SUMMARY: Fishes are often exposed to environmental sounds such as those associated with shipping, seismic experiments, sonar and/or aquaculture pump systems. While efforts have been made to document the effects of such anthropogenic(human-generated) sounds on marine mammals, the effects of excess noise on fishes are poorly understood. We examined the short- and long-term effects of increased ambient sound on the stress and hearing of goldfish (Carassius auratus; a hearing specialist). We reared fish under either quiet(110-125 dB re 1 μPa) or noisy (white noise, 160-170 dB re 1 μPa)conditions and examined animals after specific durations of noise exposure. We assessed noise-induced alterations in physiological stress by measuring plasma cortisol and glucose levels and in hearing capabilities by using auditory brainstem responses. Noise exposure did not produce long-term physiological stress responses in goldfish, but a transient spike in plasma cortisol did occur within 10 min of the noise onset. Goldfish had significant threshold shifts in hearing after only 10 min of noise exposure, and these shifts increased linearly up to approximately 28 dB after 24 h of noise exposure. Further noise exposure did not increase threshold shifts, suggesting an asymptote of maximal hearing loss within 24 h. After 21 days of noise exposure, it took goldfish 14 days to fully recover to control hearing levels. This study shows that hearing-specialist fishes may be susceptible to noise-induced stress and hearing loss.

Seismic airguns produce considerable amounts of acoustic energy that have the potential to affect marine life. This study investigates the effects of exposure to a $\text{730\hspace{0.17em}}{\mathrm{in.}}^3$ airgun array on hearing of three fish species in the Mackenzie River Delta, the northern pike (Esox lucius), broad whitefish (Coregonus nasus), and lake chub (Couesius plumbeus). Fish were placed in cages in the 1.9 m of water and exposed to five or 20 airgun shots, while controls were placed in the same cage but without airgun exposure. Hearing in both exposed and control fish were then tested using the auditory brainstem response (ABR). Threshold shifts were found for exposed fish as compared to controls in the northern pike and lake chub, with recovery within 24 hours of exposure, while there was no threshold shift in the broad whitefish. It is concluded that these three species are not likely to be substantially impacted by exposure to an airgun array used in a river seismic survey. Care must be taken, however, in extrapolation to other species and to fishes exposed to airguns in deeper water or where the animals are exposed to a larger number of airgun shots over a longer period of time.

Fishes can regenerate lateral line and inner ear sensory hair cells that have been lost following exposure to ototoxic antibiotics. However, regenerative capabilities following noise exposure have not been explored in fish. Moreover, nothing is known about the functional relationship between hair cell damage and hearing loss, or the time course of morphological versus functional recovery in fishes. This study examines the relationship between hair cell damage and physiological changes in auditory responses following noise exposure in the goldfish (Carassius auratus). Goldfish were exposed to white noise (170 dB re. 1 μPa RMS) for 48 h and monitored for 8 days after exposure. Auditory thresholds were determined using the auditory evoked potential technique, and morphological hair cell damage was analyzed using phalloidin and DAPI labeling to visualize hair cell bundles and nuclei. A TUNEL assay was used to identify apoptotic cells. Following noise exposure, goldfish exhibited a significant temporary threshold shift (TTS; ranging from 13 to 20 dB) at all frequencies tested (from 0.2-2 kHz). By 7 days post-exposure, goldfish hearing recovered significantly (mean TTS<4 dB). Increased apoptotic activity was observed in the saccules and lagenae between 0 and 2 days post-exposure. Immediately after noise exposure, the central and caudal regions of saccules exhibited significant loss of hair bundles. Hair bundle density in the central saccule recovered by the end of the experiment (8 days post-exposure) while bundle density in the caudal saccule did not return to control levels in this time frame. These data demonstrate that goldfish inner ear epithelia show damage following noise exposure and that they are capable of significant regenerative responses similar to those seen following ototoxic drug treatment. Interestingly, functional recovery preceded morphological recovery in the goldfish saccule, suggesting that only a subset of hair cells are necessary for normal auditory responses, at least to the extent that hearing was measured in this study.

Intensive aquaculture production often utilizes equipment (e.g., aerators, air and water pumps, harvesters, blowers, filtration systems, and maintenance machinery) that increases noise levels in fish culture tanks. Consequently, chronic exposure to elevated noise levels in tanks could negatively impact cultured species. Possible effects include impairment of the auditory system, increased stress, and reduced growth rates. The objective of this study was to evaluate the long-term effects of sound exposure on the hearing sensitivity, growth, and survival of cultured rainbow trout (Oncorhynchus mykiss). Two cohorts of rainbow trout were cultured for 8 months in replicated tanks consisting of three sound treatments: 115, 130, or 150 decibels referenced at 1 micropascal (dB re 1 μPa root mean square [RMS]) levels. Auditory evoked potential (AEP) recordings revealed no significant differences in hearing thresholds resulting from exposure to increased ambient sound levels. Although there was no evident noise-induced hearing loss, there were significant differences in hearing thresholds between the two fish cohorts examined. No statistical effect of sound treatment was found for growth rate and mortality within each fish cohort. There was no significant difference in mortality between sound treatments when fish were exposed to the pathogen Yersinia ruckeri, but there was significantly different mortality between cohorts. This study indicated that rainbow trout hearing sensitivity, growth, survival, stress, and disease susceptibility were not negatively impacted by noise levels common to recirculating aquaculture systems. These findings should not be generalized to all cultured fish species, however, because many species, including catfish and cyprinids, have much greater hearing sensitivity than rainbow trout and could be affected differently by noise

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 μPa) or continuous white noise. First, we examined the effect of noise sound pressure level (SPL; 130, 140, 160 or 170 dB re 1 μPa) on goldfish hearing thresholds after 24 h of noise exposure. Second, in a long-term experiment using 170 dB re 1 μPa 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.