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Toothed Whale Hearing Senstivities Can Be Turned Up and Down
Supin, Nachtigall, Breese. Hearing sensitivity during target presence and absene while a whale echolocates. J. Acoust. Soc. Am. 123 (1), January 2008, 534-541
This very interesting paper, digging deeper into a topic that had emerged in previous studies of toothed whale hearing, reveals that a pilot whale could apparently "turn up" its hearing sensitivity, so that it could listen for fainter (more distant) sounds when echolocation clicks did not reveal nearby echoes. The research used a captive whale, outfitted with suction-mounted EEG electrodes to measure evoked potentials as it echolocated; half the time, a target fish was nearby, and half the time there was no nearby fish. When no fish was present, hearing sensitivity was 20dB higher (ie it could hear sounds 20dB more faint).
Deep Ocean Vents "Sing" With Tones
Crone TJ, Wilcock WS, Barclay AH, Parsons JD (2006) The Sound Generated by Mid-Ocean Ridge Black Smoker Hydrothermal Vents. PLoS ONE 1(1): e133 doi:10.1371/journal.pone.0000133 [READ PAPER(PLOS)]
Hydrothermal vents along deep ocean ridges, over 2000m below the surface, have been recorded for the first time. The vents increase the ambient noise level by 10-30dB, and include both broad-band noise and tones that are unique to each vent, perhaps indicating resonant frequencies of the cavities. The authors speculate that such sounds coudl be used by fish and other organisms living near the "black smokers" in order to avoid the scorching water, or to seek out prey living nearby. [UNIVERSITY PRESS RELEASE, WITH VIDEO AND AUDIO]
Noise Weakens Pair Bonding in Zebra Finches
Swaddle, John and Page, Laura. High levels of environmental noise erode pair preferences in zebra finches: implications for noise pollution. Animal Behavior, 2007, 74, 363-368.
Zebra finches are a model species often used in studies of pair-bonding in birds; in the wild, only 3% of offspring are illegitimate (sorry, got unscientific there for a second, I meant to say, are the result of "extrapair preference," or EPP); in noisy and dense captive conditions, these rates can rise to 11-30%. For this study, the researchers worked with captive zebra finches, who were of the first generation of a captive breeding program. They exposed females who had already bonded with a male (having already built a nest and raised a brood), to recorded white noise of various intensities, while giving the females access to two males. In a series of tests involving 20 pairs of birds, exposed in these conditions to three levels of white noise (45dB, 75dB, 90dB), there was little change from baseline in low noise conditions, moderate EPP in moderate noise conditions, and a high level of EPP, approaching equality with preference for original mates, in high noise conditions. The researchers propose two possible explanations: masking of the paired males' gentle bond-maintaining call, or inability to distinguish the pair-bonded male's calls from the other male under noise conditions.
Using Fixed Hydrophones to Listen for Whales: An Review of Current Techniques
Mellinger, D.K., K.M. Stafford, S.E. Moore, R.P. Dziak, and H. Matsumoto. 2007. An overview of fixed passive acoustic observation methods for cetaceans. Oceanography 20(4):36-45. [DOWNLOAD PAPER(pdf)]
This is a good overview of the state of technology and procedures used in using passive acoustics (listening to hydrophone recordings) in studying whale and dolphin populations. The focus here is population assessment, but the information is relevant to other uses of passive acoustics as well. The paper also features two very clear charts showing the frequency ranges used by several species of cetaceans, including both lower frequency calls and higher frequency echolocation clicks.
Related Paper: Review of 40 Years of Acoustic Playback Experiments
Deecke, V.B. Studying marine mammal cognition in the wild - a review of four decades of playback experiments. 2006. Aquatic Mammals 32(4):461-482 [DOWNLOAD PAPER(pdf)]
Since 1964, biologists have been using playback of sounds to assess behavioral responses in marine mammals. The purposes of these studies have ranged from learning more about the response of animals to sounds of their predators, investigating kin recognition or other communication questions, and determining responses to human noise. The author found 47 such studies: 17 prior to 1990, and 30 in the past ten years. This paper summarizes the historical trends and purposes of such research, and looks at procedural and design improvements that could be warranted.
Extended Near-surface Dives May Be Cause of Beaked Whale Bends
Walter, M.X. Simmer, Peter L. Tyack. Repetitive shallow dives pose decompression risk in deep-diving beaked whales. Marine Mammal Science, Volume 23 Issue 4 Page 888-925, October 2007.
ScienceNow summary of paper: [WEBSITE]
It is apparent that beaked whales are especially sensitive to mid-frequency sonar, but the reasons have remained elusive. This study explores a new idea about how the whales' dive patterns may be disrupted enough to cause decompression sickness (DCS, ie "the bends"). Some have suspected that exposure to sonar may cause the whales to surface more quickly than usual, but since the whales' lungs are collapsed in dives deeper than 70m (thus preventing nitrogen from entering the bloodstream and infiltrating into tissues where it could cause damage), it is not readily apparent how faster surfacing would cause DCS. Instead, this study looks at the "recovery" period observed in beaked whales, during which they make a series of near-surface dives before embarking on a new deep foraging dive. The researchers modeled the possible physiological effects of having this recovery period extended longer than usual. The team incorporated known physiological data into a model that charts how the bubble size might increase in the circulatory system, brain, muscles, and fat tissues when a whale dives repeatedly to between 30 and 80 meters for as long as 3 hours. The team's model predicts that if the whales' lungs do not collapse during a long series of shallow dives, the increased pressure can cause nitrogen bubbles to diffuse into tissues, increasing the risk of bubble formation on ascent. Such behavior may result if the whales perceive sonar transmissions as a predator: repeated dives travelling horizontally to escape the percieved predator could put the animals at risk. When fleeing an orca (their primary predator), such avoidance would like be short duration, and not dangerous; however, if the sonar transmissions continued to be audible for long periods of time, the whales might continue the avoidance dives to the point where they begin to sustain injury. This could also explain the relatively unusual appearance of beaked whales close enought to shore to end up beaching: they might be chased far from their normal habitat by continuing sonar sounds (ed. note: it also begs the question of whether many other animals are similarly injured, but happened to be fleeing in other directions). The team concludes that limiting the duration of sonar testing may prevent the animals from diving in these harmful patterns.
Related Paper: Analysis of Beaked Whale Dive Patterns With Consideration of Proposed Behavioral Risks in Response to Mid-Frequency Sonar
Tyack, Johnson, Soto, Sturlese, Madsen. Extreme diving if beaked whales. Journal of Experimental Biology 209, 4238-4253 (2006) [READ PAPER ONLINE]
This detailed report examines 44 dive sequences by 10 individual beaked whales. Deep foraging dives, during which the whales attempt to eat about 30 prey fish, last 45 minutes to an hour; between deep dives, they perform a series of dives closer to the surface. It appears that the surface dives become progressively shallower over the course of the hour to hour and a half spent between deep dives. The authors examine several proposed changes in dive behavior or possible physiological responses to observed dive patterns, and conclude that the evidence does not clearly support most of the proposals, noting that "aspects of the diving behavior of beaked whales remain enigmatic." They propose the theory explored in the physiological modeling paper above: that extending or altering the near-surface dive behavior could place the animals at risk for decompression sickness, triggering gas and fat embolic syndrome and stranding deaths.
Low Frequency Active Sonar Shows Less Impact on Fish than Airguns
Popper, Halvorsen, Kane, Miller, Smith, Song, Stein, Wysocki. The effects of high-intensity, low-frequency active sonar on rainbow trout. J. Acoust. Soc. Am. 122 1 , July 2007. p. 623-635.
This study extended a previous line of research that had measured physiological impacts of seismic survey air guns on fish kept confined in a cage and exposed to the noise. This time, the research team exposed trout (which share hearing mechanisms with salmon, which are of special concern due to their endangered status) to sounds produced by low-frequency active sonar. LFA sonar uses frequencies (100-500Hz) that many fish can detect, often the range of most sensitive hearing. Fish were tested for hearing sensitivity using Auditory Brainstem Response (ABR), and some were sacrificed to check for physiological damage, including swim bladder or ear hair damage. Results indicate that fish had reduced hearing sensitivity after exposure to LFA sonar, ranging from 17-25dB at particular frequencies (i.e., sounds needed to be that much louder in order to be heard), and that the effects lasted at least 48 hours (the longest followup the study included). However, the researchers note that there was quite a lot of variability in results, with some study groups showing little shift in hearing thresholds even with longer exposure to the LFA sounds, and some frequencies of hearing being little affected; also, the fish, being captive in a cage, were exposed to constant high sound levels that are unlikely in the wild, making this a probable worst-case scenario (the levels these fish were exposed to would occur only within 100m of the sonar transmission, which would be on a moving vessel). Unlike some earlier pile-driving and explosives studies, the fish exposed to LFA sounds did not show any acute tissue or organ damage. And, unlike earlier airgun studies, there was no apparent damage to ear hair cells; such injury would likely cause a permanent reduction in hearing sensitivity. The fish did respond to the onset of the sound with a rapid burst of swimming; this will be examined in another paper.
Boat Traffic Noise High Enough in Intracoastal Waterways to Raise Concerns About Hearing Damage and Making; Dolphins Seem to Avoid Weekends
Haviland-Howell, Frankel, Powel, Bocconcelli, Herman, Sayigh. Recreational boating traffic: A chronic source of anthropogenic noise in Wilmington, North Carolina Intracoastal Waterway. J. Acoust. Soc. Am. 122 (1), July 2007, p.151-160.
This study analyzed recordings of ocean noise during the summer season (June 21-September 1) in a 100m-wide intracoastal waterway off North Carolina. The mean received sound levels (measured as RMS re 1uPa, at frequencies averaged in steps over 0-37.5kHz) ranged from 109dB at 6am to 118dB in early afternoon and back down to 111dB by 10pm. Not surprisingly, weekends and holidays were 1-3dB louder than weekdays. The frequency spectrum shows that low frequencies (below 1kHz) dominate, with significant energy remaining up to 5kHz, and lesser but still perceptible noise up to the 37.5kHz limit of the study. Dolphin observations showed a clear reduction in numbers of dolphins observed on weekends and holidays, relative to weekdays; less than half as many dolphins were observed on weekends. Noise levels in the range of 5-25kHz, the primary range of dolphin social whistles, was of particular concern. The researchers note that "mean hourly RLs exceeded 116 dB nearly every day surveyed, indicating bottlenose dolphins in the ICW near Wilmington, North Carolina could be at risk for noise exposure on a daily basis. High mean RLs were often recorded over consecutive hours, making high sound levels the rule in this area during the summer, not the exception." Further, they note that since bottlenose dolphins feed mainly on soniferous fishes (fish that make noise), and that fish vocalizations are primarily below 1kHz, the range most dominated by boat sounds, dolphins may well find it more difficult to hear and find prey.
Current Marine Mammal Population Monitoring Effort Is Very Unlikely to Detect Even Precipitous Declines
Barbara L. Taylor, Melissa Martinez, Tim Gerrodette, Jay Barlow, Yvana N. Hrovat. Lessons from monitoring trends in abundance of marine mammals. Marine Mammal Science, 23(1): 157–175 (January 2007)
Marine mammals are notoriously hard to count, since they spend so much of their time underwater. Scientists, policy makers, and industrial and military planners all need to have accurate assessment of populations and population trends. However, many populations are not surveyed often enough or comprehensively enough to really know how many animals there are. Using statistical analysis, the researchers looked at a hypothetical situation in which populations were declining 50% over a 15 year period (the rate at which a stock can be legally classified as "depleted"). The percentage of precipitous declines that would not be detected as declines by current survey techniques and frequency was 72% for large whales, 90% for beaked whales, and 78% for dolphins/porpoises, 5% for pinnipeds on land, 100% for pinnipeds on ice, and 55% for polar bears/sea otters (based on a one-tailed t-test, ? = 0.05; this number measures the statistical significance of the results, or in lay terms, how confident we can be in the results--more on this key factor below). "Fundamentally, we cannot reliably detect even precipitous declines in most whale, dolphin, porpoise, and ice-hauling pinniped populations with present levels of investment in surveys and current survey technology and design," the researchers stressed. They recommended several alternatives to improve performance, including (to list a few) more diligent focus on indicator species, increase in survey frequency, and most fundamentally, a change in the "decision criteria." This last suggestion involves a shift in the balance of risk between over-protection and under-protection: the current standard of statistical significance of ? = 0.05 means that we will over-protect (perceive a decline where none actually exists) only 5% of the time; meanwhile, this standard, as suggested by the results above, leads us to MISS a decline up to 50-90% of the time. Softening our standard of significance to ? = 0.2 would detect a decline 80% of the time that one existed, while also over-protect 20% of the time (equal over and under). They also suggested possible changes to the math used in analyzing trends, development of better models, and designing surveys to more specifically detect trends than absolute abundance (though noted the drawback that, especially during declines, it is common for distributional shifts to occur. In many cases, for animals that range widely, repeated survey cannot be sure that they are observing the same animals/population; thus, researchers suggest that changing the decision criterion is a "more reliable" solution.
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