Picture a wild animal that makes noise. Perhaps you’re thinking of a chirping songbird, a bellowing whale, or a howling wolf. But I’d wager you’re absolutely not thinking about a fish.
Some fish species use sound to communicate, just like marine mammals or any number of terrestrial species. And these vocalizations could be key for scientists studying both fish and their freshwater ecosystems. A new paper from Australian and European scientists outlines how these vocalizations — and even the sounds of a fish’s river habitat — could be key for freshwater management and conservation.
Drumming & Stridulations
In most places on earth, every morning starts with the dawn chorus of birds and insects communicating with one another. But a similar chorus occurs beneath the surface of the world’s lakes and rivers, too. “An estimated 20 percent of fish species actively make noises to communicate,” says Simon Linke, a scientist at Griffith University’s Australian Rivers Institute and lead author on the research.
Aristotle was the first person to write about fish vocalizations in his Historia Animalium more than 2,000 years ago. Fish don’t have vocal chords, so they vocalize using two different anatomical methods. The first is when a fish uses its muscles to drum or vibrate its swim bladder, an internal organ that helps fish control their buoyancy. “It’s similar to if you were to scrape your hand across a balloon, you get a squeaking noise,” says Linke. The second method is akin to insect stridulation, where the fish produces sound by rubbing its pectoral fin against the pectoral girdle. “If you have a cheese grater and you run a spoon over it you get a gruuunk sound,” says Linke, “and that’s stridulation.”
North American anglers familiar with the weakfish know that it’s a challenging species to land. But beneath the water, the weakfish vocalizes with a low, staccato grunt. And the plainfin midshipman, a species of toadfish found along North America’s Pacific coast, courts females with a call that sounds like a low, groaning foghorn. Scientists first discovered their calls when Seattle residents complained of a strange, sinister noise waking them up in the middle of the night.
But fish aren’t the only thing you’ll hear if you stick your head — or better yet a hydrophone — underwater. Some bodies of water also have an insect chorus, produced by species like water boatmen and beetles. “And in flowing rivers and gurgling creeks, you really hear the gurgling of the creek a lot,” says Linke. “Its interesting, because you can use that gurgling as one of the indicators of river health.”
Though the field of ecoacoustics, or bioacoustics, is just emerging as a major area of scientific research, scientists are already using acoustic recorders to gather many kinds of data from nature. In the ocean, recorders floating on buoys help understand humpback whale communication and track the changing distribution of North Atlantic right whales. On land, they can detect the infrasonic rumblings of African forest elephants, help scientists study how urban lights affect migrating songbirds, and understand how road noise impacts frog communications. And the Nature Conservancy is using this technology in Papua New Guinea, Borneo, and Myanmar to understand how forest management practices affect biodiversity.
Unlocking the Potential for Freshwater Ecoacoustics
So far, most acoustic research has focused on either terrestrial or marine ecosystems and species. In their new paper, Linke and his coauthors propose that acoustic monitoring could be equally useful to scientists and managers who work on freshwater systems.
For species-specific conservation, bioacoustics could help monitor fish species of high conservation, cultural, or recreational value. Linke’s current research focuses on the sooty grunter, Hephaestus fuliginosus, an Australian species that’s widespread in slow-flowing subtropical and tropical rivers. “The grunters often vocalize when they want to warn each other, or during mating and spawning,” says Linke. The sooty grunter is a target species for recreational fishers, and is culturally important to Aboriginal peoples in its range.
Acoustics could also be used to monitor the spread of invasive fish species or judge the effectiveness of eradication efforts. Another group of researchers is working to identify the signature vocalizations of invasive tilapia in Australian waters, while Linke’s group is seeking funds to help eradicate invasive catfish in New Zealand.
While Linke and his coauthors aren’t aware of any direct use of acoustics for recreational fisheries, he says that it would be possible to use this technology to monitor populations and measure recruitment following restocking efforts.
Even the sounds of a river moving can be a clue to the ecosystem’s health. “A gurgling creek means that there is roughness on the bottom, pebbles moving around,” says Linke. “But if you have lots of sediments…. it fills in the spaces between those pebbles and it stops gurgling.” One of his co-authors, Diego Tonolla, is currently studying how river soundscapes change with increased agriculture. “You can monitor change over time in the entire soundscape,” says Linke, “and see if the condition of a river changes.” And in human-dominated environments, scientists can measure the amount of human-generated noise as a proxy for estimating anthropogenic influence. “The Thames sounds like a highway,” says Linke, who dropped hydrophones into London’s major waterway to listen to the river. “At night it’s quite silent and you can hear snapping shrimp,” he says, “but then in the morning it gets very loud, with boats zooming past all the time.”
Anthropogenic noise is also a form of pollution in its own right. In the oceans, the intense noise from military sonar can cause whales and dolphins to strand themselves en masse on nearby beaches, usually resulting in their deaths. Other research shows that anthropogenic noise also harms fish by disrupting their communication, raising their stress levels, and interfering with predator-prey interactions. Soundscape recordings offer a logical and consistent way to gather data on any anthropogenic noise pollution in a particular waterway.
A Call to Crowdsource Freshwater Soundscapes
Acoustic monitoring brings with it several advantages over more traditional data-collection methods, both on land and underwater. It’s often cheaper than intensive biodiversity sampling, can collect data continuously over long periods of time, and avoids various types of sampling bias. But there are still a few challenges that need to be overcome before this technology can fulfill its potential for freshwater science and conservation.
“One of the key challenges is that there is no big database that you can go to,” says Linke. “There are fantastic researchers around the globe doing fantastic work… but none of these calls have been submitted to a database where people can reference it.” One of the premier acoustic libraries in the world, the Macaulay Library at Cornell University, has more than 120,000 recordings of birds but just 929 recordings of fish, most of which were collected before 1980.
This means that scientists like Linke have to painstakingly work out exactly what species produce what sounds. “We have to throw insects in buckets and let them sing by themselves,” he says. Linke is currently seeking funds to create a catalogue of calls for all 34 species of Australian grunters. He says that if researchers conducting similar work contributed to a global database, it would help crowdsource valuable data for future research.
“The second big challenge is really to derive links between sounds and ecosystem health,” says Linke. For example, insects like beetles and water boatmen are already used as indicators for disturbance, including high nutrient levels. Acoustics could detect an increase in insect activity, potentially signaling a decline in river health, but first scientists would need to know just how much insect noise occurs in a healthy river. “We’re trying to construct reference conditions on how a river should sound,” says Linke.
If scientists can overcome those obstacles, ecoacoustics is poised to transform freshwater ecology, one fish grunt at a time.