Vocalizations from deep-sea fish recorded

A team of ecologists recently captured the vocalizations of deep-sea fish in a recording that includes familiar sounds, such as dolphin and humpback whale calls. These findings support long-standing hypotheses about fish communication in the under-explored mesopelagic zone.

The mesopelagic, or twilight zone, marks the boundary between the sunlit surface layers and the freezing, oxygen-poor deep water. As oxygen-rich surface waters become colder and saltier, its density increases and causes it to sink. The sharp drop in temperature also serves as a barrier between the surface and deep-sea animals.

Over 50 years ago, researchers hypothesized that sound production would be common in mesopelagic fish based on the observation that many discovered species possess the anatomy needed to produce sound. While most fishes unintentionally produce sounds from chewing and swimming, deep-sea fish may require communication for survival and reproduction.

The dimly-lit waters of the twilight zone and the great distance between fellow fish may warrant such an adaptation in pelagic fish. However, Rodney Rountree, a marine ecologist at the University of Massachusetts, notes that fish will not necessarily vocalize just because they have the anatomy to do so.

Scientists know little about deep-sea fish sounds due to the scarcity of research papers concerning this topic. Rountree notes that the vocalizations of deep-water fish cannot be recorded simply by placing the fish in tanks and eavesdropping on their conversations.

According to Rountree, pelagic fishes are very difficult to keep alive in the lab, and the fish will not produce sounds unless researchers can elicit certain behaviors. For instance, the fish will not produce reproductive sounds unless they can be made to spawn in the laboratory — a Herculean task considering the difficulty in mimicking correct environmental conditions.

Researchers have not previously recorded deep-sea fish in their natural habitat, mostly due to the lack of adequate, low-cost technology. However, Rountree and his colleagues worked with commercial fishermen to develop a simple deep-water hydrophone. The team recorded deep-water sounds from the sea floor of Welkers Canyon, at a depth of approximately 862 meters below the surface for 24 hours.

From the recording, the team successfully identified various sounds from whales and other known cetaceans, but they also detected at least 12 other unidentifiable sounds. The unique vocalizations consisted of drumming, grunts and duck-like calls, and could have originated from either whales or deep-water fish.

Rountree says that most fish only hear and produce low-frequency sounds, and the sounds that the team recorded fell in the frequency range that fish typically use. If the sounds were actually produced by deep-sea fish, then increases in man-made noises in the coming years may be problematic for these fish.

The vocalizations recorded by the team were just barely above the background noise. As a result, if the noise continues to increase, it may prevent the fishes from receiving each other's signals. Rountree's team is currently trying to develop an audio system that includes video, in hope of identifying the creatures responsible for the sounds. However, the price to develop this system will be high because of the power needed to keep the light on for deep-sea investigation. The team also hopes to figure out the meaning behind the grunts and other calls.