Eavesdropping on Echolocation
The sun shines on the Charles W. Morgan’s second day out of Provincetown, and there is already flurry of activity at the Stellwagen Bank National Marine Sanctuary. The whale-watching boats that normally frequent this area have been joined by several of the Morgan’s small wooden whaleboats, built by organizations from around the country to allow the voyagers a more intimate experience with both maritime history and these marine mammals. The air is filled with the sounds of slapping fins and distinct “whooshes” of breath as humpbacks surface alongside ships and boats alike. Today they are accompanied by another species: a fin whale.
Like the minke and humpback whales observed the day before, the fin whale is a baleen whale, drawn to the area to feed on its abundant copepod population. In terms of size, however, it dwarfs even the humpbacks in comparison. The fin whale is one of the largest species of whales in the world, second only to the blue whale. It can weigh between 40 and 80 tons and grow longer than 80 feet. Only eighty years ago, the Morgan’s purpose on Stellwagen Bank would be to hunt and harvest this massive animal. Since then, however, petroleum products and plastic have replaced whale oil and baleen as sources of illumination and lubrication. Technology has moved on.
The Stellwagen Bank National Marine Sanctuary is a poignant example of how our changing perspective of the ocean inspires new forms of science. An underwater plateau at the mouth of the Massachusetts Bay, Stellwagen Bank was named after Henry S. Stellwagen, who visited the area in 1854. Before the invention of GPS or ship sonar, this underwater feature would warn warn ships that they were leaving the Gulf of Maine to enter the more dangerous waters of the Boston Harbor. For centuries, the bank was also used as a rich fishing and whaling ground.
In 1992, Stellwagen Bank was classified as a National Marine Sanctuary, regulated and protected by the US government. Since then, scientists have taken advantage of the abundance of wildlife to conduct several studies: fish ecology, water quality, the history of marine animal populations, and of course, whale research. Using historical and current observations, scientists have focused on understanding whale behavior so as to reduce whale mortality caused by entanglement in fishing gear or direct ship strikes. However, there has been recent concern about human-induced sources of underwater noise, which can also affect marine animals.
In order to overcome the distance and visibility obstacles in the ocean, many animals rely on sound to monitor their surroundings. Among highly social species, such as whales, sound is also crucial to communication. Whales use a type of biological sonar known as echolocation to find food, safely travel along unfamiliar coastlines, and migrate between breeding and feeding grounds. These activities become increasingly difficult as anthropogenic noise enters the ocean through ship traffic, oil and gas extraction, and the use of military sonar equipment. Some scientists suggest that noise pollution may not only impede echolocation sound waves that travel through the water, but can also damage hearing or cause negative behavioral changes.
The baleen whales on Stellwagen Bank are known to use low-frequency sounds to communicate. Minke whales vocalize via clicks, grunts, and recently, “boings.” Humpbacks produce long, complex songs. Ironically, the best equipment to examine the effect of noise pollution on these species is a former instrument of war: a hydrophone. This passive underwater listening device converts sound into electricity. It was first invented in 1914 by Canadian inventor Reginald Fessenden, who sought a way to locate icebergs following the sinking of the Titanic. In World War I, hydrophones were used on both ships and submarines alike to detect submerged and floating targets. Although a single hydrophone can record sounds arriving from any direction, several hydrophones can be simultaneously deployed in multiple locations (a system known as a hydrophone array). The resulting signals can then be manipulated to listen in a specific direction, allowing greater sensitivity to noise location than a single hydrophone.
With access to such equipment, modern ocean research has started to focus on underwater noise. Within the past 10 years, the governmental organization NOAA has established several passive monitoring arrays of these hydrophones throughout United States waters. The arrays detect sound produced by living marine organisms and natural sources of noise from physical oceanographic processes, as well as monitor anthropogenic noise sources that enter the environment. Perhaps nowhere is this new technology more appropriate than on Stellwagen Bank, where hydrophones of the twenty-first century have replaced the harpoons of old.