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Wu-Jung Lee, Applied Ocean Physics & Engineering*

Broadband acoustic prey recognition by Toothed Whales

The broadband nature of the echolocation signals of toothed whales has been hypothesized to serve the function of prey recognition. The final goal of this project is to explore the signal processing techniques used by toothed whales to discriminate their prey, through a combination of behavioral and acoustic observations in a controlled captive environment. The Student Research Funds provided by the Coastal Ocean Institute was used to support Wu-Jung Lee’s travel to two potential experimental sites, the Roatan Institute for Marine Science, and the Hawai’ian Institute of Marine Biology, to conduct pilot studies.

I. Hawai’ian Institute of Marine Biology (HIMB)
Both the traditional stationary experimental setup and the proposed dynamic setup were tested in  HIMB using an adult female false killer whale. There were two components of acoustic recordings: 1) far-field echolocation signals recorded by a hydrophone array, located in front of the animal but behind the target; and 2) target echoes recorded by the digital acoustic and behavior archival tag (Dtag) attached on the back of the animal. The behavioral recordings were primarily from the underwater cameras installed on two sides of the pen pointing at the target, with the aid of the orientation records in the Dtag.

It was found that the echoes from artificial targets, such as spheres and cylinders, can be easily identified on the Dtag recordings; while echoes from the whale’s daily diet, including capelins and herrings, could not be clearly identified. However, the primary sound-reflecting organ in fish, the swimbladder, is usually collapsed after death, and can result in the lack of echoes in the Dtag recording. It was also observed that the experimental whale might have been using visual, rather than acoustic cues, to find the prey. The whale missed several fish thrown into water when one of her eyes was covered by an eye-cup. This was later associated with the finding of two recently-published papers on the high-frequency hearing loss of this particular animal [1], [2].

II. Roatan Institute for Marine Science (RIMS)
Due to the limitation of being a commercial facility rather than a research facility and the fact that the dolphins at RIMS have not been trained to wear any kind of suction cups before, only two simple tests were conducted at RIMS. The first test constituted a two-hydrophone recording of a group of free-swimming bottlenose dolphins. The second test constituted two Dtag recordings made by mounting the Dtag on top of a hand-held underwater video camera, with the swimmer interacting with the dolphins. Both tests showed that the experimental site at RIMS is very quiet compared to HIMB in terms of broadband snapping shrimp noises, and that echolocation signals with much high frequency content were recorded on the
hydrophones.

Conclusion
The results of the preliminary experiments showed that each of the two sites has its
advantages and disadvantages. HIMB benefits from being a dedicated research facility with professional research trainers. However, three out of four of the toothed whales housed in this facility suffer from serious high-frequency hearing loss [3], and only a three-year-old bottlenose dolphin might be able to participate in our future experiment. The progress in RIMS is generally much slower, owing to the limitation of being a commercial facility and the difficulty to communicate with the trainers in terms of research requirement. However, the twenty-four bottlenose dolphins, ranging from newborn to older adults, provide an excellent opportunity to investigate the echolocation behavior for more than one subject. The much lower contamination of snapping shrimp noise is also advantageous for detecting subtle
echoes from realistic prey items, such as fish and squid.

With the preliminary data collected and the relationship established using the funds provided by the Student Research Funds of the Coastal Ocean Institute, the continuing work of this project is being funded by the Ocean Life Institute in 2010.

References
[1] L. N. Kloepper, P. E. Nachtigall, and M. Breese, “Change in echolocation signals with hearing loss in a false killer whale (Pseudorca crassidens),” The Journal of the Acoustical Society of America, vol. 128, no. 4, pp. 2233-2237.
[2] L. N. Kloepper, P. E. Nachtigall, R. Gisiner, and M. Breese, “Decreased echolocation performance following high-frequency hearing loss in the false killer whale (Pseudorca crassidens),” J Exp Biol, vol. 213, no. 21, pp. 3717-3722.
[3] S. D. Ibsen, W. W. L. Au, P. E. Nachtigall, and M. Breese, “Functional bandwidth of an echolocating Atlantic bottlenose dolphin (Tursiops truncatus),” The Journal of the Acoustical Society of America, vol. 125, no. 2, pp. 1214-1221.


Last updated: April 18, 2012