"canned" acoustic software packages available on the market, but each have one or more significant drawbacks for the field ecologist: 1) lack of automatic calibration to determine received source levels, 2) require full windows or Mac operating systems (i.e., cannot easily be integrated into specialized hardware), 3) multichannel recording systems often lack options for separate channel calibration and playback, 4) program screens are optimized for desktop use and are difficult to navigate in harsh field conditions with highly variable light conditions, 5) often lack options to incorporate good signal autodetection algorithms (i.e., ones that actually work for fish sounds, especially in noisy environments), and 6) often lack robust sound source localization options. In short, most systems are designed for laboratory or terrestrial field conditions and for biologists working with bird sounds, and not for underwater sounds and marine field conditions. To illustrate an example of a fish ecologist application, I recently conducted a shore-based survey of rivers in New England where I sampled from docks, sea walls, river
banks, bridges, mountain streams, large lakes, small ponds, large estuaries, etc. I often had to scramble over large rocks, down steep banks, through streams, through heavy woods, etc., all carrying a laptop and other cumbersome equipment. Often I had to toss the hydrophone some distance across a bank or off a bridge to reach the water. Did I mention I was sometimes working in either complete darkness, or in sun so bright I could not see the laptop screen? This type of work demands equipment that is highly mobile, flexible, and protected from the elements.
Portable Arrays and Localization of Sound Sources
Another exciting and challenging area of passive acoustic technology development is of portable hydrophone array systems. Hydrophone arrays are needed to localize sound sources so that ecologists can collect data on animal association with habitat and environmental conditions on course and fine scales. In some cases arrays can be used to count, and hence, census marine animals, though the high mobility, intermittent calling, and other aspects of fish behavior make this difficult or impossible for many fishes. One often overlooked, but important byproduct of sound source localization is the determination
Figure 4. Illustration of use of a fixed array to guide an ROV to a target sound source. 44 MTR November/December 2008
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