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Photo: Iain Kerr |
VOYAGE OF THE ODYSSEY - BIOACOUSTICS
Marine bioacoustics concentrates on understanding the acoustic behaviors of marine animals with particular focus on marine mammals (whales and dolphins). As such it serves as the focal point for a wide range of disciplines including behavioral ecology, biological oceanography, population biology, psychoacoustics, neuroethology, electrical engineering, statistics, applied mathematics, and applied physics.
Marine Bioacoustics Program - Cornell University
The research deals with basic descriptions of acoustic behaviors of whales on an ocean scale. The objective is to understand the relationships between these animals and the physical and biological features of the ocean in which they evolved and survive. To accomplish this, Dr. Clark is actively leading several major research efforts in the North Pacific and North Atlantic. This effort requires development and implementation of advanced digital signal processing systems deployed either remotely or linked back to Cornell or to Navy facilities. It also requires the constant integration of the physical and biological sciences. This research has attracted a great deal of attention because of its scope and the fact that it is revolutionizing the way we think about the movements, distributions, populations, and behaviors of whales. As an example of scale, Clark's laboratory is presently acquiring daily data from ocean deployed systems such that the total numbers of whales they detect and track (using passive acoustics) are greater on a quarterly basis than the summed total of all whale survey efforts over the past twenty years. An example of an intriguing insight comes from the work on blue whales which make extremely loud, low frequency (infrasonic) signals. These signals have an uncanny suite of features superbly designed not only for long range communication (Clark can detect a blue whale's sounds at a distance of 1000 nautical miles), but also for acoustic navigation and tomography. This is to say that the sounds, and patterns of sounds, made by blue whales are such that we, as scientists, can even use them as rudimentary acoustic signals to probe the physical structures of the ocean. In fact, at this point, humans can not produce, even with our most advanced machines, the kinds of sounds that a blue whale routinely makes. However, we have in the last 5 to 10 years, begun to learn how to make loud low frequency sounds for probing the ocean. A deep concern is that unless we are very careful, these human-made acoustic probes could prove to be a threat to the survival of species like the blue whale.
Global warming will eventually affect everyone, and everyone will eventually recognize that the rate at which the earth's average temperature is changing is of great relevance to their lives. The problem at present is that no one knows the rate at which global warming is proceeding. To end this ignorance it is important to devise a way to take the earth's temperature. The ATOC Experiment (Acoustic Thermometry of Ocean Climate) is perhaps the most notable attempt to do this. It takes advantage of the fact that sound travels faster through warm water than through cold water. Thus, by making a loud sound underwater on one side of an ocean and measuring accurately the time the sound takes to reach a hydrophone (underwater microphone) at a known distance on the other side, one can determine the average temperature of the water between source and receiver. When the average water temperature is hotter the trip will be shorter than when the water is colder. (It is the great length of the trip that averages out the different speeds of travel associated with areas of hotter and colder ocean water hence the need, if one is to take advantage of this integrating effect, to make a sound loud enough to cross an entire ocean.) Such measurements made repeatedly, a few times a day over many days make it possible to cancel out confounding variables and after several months or years to determine with great accuracy an average temperature for the entire ocean. The oceans are 71% of the earth's surface and contain almost all of its surface heat capacity, therefore knowing the average ocean temperature over several years enables us to know the rate of global warming. As global warming proceeds, the temperature of the oceans will slowly rise. With ATOC measurements we will finally know how bad the problem is and therefore what steps can be taken to avert disaster.
The results of such research are of fundamental importance to humanity. However, because the ATOC sound signal has to be loud enough to be heard across oceans, many people are concerned that these sounds may pose serious problems for whales. But most whale species make very loud sounds themselves and some can even be heard across ocean basins. Whales must have some way to deal with the fact that these very loud sounds are occurring so close to their ears. Which is to say that we do not know whether sounds like the ATOC signals really pose a serious threat to whales. The objections raised to the ATOC experiment have delayed its beginning. But because the results it promises are so important to humanity we need to know as soon as possible whether whales are significantly disturbed by ATOC sounds. Whenever possible during the Voyage, the Ocean Alliance will make its research vessel Odyssey available to Dr. Clark to assist in his research on this question. We anticipate that the collaboration between TOA and Clark will produce work of fundamental importance.
A more recently divulged threat involving loud, low-frequency sounds is SURTASS LFA (Surveillance Towed Array Sonar System - Low Frequency Active), a ship-based sonar system under development by the U.S. Navy for detection and tracking of submarines and for supporting fleet and anti-submarine warfare exercises. It will use an array of acoustic transmitters suspended, on average, 100 meters beneath a ship and will transmit at frequencies between 100 and 1,000 Hz (a band in which many whales produce sounds, and where therefore their hearing is thought to be sensitive). The transmitters may be turned on as much as ten percent of the time, during which they will broadcast at classified sound intensities (the National Resources Defense Council has estimated that the intensities may be as much as two hundred times the intensity of ATOC sounds). There is widespread concern that such sounds might seriously damage the hearing of whales located close to the source, or that it could seriously mask communications between whales located at safer distances.
Accumulating data and knowledge about natural ambient noises provides the background for understanding the biological significance and survival value of a rich variety of sounds produced by whales, especially larger whales communicating with low frequency (LF) sounds. The National Marine Fisheries Service consistently monitors, by shipboard and aerial surveys a band of North American coastal waters roughly 300 miles wide, running from southern Mexico to central California. From these surveys estimates for eastern Pacific blue whale populations are derived for some species in California. These are independently corroborated through photo identification work by John Calambokidis. However, neither of these studies takes into account the fact that throughout their sampling periods other blue whales are being detected acoustically in deep ocean between California and Hawaii. These animals are not included in the estimates. The same is true for humpbacks in the Caribbean. According to Christopher Clark this is the kind of underestimation going on throughout the worlds oceans. Acoustic censuses thus indicate that we are consistently undercounting whales to an unknown extent. Such uncertainty accounts for the biases and poor confidence intervals in our estimates of populations and consequently of their rates of recovery. We will combine acoustic survey and visual survey techniques to improve censusing techniques. We will also work in conjunction with Clark when he is monitoring the US Navy's SOSUS arrays. Positions of animals tracked on SOSUS will be relayed to the Odyssey, which will travel to the area and confirm species identification. Odyssey will also provide comparative data on species abundance in the locale indicated.
Marine mammal responses to low frequency (LF) sound
The uncertainties concerning potential impact of human-made LF sounds on whales are large, and under these conditions the procedure is to err on the side of caution. This has led to decisions being made based on lack of knowledge rather than from experience. Response thresholds and levels of impact are best determined through scientific research, not legal compromise, and any improvement in our understanding of how and under what conditions marine mammals respond to low frequency sounds will be extremely valuable.
The work we plan in conjunction with Voyage will largely involve a limited number of species that are known to produce and/or hear LF sounds. Certain species of whales are known to produce loud (as great at 190 dB re 1µPa) long, patterned sequences of low frequency sounds. Some, like the songs of male humpbacks definitely appear to be part of breeding activity, while others, like the infrasonic pulses of blue and fin whales, though most probably used for communication, could also function for navigation and as a form of low frequency active sensing. The predominate eastern Pacific blue whale signal consists of a pair of contrasting sounds. The first is a 10 second sequence of amplitude-modulated tones at 17-90 Hz, followed by 30 seconds of silence, followed by a 20-25 second constant frequency (CW) tone at 17.5 Hz that changes into a frequency-modulated (FM) chirp (called so because it resembles a bird 'chirp' when greatly speeded up). Such pairs of signals are repeated every two minutes. Signal bandwidth and variability appear to be associated with different bathymetries (shallow vs. deep). Under optimum ambient conditions, these signals are detectable at ranges out to 1000 miles. We have examples of whale FM chirps exciting the first 4-5 modes in a surface-ducted, shallow water environment. Such patterns of ocean-modified whale signals have even been used to perform rudimentary acoustic tomography. If humans can use whale signals as acoustic probes (and we are now implementing our own probes that have remarkably similar features to blue whale sounds) it seems likely that whales having had 30 million years of lead time, are using their own sounds to investigate the ocean.
As for hearing LF sounds, recent anatomical evidence indicates that, not surprisingly, the ears of LF whales are specialized for hearing LF sounds. In contrast, the ears of dolphins and pinnipeds (except for the elephant seal for which there is evidence that it has good LF hearing) are adapted for high frequency hearing, extending in the case of dolphins well into the ultrasonic range.
The greatest potential risk of acoustic impact from human generated LF noise such as ships traffic noise or the sounds associated with seismic profiling or even signals like those used in acoustic thermography (the ATOC experiment) can be confined to large whales. This potential risk is real, but there is extremely limited evidence by which to estimate the risk. Instead we are reduced to invoking the 120 dB rule of thumb, and inferring impact based on comparisons with levels that produce auditory impairment in terrestrial animals including humans. This is not a very responsible way to define risk and impact. What is needed is more real, empirical scientific data about what constitutes a sound having a deleterious impact on whales, so decisions based on reality can be made.
Additional relevant use of bioacoustic data
TOA houses the world's largest library of acoustic recordings for two whales species (humpback and right whales). It stems from Dr. Roger Payne and Katharine Payne's work on humpback whale vocalizations. We will extend this collection from the R/V Odyssey. Analysis will be carried out using the analysis program "Canary" (developed by Christopher Clark at Cornell University).
Throughout the course of the Voyage, TOA personnel will use directional hydrophones and acoustic arrays to record and monitor natural acoustic behaviors of cetaceans. Analysis of these recordings will provide deeper understanding of the natural variability in ambient noise from marine mammals and other sources on global, regional and site-specific scales. Data of this kind from the southern oceans will be especially valuable.
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