Introduction
Anthropogenic underwater noise is an increasing stressor in marine environments, largely driven by maritime activities. Vessel traffic represents the dominant source of low-frequency noise in coastal areas, overlapping with the hearing range of many fish species, including European seabass (Dicentrarchus labrax) (Popper and Hawkins, 2019). As seabass rely on acoustic cues for communication and environmental perception, vessel noise may disrupt these processes and induce physiological stress. Previous studies have shown that boat noise can activate the stress response in fish, increasing cortisol levels and altering metabolic activity (��irovi�� et al., 2026). While these effects may be adaptive in the short term, repeated exposure could impair welfare and performance. This is particularly relevant in aquaculture, where seabass is often reared in coastal areas with frequent vessel traffic. However, the long-term effects of noise exposure under real farming conditions remain poorly understood. This study aims to investigate the physiological responses of European seabass to chronic vessel noise in offshore sea cage systems.
Materials and methods
Adult European seabass (n=144; mean weight±SD: 336.3±78.6g) were chronically exposed to daily 1-hour playbacks of sudden unfamiliar boat noise in three offshore sea cages at the fish farm Valle Ca' Zuliani srl (Monfalcone, Italy). Sampling was conducted at predefined time points to assess physiological responses over time. A representative number of fish (n = 12 per cage at each sampling point; three cages in total) were collected after the first sound exposure (T1), after 1 month of daily exposure (T2), and after 2 months of daily exposure (T3). A baseline sampling (control, T0) was performed 30 days prior to the onset of experimental noise exposure. Fish sampling was carried out after the sound exposure and, immediately after capture, fish were euthanized and blood and scales collected. Cortisol concentrations were quantified by radioimmunoassay (RIA) in plasma and scales to assess the primary stress response (Meloni et al., 2025). Secondary stress indicators, including plasma glucose, lactate, and heat shock protein 70 (HSP70) levels, were measured using an automated analyser (BIONSEN, EKF Diagnostics) and a commercial ELISA kit (Cusabio), respectively. Total proteins (TP) as additional stress indicator and Advanced Oxidation Protein Products (AOPP) as oxidative stress marker were assessed using a BCA kit (ThermoFisher) and the protocol outlined by Witko-Sarsat et al. (1998), respectively. Blood smears were prepared from heparinized entire blood and the differential leucocyte was performed to check for the fish health. Data were analyzed using a linear mixed model with condition and sex as fixed factors, weight as covariate and tank as random factor.
Results and discussion
Chronic vessel noise influenced physiological responses in European seabass, with different results according to the explored stress indicators. Plasma cortisol increased at all exposure times compared to the baseline period (T0), with the highest levels at T3 (after two months of exposure) (147 ng/ml; p<0.001). In contrast, scale cortisol showed a different pattern, with a significant increase only at T2 (after 1 month of exposure) (0.92 ng/g vs. 0.14 ng/g; p<0.05) and remained stable up to T3. Plasma glucose decreased at T2 compared to T0 (210 mg/dL vs. 103 mg/dL; p<0.05), followed by a marked increase from T2 to T3 (103 mg/dL vs. 224 mg/dL; p<0.05), which may indicate a shift in energy allocation along exposure. Lactate increased significantly from T0 to T2 (9.09 mmol/L vs. 13.7 mmol/L; p<0.05), suggesting potential elevated metabolic demand during exposure. Cellular stress response (HSP70) showed a significant increase only at T2, while the AOPP/TP ratio increased after the first exposure. Neutrophil-to-lymphocyte ratio (NLR), an indicator of chronic stress, was not significantly affected (0.11). Overall, these results indicate that chronic vessel noise exposure may influence endocrine and metabolic homeostasis. However, no clear evidence was found to indicate a chronic stress.
Conclusions
The data indicate an initial physiological stress response, reflected by early changes in oxidative stress markers, immune parameters, glucose, and lactate, which partially attenuate over time. Low scale cortisol levels in the 2-month exposure groups, along with the absence of an elevated neutrophil-to-leukocyte ratio, do not support the development of chronic stress. Some variations in acute indicators (plasma cortisol, glucose, and lactate) may instead reflect transient disturbances from cage farming and handling rather than noise exposure per se. Overall, chronic vessel noise, under the tested conditions, induces measurable early physiological responses in farmed European sea bass without clear evidence of chronic stress. Future studies are needed to investigate whether different types of vessel noise, as well as varying modes and timing of exposure, may produce different effects in fish.
Acknowledgment
The Project was funded by the European Union-Next Generation EU, Mission 4 Component 1 (CUP C53D23005310006).
References
Meloni, A., et al. (2025). DHEA and Cortisol in Rainbow Trout (Oncorhynchus mykiss): Effect of Sex, Sexual Maturity, and Acute Stress Exposure. Animals, 15, 2710. https://doi.org/10.3390/ani15182710
Popper, A.N., Hawkins, A.D. (2019). An overview of fish bioacoustics and the impacts of anthropogenic sounds on fishes. Journal of Fish Biology, 94, 692–713. https://doi.org/10.1111/jfb.13948
��irovi��, A., et al. (2026). Physiological effects of anthropogenic sound on aquatic animals: where are we and what is next? Journal of Experimental Biology, 229. https://doi.org/10.1242/jeb.250800
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