Bob Freeman works in the Office of the Oceanographer of the Navy.
In March 2000, 17 cetaceans stranded and seven of the animals died on an island in the Bahamas following a nearby naval exercise that employed active sonar. Since then, the Navy has been sponsoring research to better understand the nature of marine mammal behavioral response to anthropogenic (human-made) sound in the ocean.
Some of that research is taking place at Navy acoustic ranges like the Atlantic Undersea Test and Evaluation Center, or AUTEC, in the Bahamas.
“The goal of our program is to study animals in their natural environment through the application of passive acoustics, which means we listen for the vocalizations that are made by animals and then try to use detections of vocalization as a proxy for the behavior,” said Mr. Dave Moretti, the principal investigator for marine mammal monitoring on Navy ranges, in an April 21 interview on Pentagon Web Radio’s audio webcast “Armed with Science: Research and Applications for the Modern Military.”
Moretti described AUTEC as an area in the ocean that’s been outfitted with hydrophones (listening devices similar to underwater microphones) for tracking submarines and other undersea vehicles. “We’re trying to take the infrastructure of these facilities and apply it to passive acoustics for the study of marine mammals,” he explained.
The AUTEC research, which is sponsored by the Chief of Naval Operations Environmental Readiness Division, is situated about 40 miles from the location of the Bahamas stranding. “It’s really one of the things that make AUTEC so interesting for this study,” Moretti noted.
In the 2000 incident, a pod of beaked whales appear to have stranded in response to a naval exercise conducted over the Northwest Providence Channel. “It’s really a deep ocean canyon that sort of splits the upper Bahamas,” Moretti explained, “and AUTEC is on a branch of that…a separate canyon…and together they make the up the Great Bahamas Canyon,” he said.
Moretti said that there are several species of whales on the range, with beaked whales being the most prevalent. “At AUTEC, to our knowledge, there haven’t been any mass strandings of beaked whales…even though they use active sonar repeatedly throughout the year,” he noted. Moretti defined “mass stranding” as when two or more adult mammals swim up onto a beach.
“[At AUTEC] you don’t have mass strandings even when sonar operations take place,” Moretti said, “and it’s one of the things that we’re trying to tease out of the data, to try to understand why that is.”
Moretti said that ships are on the range at scheduled times and are precisely tracked. “The sensors are really set up to track anything under the water, but anything on the surface is also tracked.”
One of the advantages of AUTEC, Moretti suggested, is the ability to correlate data related to the mammals with vessel traffic. “So now we can get a record of both animal behavior and also a precise record of ship tracks in open ocean waters,” he noted, “and in cases like the Northwest Providence Channel, putting those data sets together is very, very difficult.”
Moretti described the range as an array of bottom-mounted sensors placed two nautical miles apart over some 500 square nautical miles. Signals from the sensors are sent via cable to a shore-based command and control center where the signals are digitized and sent to signal processors. “If an animal makes a call, we have the ability to detect it on the sensors and gain information as to where the animals are distributed,” he explained, “and we can make inferences as to how they’re moving and what they’re doing.”
To ensure continuity of data when the animals are not vocalizing or out of range of the sensors, Moretti said that selected animals are tagged with transponders that allow satellites to track their movement. This is a cooperative effort with representatives from the National Marine Fisheries Service and the Bahamas Marine Mammal Research Organization. In addition, he said, representatives from the Woods Hole Oceanographic Institution and St. Andrews University of Scotland are assisting in the program.
Another type of tag used in the program is the D-tag, a digital recorder developed at Woods Hole that provides data on the pitch, roll, and depth of the animal’s movement. “Those tags stay on about 19 to 20 hours,” Moretti said, “but they give very precise…very pristine data on the movement of animals within that time span.”
Moretti explained that the data they are gathering has provided much needed insights into the private lives of whales, particularly the elusive, deep-diving beaked whales. “I think what people forget is that we’re trying to study animals in their natural environment, but these animals live in the deep ocean and they don’t come to the surface often,” he said. “It’s extremely difficult to study their behavior because it’s an environment we’re just not equipped to deal with well.”
In addition to understanding the behavioral response of whales to anthropogenic sound, Moretti said, the research is designed to understand the health of the populations. “With marine mammals, a lot of the focus has been on individual animals,” he noted, “and we’d like to really shift that so we start thinking in terms of populations.”
Moretti said that the research thus far has suggested that the beaked whales may constitute a resident pod on the range with a fairly robust population, but whether the pod is healthy or negatively impacted by activity on the range is still being studied.
“We’d like these tools to evolve to the point where we can say something about population health and there’s an effort underway to start some programs for the study of these animals on a population level,” Moretti said. “It’s actually been dubbed the PCAD [Population Consequences of Acoustic Disturbance] modeling,” he added, “but that’s in its infancy, actually.”
Moretti said that complementary research is being conducted at a similar Navy range off the coast of California, with assistance from specialists at Cascadia Research, University of California, San Diego, and the University of Oregon. A third program is currently being developed at the Pacific Missile Range Facility off of Hawaii.
“It’s interesting to compare the three areas that we’re working in because it turns out they are really different environments with different species of differing densities,” Moretti said.
Moretti also applauded the cooperative work of the various federal offices and institutions, stating that the project required a broad range of expertise in marine acoustics, signal processing, statistical modeling, marine biology, and animal behavior. “It brings expertise together to help make sense of these data sets that are being collected,” Moretti said, “and the data have proliferated out into several other studies.”
Moretti stated that Navy leadership is trying hard to balance the requirements of their military mission with good environmental stewardship. “It’s to that end that we are funded,” he said, “so hopefully with time, we’ll be able to give some understanding and some guidelines for future environmental compliance.”