Plumbing Depths for Fish Sounds

by Andrea Cohen, MIT Sea Grant

"Until now, scientists have largely ignored sound in their explorations of the seas. It's like a terrestrial scientist trying to understand rainforest ecosystems without ever listening to insects, amphibians, and birds," says Rodney Rountree, director of Marine Ecology and Technology Applications, Inc. of Falmouth, Mass. and adjunct professor at UMass Amherst. The researcher adds: "On land, sound helps us navigate and find things. We do it without a thought. It's implicitly incorporated into all our terrestrial survey methods. You walk though the woods searching for breeding birds or frogs and you are guided both by sights and sounds. Underwater, we have largely been restricted to sights. Now we are trying to convince scientists from many disciplines that it makes sense to listen too."

The greatest obstacle to studying fish in the ocean, says Rountree, is finding them in the first place. Because fish rely on sound to communicate - more than 800 species of fish are known to be vocal - passive acoustic techniques present the opportunity to study fish in a non-invasive, non-destructive manner. Whereas video requires light, passive acoustics, i.e., recording sound, does not. Additionally, this technique allows researchers to conduct continuous monitoring to determine the daily and seasonal activities of fishes.

Yet, for all these benefits, passive acoustic monitoring has rarely been employed to study the commercially important fishes of the Northeast Atlantic. To counter that trend, Rountree has been collaborating with Cliff Goudey, director of MIT Sea Grant's Center for Fisheries Engineering Research, Ken Ekstrom, MITSG research associate, and Francis Juanes, associate professor at UMass Amherst. With funding from the Northeast Consortium and the Northeast-Great Lakes National Undersea Research Program, the researchers made the first field recordings of Atlantic cod, haddock and cusk in the Western North Atlantic. In an MITSG-funded project, UMass Amherst graduate student Katie Anderson has focused on haddock reproduction and associated sounds in the Gulf of Maine, and particularly in the Stellwagen Bank National Marine Sanctuary.

Rountree points out that the sounds of fish in deep water have rarely been recorded, and that understanding those fish becomes increasingly important given concerns that commercial fishing has begun to exploit certain species in deeper waters.

In a pilot study funded by MIT Sea Grant, Rountree and Goudey have been deploying autonomous underwater listening stations (AULS) from commercial fishing vessels. Designed by Goudey, the AULS employs an external hydrophone, which picks up acoustic energy underwater and converts it into electrical energy for listening. At less than $1,000 per unit, the AULS uses state-of-the-art digital recording devices and an MP3 player in a pressure housing that can descend to 1,100 meters and store more than 60 hours of acoustic data on a 10 GB hard drive. Working with commercial red crab fishery captain Jon Williams, the researchers deployed three AULSs in crab pots for 24-hour periods in waters 680 meters deep, in Welkers Canyon, south of Georges Bank.

And what did they hear?

"There's significant biological sound production in deep water in Welkers Canyon," says Rountree. These include abundant fin whale and dolphin sounds, as well as some humpback whale calls. And, "most importantly," says Rountree, "we recorded several different types of sounds that are most likely produced by fishes." The trouble with identifying that sound, he explains, is that no catalogue of fish sounds from the ocean depths exists. "We have nothing to compare it to," he states.

With further funding, Rountree hopes to develop a methodology for identifying fish sounds that have not yet been recorded. "There are several sounds which we heard that we suspect are fish," he says, "but we can't prove that until we have more data."

While Rountree notes that many scientists don't appreciate the value of recording unknown deep sea sounds, he explains that he and his colleagues see this as exciting, cutting-edge research. And documenting the time and location where unknown sounds occur is the first step in studying sound production in the deep sea, says Rountree. For example, if a particular unknown sound is always heard at a certain time of day in a particular habitat or location, then scientists can design studies to identify what type of animal is making the sound.

"The alternative is to throw up our hands and not do anything," says Rountree. "This study demonstrated that commercial fishing vessels can serve as an important platform for research on deep water fishes beyond the normal sampling range of government and academic surveys."

Along with its potential for identifying essential fish habitat for spawning, Rountree sees passive acoustics as a new way of monitoring remote environments. This could include monitoring ship activities in areas closed to fishing, such as remote coral reefs in the Pacific. And the findings could also improve our understanding of distribution patterns of commercial fish in deep waters, thereby aiding in industry management.

In the meantime, the pilot study has also had some unexpected benefits. Acoustic data from the deployments with crap pots indicates that the fishing gear is not as stable as believed, with pots shifting for several hours after deployment and potentially hauling in a smaller catch. So until the crab fishermen deploy their pots for more research, they may be looking into how to better stabilize their gear.

To hear what the fish have to say, go to http://www.fishecology.org/....