Controllable acoustic deterrent based on the warning signals generated by nonel detonators.

Acoustic deterrents are a practical strategy to mitigate the impact of underwater noise on marine mammals. However, their safety and effectiveness are still debatable. This study proposes a controllable acoustic deterrence method to protect marine mammals threatened by underwater blasting noise. The method creates strong-randomness warning signals using nonel detonators and establishes an escape time for animals protected. Combining the BELLHOP ray-based acoustic model with the marine environmental parameters and animals' auditory characteristics, we built a prediction model to establish a link between the acoustic fields and the adjustable source parameters, and provide a Risk zone and Deterrent zone for animals. The simulation and experimental results demonstrated that the root mean squared error between the simulated and measured sound pressure spectral density levels did not exceed 4.5 dB and the coefficient of determination remained at approximately 0.8, indicating that the new deterrent is an effective method with good controllable performances.

Acoustic Properties of the Otolith of the Large Yellow Croaker Larimichthys crocea (Perciformes: Sciaenidae).

The inner ears of fish contain three pairs of otoliths-lapilli, asterisci and sagittae-which play important roles in hearing and balance. However, acoustic properties and dynamic responses of fish otoliths are poorly understood. The large yellow croaker (Larimichthys crocea), like many species in the family Sciaenidae, is extremely sensitive to sound. The present study used L. crocea sagittae as the research subject and examined the variation in shear stress on sagittae under different acoustic stimuli. For the first time, the sound speed of the sagitta was measured using ultrasonic pulse-echo techniques, and the acoustic impedance and natural frequency of the sagitta were calculated. Larimichthys crocea adults (20-22 cm standard length, n = 10) had a sagitta density of 2781.5 ± 28.06 kg/m3, sound speed of 4828-6000 m/s and acoustic impedance range of 13.4-16.7 MPa·s/m, approximately 9-11 times that of seawater (1.48 MPa·s/m). The natural frequency of the sagitta was 76.4-95.5 kHz. The shape and structural details of sagittae were reconstructed by 3D scanner and the shear stress responses of sagittae under different acoustic stimulus were investigated based on a finite element model. The simulation results showed that the shear stress responses tended to increase and then decrease in the range of sciaenid hearing frequency from 200 to 1300 Hz, peaking at 800 Hz. The shear stress responses varied with the direction of acoustic stimulus and peaked when the incident direction was perpendicular to the inner surface of the otolith. These results provide important parameters that may be used to protect L. crocea from possible underwater noise damage, particularly during their spawning aggregations and over-wintering aggregations.