The effects of continuously acoustical stress on cortisol in milkfish (Chanos chanos)

Strong underwater acoustic noise has been known that may cause hearing loss and actual stress in teleost. However, the long-term physiological effects of relatively quiet but continuously noise on fish were less understood. In present study, milkfish, Chanos chanos, were exposed to the simulated-wind farm noise either quiet (109 dB re 1 μPa/125.4 Hz; approx. 10-100 m distant from the wind farm) or noisy (138 dB re 1 μPa/125.4 Hz; nearby the wind farm) conditions for 24 h, 3 days and 1 week. Comparing to the control group (80 dB re 1 μPa/125.4 Hz), the fish exposed to noisy conditions had higher plasma cortisol levels in the first 24 h. However, the cortisol levels of 24 h spot returned to the resting levels quickly. The fish exposed under noisy condition had significantly higher head kidney star (steroidogenic acute regulatory) and hsd11b2 (11-β-hydroxysteroid dehydrogenase 2) mRNA levels at the following treatment time points. In addition, noise exposure did not change hypothalamus crh (Corticotropin-releasing hormone) mRNA levels in this experiment. The results implied that the weak but continuously noise was a potential stressor to fish, but the impacts may be various depending on the sound levels and exposure time. Furthermore, this study showed that the continuous noise may up-regulate the genes that are related to cortisol synthesis and possibly make the fish more sensitive to ambient stressors, which may influence the energy allocation appearance in long-term exposures.