Earthquake Sensors Help Track Endangered Fin Whales
April Flowers for redOrbit.com – Your Universe Online
The second largest, and one of the least understood, animals to ever live on the Earth is the fin whale. The animal has a huge body and a global range that makes its movements and behavior hard to study under normal circumstances.
This spring, a fin whale carcass washed up on a Seattle-area beach, reminding researchers that the sleek animals, sometimes called the “greyhounds of the sea,” are vulnerable to collision when they strike fast-moving ships. If more were understood about their swimming behaviors, vessels could avoid the animals. A deeper understanding of where and what they eat could also help researchers support the fin whale´s slowly rebounding populations.
A new study by University of Washington oceanographers addresses such questions using a growing number of seafloor seismometers, which are devices that record vibrations. The team published their findings in a series of three papers in the Journal of the Acoustical Society of America this past winter, interpreting whale calls found in earthquake sensor data. This inexpensive and non-intrusive way to monitor the whales is the first study to match whale calls with fine-scale swimming behavior, providing new clues about their movements and communication patterns.
Ten years ago, a project began to monitor tremors on the Juan de Fuca Ridge, a seismically active zone more than a mile deep off the Washington coast. For the first time, UW researchers were able to collect an entire year´s worth of seafloor seismic data.
“Over the winter months we recorded a lot of earthquakes, but we also had an awful lot of fin-whale calls,” said William Wilcock, a UW professor of oceanography. Initially, the fin whale calls, which overlapped the seismic data at 17 to 35 vibrations per, “were kind of just a nuisance,” he said. By 2008, however, Wilcock received funding from the Office of Naval Research to study the previously irritating whale calls.
The calls recorded by eight different seismometers were compared by Dax Soule, a UW doctoral student in oceanography. The UW researchers automated the work to analyze more than 300,000 whale calls, compared to previous studies which have done this for just two or three animals at a time.
The method used by the team is similar to a smartphone´s GPS using triangulation — measuring a person´s location by comparing paths to different satellites. The UW team calculated an animal´s position by looking at the whale´s call from eight different seismometers. This allows the scientists to follow a particular animal´s path through the instrument grid and within 10 miles of its boundaries.
Soule charted 154 individual fin whale paths through the sensor grid and discovered three categories of vocalizing whales that swam south in the winter and early spring of 2003. A fourth category of rogue whales was also discovered traveling north in the early fall. These rogues move faster and emit a slightly higher-pitched call than the other groups.
“One idea is that these are juvenile males that don´t have any reason to head south for the breeding season,” Soule said. “We can´t say for sure because so little is known about fin whales. To give you an idea, people don´t even know how or why they make their sound.”
Unlike the humpback whale, the call of a fin whale is not melodic. For this approach, however, that is a plus. The fin whale emits a second-long chirp approximately every 25 seconds that is consistently loud and at the lower threshold of human hearing, putting it within range of earthquake monitoring instruments. The loud, repetitive bleeps are also ideally suited for computer analysis.
Soule´s triangulations were used by Michelle Weirathmueller, a UW doctoral student in oceanography, to determine the loudness of the call — a surprisingly consistent 190 decibels. This translates to about 130 decibels in air, or about as loud as a jet engine.
The consistent amplitude of the whale´s song will allow the team to better track the whales with a more widely spread seismometer grid that will record the calls with only one instrument at a time. These sensor networks include the Neptune Canada project, the US cabled observatory component of the Ocean Observatories Initiative, and the huge 70-seismometer Cascadia Initiative array off the Pacific Northwest coast.
“We´d like to know where the fin whales are at any given time and how their presence might be linked to food availability, ocean conditions and seafloor geology,” Weirathmueller said. “This is an incredibly rich dataset that can start to pull together the information we need to link the fin whales with their deep-ocean environments.”
Sound Byte: A fin whale call and earthquake recorded in September 2010 by a seismometer on the continental shelf off Vancouver Island in about 1,200 meters depth. The recording was originally 5 minutes long but has been sped up by a factor of 10 so that human can hear it. The rumbling that begins at the 18-second mark is an earthquake. Credit: Michelle Weirathmueller, Univ. of Washington.