Brandon M. Casper

Evoked Potential Audiograms of the Nurse Shark (Ginglymostoma cirratum) and the Yellow Stingray (Urobatis jamaicensis)

The hearing thresholds of the nurse shark, Ginglymostoma cirratum, and the yellow stingray, Urobatis jamaicensis, were measured using auditory evoked potentials (AEP). Stimuli were calibrated using a pressure-velocity probe so that the acoustic field could be completely characterized. The results show similar hearing thresholds for both species and similar hearing thresholds to previously measured audiograms for the lemon shark, Negaprion brevirostris, and the horn shark, Heterodontis francisi. All of these audiograms suggest poor hearing abilities, raising questions about field studies showing attraction of sharks to acoustic signals. By extrapolating the particle acceleration thresholds into estimates of their equivalent far-field sound pressure levels, it appears that these sharks cannot likely detect most of the sounds that have attracted sharks in the field.

Dipole hearing measurements in elasmobranch fishes

SUMMARY The hearing thresholds of the horn shark Heterodontus francisci and the white-spotted bamboo shark Chiloscyllium plagiosum were measured using auditory evoked potentials (AEP) in response to a dipole sound stimulus. The audiograms were similar between the two species with lower frequencies yielding lower particle acceleration thresholds. The particle acceleration audiograms showed more sensitive hearing at low frequencies than previous elasmobranch audiograms, except for the lemon shark Negaprion brevirsotris. Auditory evoked potential signals were also recorded while the dipole stimulus was moved to different locations above the head and body. The strongest AEP signals were recorded from the area around the parietal fossa, supporting previous experiments that suggested this region is important for elasmobranch hearing. This is the first time that hearing experiments have been conducted using a dipole stimulus with elasmobranchs, which more closely mimics the natural sounds of swimming prey.

Intraspecific and geographic variation of West Indian manatee (Trichechus manatus spp.) vocalizations (L)

Recordings of manatee (Trichechus manatus spp.) vocalizations were made in Florida and Belize to quantify both intraspecific and geographic variation. Manatee vocalizations were relatively stereotypical in that they were short tonal harmonic complexes with small frequency modulations at the beginning and end. Vocalizations ranged from almost pure tones to broader-band tones that had a raspy quality. The loudest frequency was typically the second or third harmonic, with average received levels of the peak frequency of about 100 dB re 1 μPa. Signal parameters measured from these calls showed the manatees from Belize and Florida have overlapping distributions of sound duration, peak frequency, harmonic spacing, and signal intensity, indicating no obvious distinguishing characteristics between these isolated populations.

The directional hearing abilities of two species of bamboo sharks.

SUMMARY Auditory evoked potentials (AEPs) were used to measure the directional hearing thresholds of the white-spotted bamboo shark Chiloscyllium plagiosum and the brown-banded bamboo shark Chiloscyllium punctatum at four frequencies and seven directions, using a shaker table designed to mimic the particle motion component of sound. Over most directions and frequencies there were no significant differences in acceleration thresholds, suggesting that the sharks have omni-directional hearing abilities. Goldfish Carassius auratus were used as a baseline to compare a species with specialized hearing adaptations versus sharks with no known adaptations, and were found to have more sensitive directional responses than the sharks. Composite audiograms of the sharks were created from the average of all of the directions at each frequency and were compared with an audiogram obtained for C. plagiosum using a dipole stimulus. The dipole stimulus audiograms were significantly lower at 50 and 200 Hz compared to the shaker audiograms in terms of particle acceleration. This difference is hypothesized to be a result of the dipole stimulating the macula neglecta, which would not be stimulated by the shaker table.

Effects of Impulsive Pile-Driving Exposure on Fishes

Six species of fishes were tested under aquatic far-field, plane-wave acoustic conditions to answer several key questions regarding the effects of exposure to impulsive pile driving. The issues addressed included which sound levels lead to the onset of barotrauma injuries, how these levels differ between fishes with different types of swim bladders, the recovery from barotrauma injuries, and the potential effects exposure might have on the auditory system. The results demonstrate that the current interim criteria for pile-driving sound exposures are 20 dB or more below the actual sound levels that result in the onset of physiological effects on fishes.

Evoked potential measurements of the West Indian manatee modulation rate transfer function

Evoked potential measurements of two Florida manatees (Trichechus manatus latirostris) suggest that these herbivores have evolved an auditory system with high temporal resolution. The manatee modulation rate transfer function (MRTF) is maximally sensitive to a 600‐Hz amplitude modulation (AM) rate. This modulation rate is midway between the AM sensitivities of terrestrial mammals (chinchillas, gerbils, and humans) (80–150 Hz) and dolphins (1000–1200 Hz). We also demonstrate evoked potential responses to carrier frequencies up to 40 kHz. These results suggest that manatees may have reasonable underwater localization abilities despite the high speed of sound underwater, which could be important in enabling them to localize oncoming boats.

Comprehensive summary of the impulsive pile driving sound exposure study series

The high intensity controlled impedance fluid filled wave tube (HICI-FT) was used to expose fishes, in the laboratory, to impulsive sound signals under controlled conditions. Fish species were exposed to pile driving signals under different experimental paradigms, followed by detailed investigations of their physiological tissue response (aka, barotrauma injuries). Most often the cumulative sound exposure level (SELcum) was held constant while varying the number of pile strikes and the single strike sound exposure level (SELss). Altering these three variables proved the equal energy hypothesis irrelevant; the higher SELss values with fewer number of strikes caused a highest injury levels. The first dose-response curve was generated for fish responses. Comparisons between different species showed fish with no swim bladder at low injury risk, fish with a closed swim bladder (physoclists) at a high injury risk, and fish with an open swim bladder (physostomes) at a moderate injury risk. If fish find safe haven in the wild, they have potential to heal within 10 days of receiving moderate injuries. Tissue injury appears to occur before damage to hair cells occurs. And most recently, fish show injury after exposure to as few as 8-impulsive signals that have a high SELss value.The high intensity controlled impedance fluid filled wave tube (HICI-FT) was used to expose fishes, in the laboratory, to impulsive sound signals under controlled conditions. Fish species were exposed to pile driving signals under different experimental paradigms, followed by detailed investigations of their physiological tissue response (aka, barotrauma injuries). Most often the cumulative sound exposure level (SELcum) was held constant while varying the number of pile strikes and the single strike sound exposure level (SELss). Altering these three variables proved the equal energy hypothesis irrelevant; the higher SELss values with fewer number of strikes caused a highest injury levels. The first dose-response curve was generated for fish responses. Comparisons between different species showed fish with no swim bladder at low injury risk, fish with a closed swim bladder (physoclists) at a high injury risk, and fish with an open swim bladder (physostomes) at a moderate injury risk. If fish find safe have...

Onset of barotrauma injuries related to number of pile driving strike exposures in hybrid striped bass

Previous studies exploring injury response to pile driving in fishes presented exposure paradigms (>900 strikes) that emulated circumstances where fish would not leave an area being ensonified. Those studies did not, however, address the question of how many strikes are needed before injuries appear. Thus, the number of strikes paired with a constant single strike sound exposure level (SELss) that can cause injuries is not yet clear. In order to examine this question, hybrid striped bass (white bass Morone chrysops × striped bass Morone saxatilis) were exposed to 8-384 strikes in three different SELss treatments that generated different cumulative sound exposure level values. The treatment with the highest SELss values caused swim bladder injuries in fish exposed to as few as eight pile strikes. These results have important implications for pile driving operations where SELss values meet or exceed the exposure levels used in this study.

Barotrauma effects on fishes in response to impulsive pile driving stimuli

We report on new results from controlled exposure studies of impulsive pile driving stimuli using the High Intensity Controlled Impedance Fluid Filled Wave Tube (HICI-FT). Following upon initial investigations focusing on injury thresholds and recovery from injuries in the Chinook salmon, experiments have been expanded to include lake sturgeon, Nile tilapia, hybrid striped bass, and hogchoker. Several key questions concerning pile driving exposure in fishes have been explored utilizing species with different types of swim bladders as well as a species without a swim bladder. Injury thresholds were evaluated in all species, with recovery from injuries measured in the hybrid striped bass. Other pile driving variables measured with the hybrid striped bass include difference in response between fish less than or greater than 2g as well as the minimum number of pile strikes needed for injuries to appear. A study to evaluate potential damage to inner ear hair cells was also conducted on hybrid striped bass as w...

The implications of long‐term increases of anthropogenic noise on fish.

Most of the focus regarding environmental sound and fishes has been on (potential) effects of intense sounds such as pile driving and seismic air guns. There has been less concern, however, about the (potential) effects on fishes of less intense, but continuous increases in the overall noise environment in which fishes live. Such sources may include increased shipping in harbors, operations of off‐shore wind farms, added noise in aquaculture facilities, etc. While the increase in ambient noise may be only a few decibels, such increases could have substantial effects on fishes resulting from the masking biologically relevant sounds and/or the overall acoustic scene. Moreover, and in contrast to intense sounds which generally ensonify any area for only a short period of time (e.g., sonar on a moving ship) or only ensonify relatively small areas (e.g., pile driving) that fish can leave, longer term sources may ensonify such large areas that many fish species cannot move from the area or would have to leave f...