Future Tech Development in Underwater Observation
Passive Acoustics
By Dr. Rodney Rountree
Observational Technologies
As an ecologist interested in observing the behavior of marine animals in their natural environment, there appears to be a disconnect between the technology industry and the needs of ecologists. Basically, ecologists often have to settle for technologies developed for scientists/users from other disciplines, getting what could be called "hand-me-down" technology ... "hand-me-down" not in the sense of its being old or used, but rather just not quite what the ecologist wants. First they are often limited to a single observation modality. There are many types of observations I may want to make underwater in support of behavioral studies (e.g., spawning), ecological studies (e.g., census of populations or species assemblages), habitat mapping (e.g., percent coverage of a habitat type), etc. In nature there are many ways, or modalities, in which humans or other animals can observe their environment. Humans are highly biased towards visual observations that correspond to optic technologies. Another important way to make observations is through hearing, which translates to acoustic technology. Other ways to observe nature have rarely been translated into observational technologies: chemoreception (smell and taste), mechanoreception (complex of related senses, including equilibrium and balance, touch or tactile, "distance-touch", and hearing), electroreception (detection of electric fields), and magnetoreception (detection of magnetic fields). The development of observational technologies designed to emulate human sensory systems and observational strategies will become increasingly important in the coming decades and will greatly enhance our understanding of underwater ecosystems.
Passive Acoustics
Considering acoustics more closely, note that there are at least three different types of acoustic observations that can
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be made, each of which conveys different information to animal observers: 1) simple hearing or listening, 2) echolocation via projecting a sound and listening for its reflection, and 3) distance-touch, or detection of mechanical disturbances in water through the lateral line. The listening acoustic modality corresponds to passive acoustic technology, while echolocation corresponds to active acoustic technology (e.g. sonar, bioacoustics). Interestingly although echolocation is relatively rare in the animal kingdom, it is the most frequently applied acoustic technology in the marine sciences. In contrast, passive acoustic technologies have only recently been recognized as both underutilized and of great potential in marine ecology fields of study, particularly in fisheries science. In its simplest form, passive acoustics is the act of listening to the underwater sounds made by fishes and other animals in order to observe their behavior and environmental requirements. This new technology is appealing because it provides scientists with an additional tool to put into their toolbox along with more traditional optic and active acoustic technologies, and because it provides the potential for remote observation over extended time periods (i.e., 24/7 realtime remote observations) at a relatively low cost. Fisheries scientists and ecologists can use passive acoustic technology in many ways. One of the most important uses is as a non-invasive method of finding fish and other noise making animals. It is important to realize that sounds can be produced both actively and passively, that is deliberately as a form of communication (e.g., courtship and disturbance calls), and incidentally as a by product of a particular behavior (e.g., swimming and feeding). In either case, those sounds can be used to detect the presence of the animal. In cases where the behavior associated with sound production is known, passive
November/December 2008
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