Page 11 - Acoustics Today

Many commercially important fishes are from families that are prolific sound-producers. The cods and haddocks (Gadidae) are among the most important marine food fish species. Many of these species have been shown to produce sounds associated with courtship and spawning. The had- dock produce relatively simple repeated pulsed sounds. The pulse rate increases as the reproductive sequence moves from courtship through reproduction (Hawkins and Amorim, 2000). Thus, passive acoustic eavesdropping on haddock would provide a means to locate spawning sites.
The great unknowns
Despite all of the research in fish bioacoustics, the answer to the question, “What is making that sound?” is typ- ically, “We don’t know.” Walleye pollock, a member of the cod family, comprises the largest single-species finfish fishery in the United States with catches of nearly 1 million tons a year. Sound production by walleye pollock has yet to be investi- gated. Other families which are well known sound producers have only had the sounds of a few species identified. For instance, worldwide there are about 275 species of croaker and drums. The sounds of only about 10 species have been recorded and identified.
The deep-sea is the largest ecosystem by area in the world. Hundreds of deep-sea fish species have sonic muscu- lature, including the grenadiers, cusk eels, deep-sea cods, and roughies (Marshall, 1967). Still, no deep-sea fish species sound has been identified. The only published report is of a fish-like sound recorded on the Navy Atlantic Undersea Test and Evaluation Center (AUTEC) hydrophone array in the Tongue of the Ocean, Bahamas (Mann and Jarvis, 2004). This sound was a pulsed, stereotyped sound produced at a depth of about 600 m, where the bottom depth was approximately 1,600 m. These species are not just of scientific interest. As coastal fisheries become depleted, fleets have begun exploit- ing species such as the hoki, a deep-sea hake species found off New Zealand. Have you had a fish sandwich at McDonald’s? There is a good chance that you have eaten hoki.
Remote sensing of fish populations with passive acoustics
For species whose sounds have been documented, we can use passive acoustic recording to learn about the ecology of these species. Passive acoustics provides a near perfect ocean observatory sensor for biological activity in fishes. It is not susceptible to bio-fouling and is very low power. Since sound production is often linked to reproductive activities, passive acoustics provides an indirect way to determine spawning seasons and identify areas where fish may migrate to spawn. Development of passive acoustics as a tool in fish- eries science has expanded greatly in the past 10 years. This is largely due to the availability of relatively inexpensive hydrophones and digital recording systems. Efforts have been made to bring the recent advances to the attention of fisheries biologists. Rountree et al. (2006) recently published an article in Fishery magazine to introduce fisheries scientists to passive acoustics as a tool. A special issue of the
Transactions of the American Fisheries Society was recently devoted to new developments and research (Luczkovich et al., 2008).
The beginning of the scientific use of eavesdropping on fish sounds to study behavioral patterns in fishes can be traced to Charles Breder’s pioneering study listening to the sounds produced by fishes in Lemon Bay, off the Gulf of Mexico (Breder, 1968). Breder lowered a hydrophone off of a dock, and listened for fish sounds that had been previously identified, such as by the gulf toadfish and marine catfish (Tavolga, 1958). By listening for a period of five years he quantified patterns in sound production that likely reflect the seasonal patterns of reproductive activity of these species. One of the most interesting aspects of this paper was that he described other fish sounds, which were named the “gallop- er” and “repeater,” but whose identities remain unknown.
Most studies in the 1960’s–1970’s were devoted to under- standing mechanisms of fish sound production and behav- ioral analysis of sound production. Important work during this period showed that coexisting species of damselfish could distinguish each other’s sounds, even though they were quite similar (Spanier, 1979). Foundational work identified sound production by three croaker and drum species and their patterns of sound production in the Indian River Lagoon on the Atlantic coast of Florida (Mok and Gilmore, 1983).
Up until this point, recording and analyzing fish sounds were creative endeavors. Scientists often had to create their own hydrophones, and used the Kay spectrograph machine to burn spectrograms into images. Breder used his brain’s spectrograph to identify fish sounds. While still the best tool available, it is extremely time consuming to manually listen to fish sounds.
In the 1980’s the development of personal computers made it possible to plot a spectrogram on a computer, and greatly increase the amount of data that could be analyzed. I began graduate school in 1990, and joined the laboratory of Phil Lobel, who impressed me with underwater video record- ings showing sound production by the domino damselfish. This was exciting to me, because behavioral studies often seemed qualitative. With the ability to record behavior and sound production, we could become quantitative.
The domino damselfish is an ideal species with which to work because it lives on coral reefs and lays eggs in a nest, which makes it easy to determine spawning patterns just by looking for these nests. Furthermore, damselfish swim downward while producing a characteristic pulsed sound, so it is easy to identify the sound producer. Phil and engineers at Woods Hole Oceanographic Institution developed an automated detector system that automatically processed sounds received from a wireless hydrophone at Johnston Atoll. Using this system, we were able to show that patterns in sound production mirrored patterns in spawning (Mann and Lobel, 1995). This showed, at least for this species, that we could use passive acoustics to identify patterns in spawning.
Croakers and drums are more typical of marine species in that they spawn planktonic eggs, which float with the cur- rents. Thus, it is a lot trickier to link sound production to
10 Acoustics Today, July 2012