Hearing thresholds of swimming Pacific bluefin tuna Thunnus orientalis.

Pacific bluefin tuna (Thunnus orientalis) is a highly migratory, commercially valuable species potentially vulnerable to acoustic noise generated from human activities which could impact behavior and fitness. Although significant efforts have been made to understand hearing abilities of fishes, the large size and need to continuously swim for respiration have hindered investigations with tuna and other large pelagic species. In this study, Pacific bluefin tuna were trained to respond to a pure tone sound stimulus ranging 325-800 Hz and their hearing abilities quantified using a staircase psychophysical technique. Hearing was most sensitive from 400 to 500 Hz in terms of particle motion (radial acceleration -88 dB re 1 m s(-2); vertical acceleration -86 dB re 1 m s(-2)) and sound pressure (83 dB re 1 µPa). Compared to yellowfin tuna (Thunnus albacares) and kawakawa (Euthynnus affinis), Pacific bluefin tuna has a similar bandwidth of hearing and best frequency, but greater sensitivity overall. Careful calibration of the sound stimulus and experimental tank environment, as well as the adoption of behavioral methodology, demonstrates an experimental approach highly effective for the study of large fish species in the laboratory.

Coding of sound direction in the auditory periphery of the lake sturgeon, Acipenser fulvescens.

The lake sturgeon, Acipenser fulvescens, belongs to one of the few extant nonteleost ray-finned fishes and diverged from the main vertebrate lineage about 250 million years ago. The aim of this study was to use this species to explore the peripheral neural coding strategies for sound direction and compare these results to modern bony fishes (teleosts). Extracellular recordings were made from afferent neurons innervating the saccule and lagena of the inner ear while the fish was stimulated using a shaker system. Afferents were highly directional and strongly phase locked to the stimulus. Directional response profiles resembled cosine functions, and directional preferences occurred at a wide range of stimulus intensities (spanning at least 60 dB re 1 nm displacement). Seventy-six percent of afferents were directionally selective for stimuli in the vertical plane near 90° (up down) and did not respond to horizontal stimulation. Sixty-two percent of afferents responsive to horizontal stimulation had their best axis in azimuths near 0° (front back). These findings suggest that in the lake sturgeon, in contrast to teleosts, the saccule and lagena may convey more limited information about the direction of a sound source, raising the possibility that this species uses a different mechanism for localizing sound. For azimuth, a mechanism could involve the utricle or perhaps the computation of arrival time differences. For elevation, behavioral strategies such as directing the head to maximize input to the area of best sensitivity may be used. Alternatively, the lake sturgeon may have a more limited ability for sound source localization compared with teleosts.

The effects of high-intensity, low-frequency active sonar on rainbow trout.

This study investigated the effects on rainbow trout (Oncorhynchus mykiss) of exposure to high-intensity, low-frequency sonar using an element of the standard Surveillance Towed Array Sensor System Low Frequency Active (LFA) sonar source array. Effects of the LFA sonar on hearing were tested using auditory brainstem responses. Effects were also examined on inner ear morphology using scanning electron microscopy and on nonauditory tissues using general pathology and histopathology. Animals were exposed to a maximum received rms sound pressure level of 193 dB re 1 microPa(2) for 324 or 648 s, an exposure that is far in excess of any exposure a fish would normally encounter in the wild. The most significant effect was a 20-dB auditory threshold shift at 400 Hz. However, the results varied with different groups of trout, suggesting developmental and/or genetic impacts on how sound exposure affects hearing. There was no fish mortality during or after exposure. Sensory tissue of the inner ears did not show morphological damage even several days post-sound exposure. Similarly, gross- and histopathology observations demonstrated no effects on nonauditory tissues.

Acoustical stress and hearing sensitivity in fishes: does the linear threshold shift hypothesis hold water?

Mammals exposed to loud aerial sounds exhibit temporary threshold shifts (TTS) that are linearly related to increases of sound pressure above baseline hearing levels. It was unknown if this relationship held true for aquatic ectotherms such as fishes. To test this linear threshold shift hypothesis (LINTS) in fishes, we examined the effects of increased ambient sound on hearing of two species differing in hearing capabilities: goldfish (Carassius auratus; a hearing specialist) and tilapia (Oreochromis niloticus; a hearing generalist). Fish were exposed to 1-28 days of either quiet (110 dB re 1 microPa) or continuous white noise. First, we examined the effect of noise sound pressure level (SPL; 130, 140, 160 or 170 dB re 1 microPa) on goldfish hearing thresholds after 24 h of noise exposure. Second, in a long-term experiment using 170 dB re 1 microPa white noise, we continuously exposed goldfish and tilapia for either 7 or 21-28 days. In both experiments, we measured alterations in hearing capabilities (using auditory brainstem responses) of noise-exposed fish. While tilapia exposed to noise for 28 days showed little or no hearing loss, goldfish exhibited considerable threshold shifts that reached an asymptote of up to 25 dB after only 24 h of exposure. There was a positive linear relationship between noise-induced TTS and the sound pressure difference between the noise and the baseline hearing thresholds in goldfish but not in tilapia. A similar relationship was found for published noise-induced threshold shifts in birds and mammals, but the slope of the linear relationship was greater in these groups than for fish. The linear threshold shift relationship provides insights into differential susceptibility of hearing specialist and generalist fishes to noise-induced hearing loss for a given SPL and provides a framework for future research on noise-induced threshold shifts in fishes and other animals.