Many fish in the mesopelagic layer have eyes that point upwards, to detect possible prey highlighted by the small amount of light from the surface. At greater depths, the surroundings are totally dark. |
The different layers in the ocean. Courtesy Finlay McWalter. |
Fish biologists from the University of Massachusetts Amherst have published one of the first studies of deep-sea fish sounds in more than 50 years, collected from the sea floor about 2,237 feet (682 meters) below the North Atlantic. With recording technology now more affordable, Rodney Rountree, Francis Juanes and colleagues are exploring the idea that many fish make sounds to communicate with each other, especially those that live in the perpetual dark of the deep ocean.
Though little is known at present about the significance of sounds made by deep-sea fishes, Rountree and Juanes say that if, as their pilot study suggests, these tend to be low-amplitude, then man-made noise in the oceans may turn out to be a particular problem for some important species.
A noisy environment
Using hydrophones deployed by fishermen during normal fishing operations, Rountree, Juanes and colleagues obtained a 24-hour recording in Welkers Canyon south of Georges Bank that yielded "a wealth of biological sounds" including sounds of fin, humpback and pilot whales, dolphins and examples of at least 12 other unique and unidentified sounds that they attribute to other whales or fish.
The team has now published a series of graphics showing the number of these grunts, drumming and duck-like calls recorded per minute by time of day, plus peak volume and frequencies of various noises. Some of the sounds exhibited strong temporal patterns; for example, fin whale and dolphin sounds dominated the recording and peaked at night.
Rountree explains, "We think work to describe underwater sounds is extremely valuable. The importance of sound in the ecology of both freshwater and marine systems is poorly understood. At this point, in fact, most of our work consists of making careful observations, which of course is the first step in the scientific process."
He adds, "If sound is important to these deep sea fishes, it's a whole area of ecology we need to know about. One reason is that fishermen are exploring deeper and deeper water to make their catch, and we need to know such things as the baseline populations of food fish, their requirements for spawning, their essential habitat and other key aspects of their lives. We believe passive acoustic monitoring is an important tool in this study. And, it doesn't harm the fish or their habitat."
Listening in
Unlike active acoustic studies that bounce sound waves out and back, passive acoustic studies involve just listening. Rountree and Juanes have been promoting underwater passive acoustic studies for more than a decade. They hope to create a census of sounds and behavior observed concomitant with sounds from many different aquatic and marine habitats.
Juanes says some fish use special "sonic muscles" to produce some sounds, and different sounds have different meanings or functions. Many are believed to be related to reproductive behavior. Some fish use a "sound map" for orientation in their immediate environment, and may even use sound waves returning from distant beaches to help them navigate over longer distances. "There is a fascinating acoustic soundscape out there just waiting to be explored."
The scientists are continuing their studies by conducting pioneering passive acoustic surveys of sound in many different habitats, such as freshwater ponds, rivers and streams and coastal estuaries of New England, as well as on the commercial fishing grounds in the Gulf of Maine.
Hear some sounds here.
Reference: This research appears in the new book, Effects of Noise on Aquatic Life, from Springer Science+Business Media in its "Advances in Experimental Medicine and Biology" series. Further details can be found at here.