By Russel Barsh and Madrona Murphy

In literature, the depths of the sea are dark and “silent”. In reality, marine ecosystems are very noisy. At the frequencies heard by most fish, the oceans have been as noisy as busy city streets for hundreds of millions of years. Human activities have nonetheless recently added new layers of noise to an already noisy ocean.

Eerie marine mammal “songs” were popularized in the 1960s, but from a human point of view, fish songs are boring series of grunts, barks, chirps, and buzzes. Very sexy to other fish, however; and as more fish are subjected to acoustic studies, it appears that song is as widespread a tool for attracting mates and demarcating territory in fishes, as among birds. In addition, many underwater predators eavesdrop on fish songs to target their prey.

Fish are the oldest surviving class of vertebrate animals, emerging over 400 million years ago, and at an early stage they evolved two kinds of acoustic organs: internal ears with a covering of fine hairs that sense high-pitched vibrations, and rows of pressure-sensitive skin cells (the lateral line) that sense very low-pitched sounds and movement. Fish “hear” in two ranges—one that overlaps with the lower octaves of human hearing, and another that is sensitive to “low bass” sounds that humans tend to feel rather than hear.

Sound propagates better in water than air, and the deep bass singing of some whales and fish can be heard and understood by animals miles away. Indeed, hearing in air posed a serious challenge for the first vertebrate animals to explore the land. Significant changes in vertebrate ears can be seen in the fossil record about 250 million years ago, associated with the initial transition from water to air. Reptiles, birds and mammals have ears with several tiny interlocking bones that help amplify high frequencies that carry farther in air.

The ears of a bat can detect the faint echo of a hypersonic chirp bouncing off the wings of a moth a hundred feet away, but not the deep bass thrumming of a lovesick codfish!

In light of the pervasive role of sound in marine animals’ feeding and reproduction, it should not be surprising that human noise has significant impacts. From the creaking of the boards and pulleys on old wooden sailing ships, to the engines and sonar of modern ships, humans have been making the oceans noisier for centuries.

Modern propulsion systems, with petroleum-burning engines and electrical motors, pose particular problems because they emit mainly low-frequency infrasound, just about in the range most widely utilized by fish. Marine engines may be broadcasting confusing love notes to fish for hundreds of miles around.

Fish and marine mammals have some ability to move away from noisy patches of ocean, although the long-distance propagation of sound underwater makes this much harder for marine animals than their terrestrial cousins. The greatest threat is to animals that cannot afford to abandon their habitat: for example, rockfish nesting in a rocky reef such as Bell Island here in San Juan County. Summer increases in engine noise conflict with crucial, site-specific warm weather activities such as nesting, spawning, and rearing young.

A great deal remains to be learned about the vocal range of fishes, the use and meaning of fish songs and acrobatic performances that sometimes accompany them, and the possible adverse impacts of competing human noise. It may turn out that damping engine noise in marine vessels is as important for protecting the marine environment, as reducing engine emissions.

Learn more about nearshore fish research in the San Juan Islands at www.kwiaht.org. Kwiaht is a partner in the Salish Sea Marine Survival Project, studying bottlenecks in the growth and development of wild Chinook salmon in the marine environment.