: (A) the noise from a boat considered "familiar" to the animals, as it was the same vessel that regularly visited the sea cages for the management; and (B) the noise from a boat with similar acoustic characteristics but not experimented by the animals so representing an "unfamiliar" sound to them.
Each tank were exposed for one hour to one of the following 5 noise conditions : background noise of fish farm’s origin (BG), familiar boat noise played in a progressive (PF) or sudden pattern (SF), unfamiliar boat noise played in a progressive (PU) or sudden pattern (SU).
After exposure, fish blood and tissues were collected following anaesthetic overdose (MS222, Sandoz) and samples were stored at -20°C until analysis. Cortisol, as main stress hormone, was measured by Radioimmunoassay (RIA ) in different matrices (plasma, mucus, fin, and scales) to assess the fish stress response avoiding sampling effects and potential fish chronic stress status before the exposure (Bertotto et al., 2010). Haematocrit as well as plasmatic glucose, lactate and heat shock protein levels (HSP70) were assessed, as secondary stress response indicators, by means of an automatic analyser (BIONSEN, EKF Diagnostics) and a commercial ELISA kit (Cusabio). P hysiological indicators were compared across treatments through Bonferroni correction test (P<0.05).
Behavioral data was collected on videos, in five non-consecutive 5-min intervals, one before and four during the exposure. DeepLabCut , a machine learning for the markless pose estimation using a deep neural network with supervised learning , was trained to track fish and collect data related to the movements, specifically the swimming speed. Behavioural data were analysed separately for each noise exposure via Generalized linear mixed model , with sampling interval as a fixed factor and the tank as a random factor.
Results and Discussion
Overall, physiological data showed mild variation across different noise exposure conditions and no stress response. Plasmatic cortisol showed higher levels in the progressive and sudden unfamiliar noise groups (PU and SU) than in the background group ( BG, P<0.00 5) and no differences with the familiar boat noise groups, both progressive and sudden ( PF and SF, P>0.05) . The latter were not different from the background group and overall, the levels were well below the basal ones indicating no stress response (concentration <88 ng/ml; Samaras, 2023). Also, the cortisol in fins and scales, indicators of previous and chronic state of stress, showed baseline values albeit with some differences between groups ( P<0.001; Bertotto et al., 2010; Samaras et al., 2021) . Hematocrit, glucose, lactate and HSP70 levels confirmed the absence of variations among groups (P>0.05) and non-stress conditions.
The analysis of movement revealed a significant increase in speed following the onset of sound exposure (P < 0.05), regardless of the condition, indicating a sudden behavioral response. When considering specific conditions, individuals in the BG group appeared to recover from the initial startle response, exhibiting a gradual reduction in speed over the subsequent hour. This recovery pattern was not observed in the other groups. In the SU group, fish initially responded to the background sound but showed a further significant increase in speed upon the onset of the sudden sound stimulus (P < 0.05).
Conclusions
Plasma levels of cortisol and swimming speed seem to identify unfamiliarity and sudden exposure as the factors that most affect the stress response in European sea bass exposed to boat noise. Basal levels found in all physiological indicators analysed indicate a lack of physiological response to stress despite significant behavioural alteration related to swimming speed. This suggests a disconnect between immediate behavioural responses and typical markers of physiological stress in the context of noise exposure, which requires further investigation.
Acknowledgement
The study is part of the SOS Bass project (PNRR M4C2 Investment 1.1 Research Projects of National Relevance – PRIN) funded by the European UnionNextGenerationEU.
References
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