Page 9 - Acoustics Today

REMOTE SENSING OF FISH USING PASSIVE ACOUSTIC MONITORING
David A. Mann
Loggerhead Instruments
6576 Palmer Park Circle, Sarasota, Florida 34238 and
University of South Florida
College of Marine Science, Saint Petersburg, Florida 33701
“Despite all of the research in fish bioacoustics, the answer to the question, ‘What is making that sound?’ is typically, ‘We don’t know’.”
Introduction
Lower a hydrophone into nearly any coastal area at dusk or night and you will be immersed in a world of clicks, knocks, and hums. After several seconds of listening and internally classifying sounds, you’ll want to ask, “What made that sound?” While we are mostly familiar with the communication and echolocation sounds of marine mammals, thousands of other animals produce sound.
Fishes are likely the largest source of biological sound in the coastal oceans. Partly this is due to some species being very loud, and partly this is due to their being so many fish producing sound. It seems that about every decade the pub- lic and press rediscover the fact that many fishes produce sounds. The town of Cape Coral, Florida was about to under- take an engineering study to discover source of mysterious late-night sounds, until it was determined that black drum were the source of the booms (Locascio and Mann, 2011). The black drum sounds were so loud, that they could be heard inside the houses adjacent to seawater canals. The mid- shipman toadfish in Sausalito, California has been the subject of many humorous news stories, as their incessant humming kept houseboat dwellers awake at night wondering whether there were nuclear submarines out back.
Fish bioacoustics
Some sound producing fishes have been known for mil- lennia, and their common names reflect their sound produc- ing nature. The fish family Sciaenidae is more commonly known as the croakers and drums. If you pick up a croaker, you will get sound.
Fish produce sounds by a variety of mechanisms. Many of the loudest species, such as the croakers and drums, have specialized muscles located on or next to the gas filled swim- bladder. Other species, such as the catfishes, can also produce sounds by stridulating bones (Parmentier et al., 2010). Clownfish, made popular by Disney’s movie Finding Nemo, produce pulsed sounds by clapping their jaws together using a specialized sonic ligament (Parmentier et al., 2007).
The acoustic characteristics of fish sounds are directly tied to the mechanisms of sound production. Toadfish twitch their sonic muscles to drive the swimbladder to produce sound. The swimbladder does not act as a resonator in these species; it is highly damped (Fine et al., 2009). Thus, the fun- damental frequency of the toadfish boatwhistle sound
reflects the muscle contraction rate. In these species, the frequency of the sound can change seasonally as the water warms, allowing faster muscle contraction rates (Fine, 1978). Fish sounds are generally below 1 kHz, with some large fishes like the goliath grouper producing sounds as low as 60 Hz (Mann et al., 2009). This frequency range generally matches the hearing range of fishes, which is best at low fre-
quencies. In some cases, like the toadfish boatwhistle, har- monic frequency components of the sound may exceed their range of best hearing.
Fish sounds are stereotypical, like insect and frog sounds. While there is some inter-individual variation, it is small compared to variation between species. Still, different species in the same family often produce similar sounds. Many toadfish produce the distinctive boatwhistle call, but the contraction rate and number of elements varies between species (Tavolga, 1958; Amorim et al., 2011). The stereotypi- cal nature of fish sounds makes it relatively easy to identify which species made which sound, once the sound has been characterized. (See Figs. 1 and 2)
Sound production is by no means limited to marine fish- es. Sound production by freshwater fishes is well described (e.g., sculpin: Kierl and Johnston, 2010; cichlids: Lobel, 1998;
Fig. 1. William Mowbray in an acoustic recording van used to record fish sounds. These studies were compiled into the only published compendium of fish sounds (Fish and Mowbray, 1970). Photo used with permission of T. Mowbray.
8 Acoustics Today, July 2012