Testing the odontocete acoustic prey debilitation hypothesis: no stunning results

The hypothesis that sounds produced by odontocetes can debilitate fish was examined. The effects of simulated odontocete pulsed signals on three species of fish commonly preyed on by odontocetes were examined, exposing three individuals of each species as well as groups of four fish to a high-frequency click of a bottlenose dolphin [peak frequency (PF) 120 kHz, 213-dB peak-to-peak exposure level (EL)], a midfrequency click modeled after a killer whale's signal (PF 55 kHz, 208-dB EL), and a low-frequency click (PF 18 kHz, 193-dB EL). Fish were held in a 50-cm diameter net enclosure immediately in front of a transducer where their swimming behavior, orientation, and balance were observed with two video cameras. Clicks were presented at constant rates and in graded sweeps simulating a foraging dolphin's "terminal buzz." No measurable change in behavior was observed in any of the fish for any signal type or pulse modulation rate, despite the fact that clicks were at or near the maximum source levels recorded for odontocetes. Based on the results, the hypothesis that acoustic signals of odontocetes alone can disorient or "stun" prey cannot be supported.     

Acoustic backscattering by Hawaiian lutjanid snappers. II. Broadband temporal and spectral structure

The characteristics of acoustic echoes from six species of deep-dwelling (up to 400 m) Hawaiian Lujanid snappers were determined by backscatter measurements at the surface. A broadband linear frequency-modulated signal and a short dolphinlike sonar signal were used as the incident signals. The fish were anesthetized and attached to a monofilament net that was attached to a rotor so echoes could be collected along the roll, tilt, and lateral axes of each fish. The temporal highlight structure of broadband echoes was determined by calculating the envelope of the cross-correlation function between the incident signal and the echoes. The echo waveforms were complex with many highlights and varied with the orientation of the fish. In the tilt plane, the strongest echoes occurred when the incident signal was perpendicular to the long axis of the swimbladder. The number of highlights was the fewest at this orientation. The number of echo highlights and the length of echoes increased as the fish was tilted from this orientation. The highlight structure of the echoes resulted in the transfer function being rippled, with local maxima and minima that changed with fish size and species. The echo structures in both the time and frequency domains were generally consistent within species and were easily distinguishable between species.     

Broadband backscatter from individual Hawaiian mesopelagic boundary community animals with implications for spinner dolphin foraging

Broadband simulated dolphin echolocation signals were used to measure the ex situ backscatter properties of mesopelagic boundary community (MBC) in order to gain a better understanding of the echolocation process of spinner dolphins foraging on the MBC. Subjects were captured by trawling with a 2-m-opening Isaacs-Kidd Midwater Trawl. Backscatter measurements were conducted on the ship in a 2000 L seawater tank with the transducer placed on the bottom pointed upwards. Backscatter measurements were obtained in both the dorsal and lateral aspects for seven myctophids and only in the dorsal aspect for 16 more myctophids, six shrimps, and three squids. The echoes from the myctophids and shrimps usually had two highlights, one from the surface of the animal nearest the transducer and a second probably from the signal propagating through body of the subject and reflecting off the opposite surface of the animal. The squid echoes consisted mainly of a single highlight but sometimes had a low amplitude secondary highlight. The backscatter results were used to estimate the echolocation detection range for spinner dolphins foraging on the mesopelagic boundary community. The results were also compared with multi-frequency volume backscatter of the mesopelagic boundary community sound scattering layer.     

Modeling the detection range of fish by echolocating bottlenose dolphins and harbor porpoises

The target strength as a function of aspect angle were measured for four species of fish using dolphin-like and porpoise-like echolocation signals. The polar diagram of target strength values measured from an energy flux density perspective showed considerably less fluctuation with azimuth than would a pure tone pulse. Using detection range data obtained from dolphin and porpoise echolocation experiments, the detection ranges for the Atlantic cod by echolocating dolphins and porpoises were calculated for three aspect angles of the cod. Maximum detection ranges occurred when the fish was broadside to the odontocete and minimum detection ranges occurred when the cod was in the tail aspect. Maximum and minimum detection ranges for the bottlenose dolphin in a noise-limited environment was calculated to be 93 and 70 m, respectively. In a quiet environment, maximum and minimum detection ranges for the bottlenose dolphin were calculated to be 173 and 107 m, respectively. The detection ranges for the harbor porpoise in a quiet environment were calculated to be between 15 and 27 m. The primary reason for the large differences in detection ranges between both species was attributed to the 36 dB higher source level of the bottlenose dolphin echolocation signals.     

Acoustic backscattering by Hawaiian lutjanid snappers. 1. Target strength and swimbladder characteristics

The target strengths and swimbladder morphology of six snapper species were investigated using broadband sonar, x rays, and swimbladder casts. Backscatter data were obtained using a frequency-modulated sweep (60-200 kHz) and a broadband, dolphinlike click (peak frequency 120 kHz) from live fish, mounted and rotated around each of their three axes. X rays revealed species-specific differences in the shape, size, and orientation of the swimbladders. The angle between the fish's dorsal aspect and the major axis of its swimbladder ranged from 3 degrees to 12 degrees and was consistent between individuals within a species. This angle had a one-to-one relationship with the angle at which the maximum dorsal aspect target strength was measured (r2 = 0.93), regardless of species. Maximum dorsal aspect target strength was correlated with length within species. However, the swimbladder modeled as an air-filled prolate spheroid with axes measured from the x rays of the swimbladder predicted maximum target strength significantly better than models based on fish length or swimbladder volume. For both the dorsal and lateral aspects, the prolate spheroid model's predictions were not significantly different from the measured target strengths (observed power >0.75) and were within 3 dB of the measured values. This model predicts the target strengths of all species equally well, unlike those based on length.     

Controlled and in situ target strengths of the jumbo squid Dosidicus gigas and identification of potential acoustic scattering sources

This study presents the first target strength measurements of Dosidicus gigas, a large squid that is a key predator, a significant prey, and the target of an important fishery. Target strength of live, tethered squid was related to mantle length with values standardized to the length squared of -62.0, -67.4, -67.9, and -67.6 dB at 38, 70, 120, and 200 kHz, respectively. There were relatively small differences in target strength between dorsal and anterior aspects and none between live and freshly dead squid. Potential scattering mechanisms in squid have been long debated. Here, the reproductive organs had little effect on squid target strength. These data support the hypothesis that the pen may be an important source of squid acoustic scattering. The beak, eyes, and arms, probably via the sucker rings, also play a role in acoustic scattering though their effects were small and frequency specific. An unexpected source of scattering was the cranium of the squid which provided a target strength nearly as high as that of the entire squid though the mechanism remains unclear. Our in situ measurements of the target strength of free-swimming squid support the use of the values presented here in D. gigas assessment studies.