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The underwater speaker (UWS) has been installed on high speed vessels; hydrofoils (HF) with low-noise during their cruises, to avoid sudden collisions with large cetaceans, while its performance has remained uncertain because of the problem in quality of the produced sound. Thus, we developed a sound source for the UWS by modulating the sound based on the audible range of major large cetaceans so as to increase its utilities. To investigate the audible sound frequency range of cetacean, we tried two procedures, (1) indirect-estimation from relationship between cetaceans audibility and vocalization, and (2) indirect-estimation from measurements on the cochlear basal membrane. We also synthesized the two new sound sources which we can potentially expect an avoidance with large cetaceans. Through several field experiments with deploy the new sounds we reached a tentative conclusion that the new sound was effective in terms of inducing the cetaceans' avoidance reaction and would be also expected to be applied to other low-noise vessels.
Content available remote Towards Safer Navigation of Hydrofoils: Avoiding Sudden Collisions with Cetaceans
Recently, sudden collisions between large cetaceans and high-speed hydrofoils have become problematic to Japanese sea transport in some localities. We therefore initiated a project to investigate ap-proaches for minimizing risk to both ships and cetaceans. Under the present project, the following three sub-projects are underway: clarifying which whale species are found near sea routes and determining their season-al variations; identifying whale species that have a high collision risk; and determining the unique acoustic characteristics of high-collision-risk cetaceans for the improvement of underwater speakers (UWS). By con-ducting acoustic surveys using novel methods, including an anatomical approach based on characteristics of the inner ear, the aim of this project is to accurately estimate the audible range of species with a high collision risk and improve the sounds generated by the UWS. Thus far, we have identified the cetacean species at high-risk in two major sea routes. In the next phase of the study we plan to develop an imaging system that recog-nizes a cetacean's unique blow using an infrared camera, in an attempt to warn of the approach of high-collision-risk whale species at an early stage by sounding an alarm.
To achieve safer navigation without sudden collisions with large cetaceans at high speed boats such as the hydrofoil, we examined its feasibility of an installation of the infrared camera. Because any ceta-ceans are of air-breathing animals, it is theoretically expected that they can be potentially detected through imaging of the infrared cameras. Thus, we examined the feasibility of detection with aiming at sperm whales in waters off Chichijima Islands (27°4'N, 142°13'E), Japan. Through the experiment, it was revealed that sperm whales could be detected stably within 200m, and detectable cue were blow, back body and fluke tails. However, boats and waves were also detected as noise images. Especially, waves greatly resemble the whale back bodies. Although potential of the infrared camera was confirmed, there are still necessities of further ex-periments including ones conducting at different temperate waters, to successfully install the infrared camera for earlier finding of large cetaceans.
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