Belugas Vocalize Less When Stressed?
Castellote, M., and F. Fossa. 2006. Measuring acoustic activity as a method to evaluate welfare in captive beluga whales (Delphinapterus leucas). Aquatic Mammals 32(3):325-333.
This innovative study was designed to see whether vocalization patterns might be a useful measure of "animal welfare" in belugas. The authors state that study of vocal behavior has proven useful in monitoring stress and general welfare in some terrestrial captive species; they here measured the acoustic activity of captive belugas before and after transportation to new facilities, and before and after the introduction of four harbour seals to their new facility. After transportation, the underwater vocalization rate dropped dramatically, remained very low during the next 4 wks, and did not reach the same level as before the transport until the 5th wk. Similarly, the vocalization rate decreased just after the introduction of the harbour seals, and it remained low for 2 wks. The observed decrease in the acoustic activity of beluga whales in both situations and the persistence of this change through time suggests that the acoustic behaviour in this species is very sensitive to environmental stressors. Ed. note: The authors emphasize the value of learning more about these effects, for use in oceanaria, though it seems likely that the results may apply to some degree in the wild as well.
Age-related Hearing Loss Measured in Captive Naval Dolphins
Houser, Dorian, and Finneran, James. Variation in the hearing sensitivity of a dolphin population determined through the use of evoked potential audiomentry. J. Acoust. Soc. Am., Vol. 120, No. 6, December 2006, pp.4090-4099.
This study used Auditory Evoked Potentials to measure hearing sensitivity in 42 captive bottlenose dolphins, ranging in age from 4 to 47 years. Researchers found patterns similar to those found in human: progressive loss of high-frequency hearing with age, generally beginning between ages 20 and 30; all animals over 27 had some degree of hearing loss. Two individuals were found to have profound hearing loss; aberrant hearing patterns were found in some of their relatives, indicating a possible genetic link to some hearing loss. The researchers note that these dolphins, part of the US Navy Marine Mammal Program, are housed in San Diego Bay, where mean ambient noise spectral density levels range from 67-87dB re 1uPa2/Hz. They suggest that future studies such as this also note ambient noise levels in the environment so that "potential relationships between chronic noise exposure and variation in hearing sensitivity might be determined." They also note that some of the dolphins in the study, including ones with notable hearing loss, had been treated with antibiotics, which are known to cause hearing loss in humans and dolphins. Ed. note: as mentioned in conjunction with similar findings regarding antibiotics, the suitability of using captive dolphins with antibiotic exposures in studies aimed at estimating hearing sensitivity of wild dolphins needs to be considered.
Sperm Whale Seismic Study Finds Long-range Acoustic "Hot Spots" That Complicate Assumptions Regarding Whales Moving Away to Find Safe Levels of Sound
DeRuiter, Tyack, Lin, Newhall, Lynch, Miller: Modeling acoustic propagation of airgun array pulses recorded on tagged sperm whales (Physeter macrocephalus). J. Acoust. Soc. Am., Vol. 120, No. 6, December 2006, pp.4100-4114.
Several studies are emerging from the 2002 and 2003 seasons of the multi-year Sperm Whale Seismic Study in the Gulf of Mexico, which uses temporary acoustic receivers placed on sperm whales to evaluate the effects of airgun sounds on their diving and foraging patterns. This study looks at sound propagation, and finds that simple models usually used to establish "safety zones" are often inadequate. As the researchers state, "Geometric spreading approximations, which have traditionally been used to determine the extent of marine animal exposure zones, are inadequate to describe transmission loss in our study environments." Sound recordings were made by tags placed on whales 8-13km from airguns, and in midst of dives that took them from the surface to roughly 500m deep (waters were of medium depth, 400-800m). They found that sound reflections from the surface and sea floor created areas of increased sound intensity at ranges of 4-8km. In an attempt to create a mathematical model to match the results obtained in the field, they used the ray-tracing program RAY, which uses complex calculations that take into account reflections of sound from sea floor and sea surface, and refraction due to sound speed variations within the water. They further developed their own "two dimensional broadband range-dependent acoustic propagation program based on Fourier synthesis", which they describe in some detail. "These results illustrate that in many cases...a simple spherical or cylindrical spreading law will not accurately predict the observed pattern of received levels. Regulation based on inappropriate application of geometric spreading law to calculate the extent of exposure zones could result in exposing animals to higher-than-intended noise levels." Key Point: The researchers also point out that some mitigation measures, such as ramp up (and even the assumption that whales will move away from the source to avoid high sound exposure), assume that moving away from the vessel will protect animals from harm; their results show, however, that they may encounter increased exposure levels as they swim away. Indeed, in some situations, animals at some distance from the airguns may actually find lower sound levels if they approach the source, where they will again find higher sound levels (potentially exacerbating any increased stress caused by the initial avoidance attempts), and perhaps increase cumulative Sound Exposure Level. This study also measured (and their model accounts for) significant higher frequency content, 500-2500Hz, in some environmental conditions (when surface duct is present). This indicates that in some situations, airgun operators should consider effects on animals with higher frequency hearing sensitivity (e.g. dolphins and other toothed whales)
Received Levels Can be as High at 12km as at 2km
Quantitative measures of air-gun pulses recorded on sperm whales (Physeter macrocephalus) using acoustic tags during controlled exposure experiments. J. Acoust. Soc. Am., Vol. 120 (4), October 2006, pp.2366-2379
This paper, also emerging from the SWSS, attempted to determine the absolute sound levels experienced by the whales. Researchers found that a single airgun pulse is multiplied into several sound pulses by reflections at various distances off the sea floor and water's surface. Received levels of analyzed pulses fell between 131 167 dB re1uPa (peak-to-peak), alternatively calculated as 111 147 dB re1uPa (rms) or 100 135 dB re1uPa2/s. Researchers state that "The relative strength of pulses arriving on different paths vary with range and depth of the diving whales, but the absolute received levels can be as high at 12km as they are at 2km." None of the whales tagged made deep dives closer than 4km from the array, where sound levels could be expected to be especially high, and the team encouraged investigations at closer range in the future.
Ship Noise Stresses Freshwater Fish
Wysocki, L.E. et. al. Ship noise and corisol secretion in European freshwater fishes. Biological Conservation 128:501-508. 2006
A striking study, confirming what many have suspected: exposure to noise significantly increases the secretion of stress hormones in fish. If this result is replicated for other species, it could dramatically shift the debate about ocean noise, because stress has been widely shown to make animals dramatically more susceptible to disease, toxins, and other health threats. The study at hand involved three freshwater fish (carp, gudgeon, perch) exposed to recordings of ships on the Danube and two Austrian lakes. The researchers measured the level of the cortisol in the water before and after noise exposure. Cortisol is the primary stress-related hormone in fish and many other animals. After thirty minutes of exposure to the ship noise, secretions of cortisol roughly doubled, ranging from 80-120% depending on the species of fish. Interestingly, there was no increase in cortisol when the fish were exposed to a steady white noise as intense as the loudest boat sounds, suggesting that the variability of the ship noise is more important than the intensity.
Travelling Dolphins Stop and Mill Together When Powerboats Pass By
Lemon, Lynch, Cato, Harcourt. Response of travelling bottlenose dolphins (Tursiops aduncus) to experimental approaches by a powerboat in Jervis Bay, New South Wales, Australia. Biological Conservation 127 (2006) 363-372.
This study documented the changes in dolphin surface behavior and whistle rates in response to the presence of a 5.6m powerboat with 90hp engine at 100m. They found that in 75% of cases, groups of travelling dolphins shifted to a "milling" behavior, and in an equal proportion of encounters, re-oriented their direction of travel to move away from the boats. A majority of the groups that shifted to milling returned to their previous direction of travel after the boat left the area. In contrast, there was no significant change in whistle rates or click duration during exposure to the boat noise. This contrasts with some previous studies that suggest changes in acoustic behavior in the presence of boats (often longer periods of vocalizing), perhaps because this study focused on dolphins that were travelling, rather than foraging (in general, dolphins are less vocal when travelling). The dolphins in this bay are exposed to regular boat traffic from commercial, military, and private boats; as researchers noted, "As demonstrated in this study, a single anthropogenic event may cause a short-term disruption in dolphin behavior, and it is possible that an accumulation of these effects may lead ultimately to long-term changes. However, long-term cumulative effects of vessel noise remain to be determined."
Dolphin Calls Can Have Effective Range of up to 20km if Background Noise is Low
Quintana-Rizzo, Mann, and Wells. Estimated communication range of social sounds used by bottlenose dolphins (Trusiops truncatus). J. Acoust. Soc. Am., Vol. 120 (3), September 2006, 1671-1683.
This study evaluated the "active space" of dolphin calls in a variety of habitats; in other words, they measured how far dolphin whistles could travel (and thus, the range within which pods and mother/calf pairs can communicate). In shallow-water sea grass habitat, relatively low pitched 7-13kHz whistles (source level 165db) could be heard for almost 500m. In shallow water mud-bottomed areas, the same whistle could be heard for up to 2km. And in channels, higher pitched 13-19kHz whistles could be detectable up to a distance of 20km. (Side note: in a classic dry statement of astounding scientific insight, the authors confirmed that "Our findings indicate that the communication range of social sounds likely exceeds the mean separation distances between females and their calves.") Perhaps the most interesting aspect of the study emerges in the discussion, where the authors note that these results are a best-case scenario, as no boats were present within 1km of the recordings. Even with these ideal conditions, the detection range was limited by more distant ambient noise, rather than by the hearing sensitivity of dolphins. Wave action, snapping shrimp, and boats can all add to ambient noise and reduce communication range and thus "active space" within which dolphins can move apart from each other. In Sarasota Bay, boats pass within 100m of any given dolphin on average of once every 6 minutes during daylight hours. Ed. note: This last piece of information has startling implications, given the findings reported above, regarding the behavioral changes caused by boats within 100m
Pile-Driving Noise Can Mask Dolphin Calls Over Several Kilometers
J.A. David. Likely sensitivity of bottlenose dolphins to pile-driving noise. Water and Environment Journal 20 (2006) 48-54
This paper is a literature survey of research that has looked at the sound profile and propagation of noise from pile-driving in water. Pile-driving is used to construct sturdy anchors for piers, bridges, and, increasingly, wind turbines. Pile-driving noise is loudest in frequency ranges that coincide with dolphin whistles, and so are likely to mask whistles at longer ranges, up to 10-15km for sounds at 9kHz. The 50kHz component of dolphin whistles can be masked at ranges of about 6km. At higher frequencies (115kHz), pile-driver noise dissipates more quickly and is loud enough to mask higher-frequency dolphin clicks only at ranges of 1.2km or less. The impact of such potential masking is likely reduced by dolphins' directional hearing, the intermittent nature of pile driving noise, and behavioral modifications in calling patterns by the dolphins (changing the frequency and amplitude of their calls, as well as the content of the signals). The author suggests several mitigation measures: avoiding pile-driving during calving season, ramped warning signals, and the use of bubble-curtains to reduce pile-driving impulsive noise.
Elephants Respond to Seismic Stimuli
C. E. O’Connell-Rodwell, J. D. Wood, T. C. Rodwell, S. Puria, S. R. Partan, R. Keefe, D. Shriver, B. T. Arnason and L. A. Hart. Wild elephant (Loxodonta africana) breeding herds respond to artificially transmitted seismic stimuli. Behavioral Ecology and Sociobiology, 59:6, 842-850. April 2006.
Seismic communication in elephants takes place using infrasonic (low-frequency) sound transmitted through the ground. This study played seismic components of elephant alarm calls, and measured the behavioral reaction of elephants at a water hole. The researchers made sure that there were no airborne component of the sounds that might cue the animals. Elephants responded to the alarm calls as might be expected, by grouping closer together and by spending less time at the water hole. The overall response rates were less than when alarm calls are received as airborne sound; researchers suspect that the seismic-only delivery of the sounds indicated that the threat was relatively distant. The study was not designed to see if elephants could distinguish meaning from the seismic signal (i.e., no non-alarm seismic calls were used), so there is also the chance that the lesser reaction was related to not being sure what the call was communicating (i.e. they responded to the existence of a seismic elephant call, but not specifically defensively). The mechanism by which elephants perceive seismic calls is not yet known, though researchers suspect bone conduction via the toes; field studies have shown that elephants can detect seismic at distances of up to 2km, and models suggest possible perception at up to 16km in favorable soil conditions.
Increased Shipping, Larger Supertankers Linked to Rising Background Noise off California
McDonald, Hildebrand, Wiggins. Increases in deep ocean ambient noise in the Northeast Pacific west of San Nicolas Island, California. J. Acoust. Soc. Am., Vol. 120 (2), August 2006, 711-718.
Confirming and extending the results of a 2002 study off the central California coast, this study examined ambient noise levels at about 1000m depth off southern California, and compared current noise levels to measurements made in 1964-6. As with the 2002 study, an increase of 10-12dB was found in low frequencies (30-50Hz), which suggests that sound levels are more than doubling each decade (2.5-3dB/decade). Above 50Hz, the difference in noise levels diminished to only 1-3dB at 100-300Hz, and above that, noise levels actually decreased, due to a daily cycle of higher frequency sound that is now missing (perhaps due to less marine life or wave noise). The increase in low-frequency sound is attributed to shipping vessel traffic; while the number of ships has not increased enough to account for the change, the introduction of supertankers (which are louder) is considered the likely primary factor.
Beaked whale studies reveal new details about behavior and response to noise
Johnson, Tyack. Measuring the behavior and response to sound of beaked whales using recording tags. NOPP CoreOcean Report, 2005. [READ PAPER(pdf)
This overview report from two Woods Hole researchers gives a good picture of the scope and goals of ongoing research projects aimed at better understanding why beaked whales seem to be especially sensitive to mid-frequency active sonar, and perhaps other ocean noise. Using acoustic tags, which remain on the whales for a few hours, the researchers can track their dive patterns, vocalization, and the noises that they hear. Among the key results are new clarity about the depth and duration of dives (the longest duration of any cetacean, with somewhat slower ascents than descents, consistent with concerns that noise may cause too-rapid ascents and cause decompression injuries). A series of short descents after surfacing are not understood. On one occasion, a tagged Cuvier's beaked whale appeared to interrupt a foraging dive when a ship passed by and significantly increased the ambient noise. Also of interest is the close coordination among groups of whales, who dive together and all vocalize while deep underwater; the researchers raise the concern that this coordination could be disrupted by masking of their vocalizations by increased ambient noise; a paper on this is in the works. Finally, the calls made by beaked whales are altogether new, unlike other whale calls; an acoustic detection system in the works, based on the new recordings, to help detect these whales via passive acoustic monitoring. Beaked whales seem to vocalize consistently (steady clicks and occasional buzzes during prey approach) after descending below 500m, and during the depth of their foraging dive; they hardly vocalize at all when ascending or resting at the surface; relative proportion of time spent vocalizing on dives appears to be under half, perhaps as little as a third of the time. The total cycle of deep dive and near-surface activity lasts close to three hours; thus, acoustic detection would need to take place for 2 to 3 hours before potentially harmful noise-making activities commenced.
Beaked whales near-surface dives may trigger decompression sickness?
Ongoing study of dive patterns of beaked whales being carried out by a team at Woods Hole suggests that the possibility of decompression issues is more complex than first suspected. Earlier analysis of the dive patterns showed that the whales ascend more slowly than they descend, which led to some speculation that exposure to mid-frequency sonar might cause them to rise too quickly, causing injuries similar to "the bends." The new paper suggests that it is likely that the whales' lungs are fully collapsed at depths below 100m, so that the rapid ascent should not cause problems below that depth. Lead researcher Peter Tyack said, "We think that beaked whales return to the surface after deep dives with an oxygen debt and need to recover before their next deep dive." Tyack said the team's analysis suggests that the normal deep diving behavior of beaked whales does not pose a decompression risk. "Rather, it appears that their greatest risk of decompression sickness would stem from an atypical behavioral response involving repeated dives at depths between 30 and 80 meters (roughly 100 to 250 feet), " Tyack said. "The reason for this is that once the lungs have collapsed under pressure, gas does not diffuse from the lungs into the blood. Lung collapse is thought to occur shallower than 100 meters (330 feet), so deeper parts of the dive do not increase the risk of decompression problems. However, if beaked whales responded to sonars with repeated dives to near 50 meters (165 feet), this could pose a risk." Source: WHOI Press Release, 10/19/06 [READ PRESS RELEASE]
Beaked whale dive patterns tracked; slow ascent is noted
Baird, Webster, McSweeney, Ligon, Schorr, Barlow. Diving behaviour of Cuvier's (Ziphius cavirostris) and Blainville's (Mesoplodon densirostiris) beaked whales in Hawai'i. Can. J. Zool. 84: 1120-1128 (2006) [DOWNLOAD PAPER(pdf)] [WEB PAGE WITH PHOTOS]
Baird, Schoor, Webster, McSweeney, Mahaffy. Studies of beaked whale diving behavior and odontocete stock structure in Hawai'i.' in March/April 2006. Report prepared for Southwest Fisheries Center, National Marine Fisheries Service, Sept. 2006. [DOWNLOAD PAPER(pdf)]
Using depth and time tracking temporary tags, researchers conducted two separate field studies of the dive patterns of beaked whales. The first study compiled 31 hours of data from 4 individuals, while the second obtained 135 hours of dive data from three individuals. Cuvier's beaked whales were found in deeper waters (typically about 2000m) than Blainville's beaked whales (around 1000m), but both showed similar overall dive patterns, including dives to depths averaging 1100m (and ranging from 800-1500m), surfacing more slowly than they descend, and spending relatively long periods of time near the surface (top 50m) between deep foraging dives. Deep dives took place about every two and a half hours. The whales tended to move up and down some during the extended time they spent at the bottom of the dives, so descent and ascent rates were measured from 85% of maximum depth; time taken for ascent in the second study was twice that of descent, a larger differential than measured in the previous shorter study (when the ascent took 38% longer). Such reduced ascent rates have also been reported for northern bottlenose whales (also members of the Ziphius family) and deep-diving beluga whales. There was no significant difference in deep dive patterns between day and night, though mid-water dives (to 100-600m) took place five times more often during the day, with a corresponding tendency to spend more time in near-surface waters (under 100m) during the night; these differences could be useful in designing operational and mitigation measures to avoid exposure to high-intensity underwater sound, and could affect the detection rates using passive acoustic monitoring. One opportunity to tag two whales in a single group suggest that they stay close together during the first 600m of a dive, then move apart to forage separately. Many of the individuals sighted during the study had been seen in the area before, suggesting a high degree of site fidelity; the overall number of sightings suggests an overall density of beaked whales that is about a third of that of the Bahamas populations. In their conclusion, the authors note a recent paper by Cox et al. (see below) that outlines several behavioral responses by beaked whales that could result in strandings such as those seen in response to active sonar, continue, "we suggest that the frequent extremely long dives push the animals' physiological limits, resulting in such behavioural mechanisms (slow ascent rates and prolonged periods of time at the surface to purge excess dissolved nitrogen from their tissues) to compensate. Indirect physical harm from surfacing excessively fast, or premature dives, seem most plausible as mechanisms for beaked whale mass strandings in relation to high-intensity sonar." (They also note the relatively small number of individuals tagged so far, and the need for a wider range of age and sex classes in future studies.)
Possible Mechanisms to Explain Beaked Whale Sensitivity to Anthropogenic Sound
T.M. Cox et al. Understanding the impacts of anthropogenic sound on beaked whales. J. Cetacean Res. Manage. 7(3):177-187, 2006. [DOWNLOAD PAPER(pdf)]
This comprehensive review paper is the result of a special workshop on beaked whales convened in 2004 by the Marine Mammal Commission, and is coauthored by an all-star team of 36 cetacean researchers. It begins by offering a good summary of a series of mass strandings stretching from Greece in 1996 to the Gulf of California in 2002. It then reviews some recent papers that have identified shared characteristics in some of the strandings, including near-shore canyons, acoustic waveguides, and certain transmission pattern similarities. The heart of the paper is an extended discussion of several possible mechanisms by which active sonar might lead to strandings. These range from simply "chasing" whales into shallow water to a variety of ways in which the whales' behavior might be altered in ways that could affect nitrogen bubble formation in their tissues. The latter include the fairly well-known concern that whales may surface too quickly and cause tissue damage much like "the bends" in human divers (some deep-diving whales seem to ascend more slowly than they descend, unlike most cetaceans, suggesting this is possible), as well as some lesser-considered variations: staying too long at depth during dives (perhaps delaying surfacing due to sound?), or having their rest time at the surface--or the poorly-understood series of shallow dives often seen between deep dives--interrupted (so that tissues may retain more nitrogen at the start of the dive than is healthy). They also summarize other physiological factors that could contribute to observed hemorrhaging, including an sensitivity to stress and disorientation caused by a vestibular response. Finally, they summarize current research into the possibility of direct tissue damage caused by exposure to intense sound. Recommendations from the panel focus on the obvious need for more research of dive patterns, beaked whale anatomy and hearing sensitivity, as well as a call for a detailed retrospective review of stranding records and for new controlled-exposure experiments to get a better sense of what received levels can trigger potentially dangerous behavioral changes.
Beaked whale hearing range measured
Cook, Varela, Goldstein, McCulloch, Bossart, Finneran, Houser, Mann. Beaked whale auditory evoked potential hearing measurements. Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology. 192:5, 489-495. May 2006.
A stranded and malnourished beaked whale was the subject for this study which took place three hours prior to its death. Researchers measured the auditory evoked potentials of its hearing (a standard technique using brain wave measurements to chart response to sound in a range of frequencies). The whale was most sensitive to high frequency signals between 40 and 80 kHz (and likely higher; 80kHz was the limit of this study), but showed smaller responses down to 5kHz, the lowest frequency tested. Mid-frequency active sonar systems operate just above and below this 5kHz range, and the concentration of beaked whales among stranding events associated with these sonars has raised questions about whether beaked whales are especially sensitive to this frequency range. The lowest Sound Pressure Level to trigger an evoked potential response at 5kHz was 132dB re 1uPa; generally, behavioral responses occur at lower levels than those measured via evoked potential.
Note: While this study did not address sonar signals, these results seem consistent with events in which strandings occurred when whales were at relatively large distances from sonar operations (e.g. Bahamas 2000). Still, the beaked whale sensitivity is similar to or less than that of bottlenose dolphins, suggesting that a distinctive auditory sensitivity is not likely to be the primary cause of beaked whales' responses to mid-frequency sonar. Other possibilities include beaked whales having a more dramatic behavioral response to sounds at the low end of their sensitivity threshold, e.g. rapidly surfacing from deep dives; or, a physiological sensitivity involving tissue lesion formation in situations where other species are not affected.
Sperm Whale Dive Patterns Suggest Long-range Echolocation
Watwood, Miller, Johnson, Madsen, Tyack. Deep-diving foraging behaviour of sperm whales (Physeter macrocephalus). Journal of Animal Ecology 2006. 75, 814-825.
This study used acoustic tags to track and listen to sperm whales during dives. The researchers compared dive and vocalization patterns in three distinct populations of sperm whales in different parts of the world: the Atlantic Ocean, Gulf of Mexico, and Ligurian Sea. During their descent, whales spent an average of 64% of the time emitting echolocation clicks, suggesting they are devoting their descent to the search for prey. As they approach prey, the emit a buzzing sound, a more detailed echolocation signal. They found that dive and click/buzz patterns are consistent throughout the world's temperate and sub-tropical oceans (this study did not include high latitude populations). One interesting conclusion they reached was that sperm whale evolutionary success (relatively unchanged for 10 million years) is largely based on the ability to successfully echolocate during their descent, locating patches of prey from a significant distance (300-500m). Most dives last 40-50 minutes and range from 400 to 1200 meters at their deepest point (most commonly bottoming out at 600-900m).
Note: The researchers made no comments about whether ambient noise might hinder such long-range echolocation, but it may deserve consideration and study. It may be that at depths, there is little propagation of noise that would interfere with the relatively high frequencies of echolocation clicks; higher frequencies do not generally travel much beyond a few hundred meters before becoming inaudible.
Acoustic Deterrence Alarm Triggers Variable Response Among Dolphins, Depending on Species
Kastelein, Jennings, Verboom, deHaan, Schooneman. Differences in the response of a striped dolphin (Stenella coeruleoalba) and a harbour porpoise (Phocoena phocoena) to an acoustic alarm. Marine Environmental Research 61 (2006) 363-378.
This study, using captive cetaceans in a floating pen, found that two different species of dolphins responded entirely differently to a common acoustic deterrence signal used to keep dolphins away from fishing nets (to avoid entanglement). The harbour porpoise reacted strongly, swimming away to the farthest reaches of the pool and increasing his respiration rate, while the striped dolphin showed no measurable reaction to the alarm. Since the two species have similar audiograms (i.e., they hear the frequencies used by the alarm similarly), this indicates that cetacean species are not equally sensitive or responsive to human-made noise. Thus, the levels of alarms should be adapted to the species they are meant to deter, and tested on the target species. One caveat is that striped dolphins have rarely been kept in captivity, so the researchers cannot be sure that the individual tested is typical. They also note that in general, striped dolphins are open-water species that often approach power boats to bow-ride, while harbour porpoises are solitary near-shore species with more concern about predation.
Human Noise Favors Frogs That Take Advantage of Lulls in Calls of Other Species
Sun, Jennifer and Narins, Peter. Anthropogenic sounds differentially affect amphibian call rate. Biological Conservation 121 (200) 419-427.
This study, apparently the first in twenty years to address the effects of human noise on amphibian call rates, suggests that human noise may increase reproductive success of some frog species, while decreasing it in its neighbors, thus perhaps changing the overall species mix in a given noisy location. The study took place in central Thailand, where researchers measured the call rates of several species of frogs, before, during, and after two noise intrusions: airplane overflights and playback of recorded motorcycle noise. Three of the most acoustically active species decreased their call rates during noise intrusions, but one increased its call rate. The species that increased its call rate, R. taipehensis, was one that uses natural lulls in local frog choruses to make itself heard to its species-mates. Thus, this study suggests that the intrusion of human noise may increase the number of lulls in the chorus of mixed species, and so offer R. taipehensis an increased number of opportunities to successfully advertise to potential mates. The implication is that human noise may differentially favor some species (such as R. taipehensis, which uses chorus lulls to be heard), while putting other species (such as ones that reduce or stop calling during noise intrusions) at a reproductive disadvantage (because, as the researchers note, "it is well established that individual reproductive success is directly proportional to calling effort in numerous frog species."
First Animal Communication: Insect Buzzes?
Hoch, Deckert, Wessel. Vibrational signalling in a Gondwanan relict insect (Hemiptera: Coleorrhyncha: Peloridiidae). Biol. Lett. (2006) 2, 222-224.
This unusual study proposes that the "first biologically meaningful sounds and vibrations ever emitted and perceived....were probably produced by anthropods making use of the mechanical properties of their exoskeleton." As an example, the researchers studied the sound-making mechanisms in an archaic, still-living insect that lives in wet moss and leaf litter, and that evidence suggests was in existence before the breakup of Gondwanaland, more than 230 million years ago. The study looked at one of the most archaic of the living species, and found, via recordings, that they do use vibrational signaling. The type of sound implies a yet-undiscovered tympanic structure; added to previous discoveries, this suggests that 230 million years ago, the three major sound-making mechanisms in insects had already evolved: percussion, stridulation, and tymbal vibration. As the authors note: "Long before the evolution of birds and mammals, the acoustic environment of a Gondwanan moss forest in the Permian must have been mainly shaped by the sounds and vibrations of insects."
Right Whale Calls Changing in Industrialized Habitats
Parks, Clark, Tyack. Acoustic communication in the north atlantic right whale (Eubaleana glacialis) and potential impacts of noise. Presented at Ocean Science 2006 conference, January 2006.
Noting that the north atlantic right whale vocal repertoire is becoming better understood (sounds are used for making long-range contact, formation of social groupings, and reproductive advertisement), and that the whales live in a highly industrialized habitat, this study investigated changes in the whales' calls over time that may be in response to rising noise levels. On short time scales (minutes), both the fundamental and peak frequency of calls increase in the presence of elevated noise levels. On longer time scales (decades), the minimum and maximum frequency of a key whale call, the "upcall", have increased between the late 1950s and 2004; this increasing frequency has been gradually noted over decades. The North Atlantic Right whale upcalls are at a significantly higher frequency than the southern right whales' calls, which may be a result of differing ambient noise conditions in their environment. These results are significant, as they present evidence for a long-term, chronic behavioral change in the North Atlantic right whale calling behavior that may be a result of increased levels of anthropogenic noise.
Discomfort Zone Suggested as Alternative to TTS
Two recent studies undertaken as part of an environmental assessment of a new underwater communications system have introduced the concept of a "Discomfort Zone" as an alternative to estimates of Temporary Threshold Shifts (TTS), for scientists and regulators assessing the biological impacts of exposure to human sounds. TTS is a temporary hearing loss, generally meaning that quiet sounds are more difficult to hear. To prevent grounding of ships and collisions between ships in shallow coastal waters, an underwater data collection and communication network is currently under development: Acoustic Communication network for Monitoring of underwater Environment in coastal areas (ACME). Marine mammals might be affected by ACME sounds since they use sounds of similar frequencies (around 12 kHz) for communication, orientation, and prey location. If marine mammals tend to avoid the vicinity of the transmitters, they may be kept away from ecologically important areas by ACME sounds. A team of researchers from the Netherlands and the UK suggest that this new behavioral criterion is a more appropriate threshold of minimal allowable disturbance than the physiological threshold of TTS, because "if animals are deterred by sounds from ecologically important areas to less favourable areas, this might effect the population size." Such a shift of emphasis is especially relevant for chronic noise sources, such as the proposed communication network, or even shipping noise, which could be presumed to cause long-term or recurring avoidance of the resulting discomfort zones.
Harbour Seal Acoustic Discomfort Zone
Kastelein, van der Heul, Verboom, Triesscheijn, Jennings. The influence of underwater data transmission sounds on the displacement behaviour of captive harbour seals (Phoca vitulina).Marine Environmental Research 61 (2006) 1939.
This study, using captive animals, was designed to determine the received sound levels at which harbour seals showed avoidance behavior, or what the researchers termed the "acoustic discomfort threshold." For all four test sounds, seals avoided zones where the sounds were louder than 107dB (SPL). These results are in an "ecologically neutral" area; that is, the seals could respond to simple discomfort; it is possible that in the wild, seals may tolerate higher levels, in order to get food, escape predators, or stay with a pup. The researchers suggest that "source levels can be selected that have an acceptable effect on harbour seals in particular areas" and recommend that "the source level of the communication system should be adapted to each area (taking into account the width of a sea arm, the local sound propagation, and the importance of an area to the affected species). The discomfort zone should not coincide with ecologically important areas (for instance resting, breeding, suckling, and feeding areas), or routes between these areas. In practice, their models suggest that the radius of the discomfort zone would range from 20m (if the source level of the ACME system is 130dB re 1Pa at 1m), to 200m (source 150dB), 2km (source 170dB), or 6.3km (source 180dB).
Harbour Porpoise Acoustic Discomfort Zone
Kastelein, Verboom, Muijsers, Jennings, van der Heul. The influence of acoustic transmissions for underwater data transmission on the behaviour of harbour porpoises (Phocoena phocoena) in a floating pen. Marine Environmental Research 59 (2005) 287307.
This study measured the change in location in a pool of two harbor porpoises exposed to three different ACME test sounds. Both dolphins moved away from the sound, though one moved farther than the other. Within seconds after the test sounds stopped, they resumed normal behavior, generally moving quickly back to the end of the pen where the sound source had been. While the animals clearly moved away from the test sound, there was a significant difference in the amount of displacement caused by the various test sounds (mean discomfort zone ranged from 23m to 18m). This study, while suggestive, is limited by the fact that only two dolphins were tested, as well as by to other key factors: source levels were relatively low, so that the discomfort zone would be small enough to be measured in a relatively small pool, and animals were not exposed to test sounds for long periods, which might have revealed the potential for habituation. A key finding was that the porpoises were much more sensitive to some of the proposed ACME sounds than to others. Low bit-rate, chirp-like sounds, with clear on-off switching, were more disturbing than high bit-rate, more random sounds.
Aerial survey confirms acoustic monitoring results
Tiemann, C., Martin, S. and Mobley, Jr., J.R. (2006). Aerial and acoustic marine mammal detection and localization on Navy ranges. IEEE Journal of Oceanic Engineering, 31(1):107-119. [READ PAPER(pdf)]
Studies in a Naval Missile Range off Kauai have compared the whale detections obtained with an acoustic monitoring system to visual observations made from aircraft at the same time. Humpback whales and sperm whales were both present at the time, and the relative numbers of whales identified by acoustic monitoring closely matched the numbers observed from the air. A new algorithm was used to determine the direction and distance to the whales, and again, the technique closely matched the time and places where whales were observed on the surface from planes. Obviously, the acoustic system is hearing whales while they are submerged and vocalizing, while the aerial detections took place when whales surfaced to breath and rest. Still, the results suggest that a high proportion of whales were detected by the acoustic system, as the visual monitors did not see significantly more whales.