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Beaked Whale Dive Patterns Studied - 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]
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.
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
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