ACS Research Committee Report

Acoustic dead zones and spatial aggregation of stranded cetaceans
Source: Sundaram, B., Poje, A.C., Veit, R.R., and Nganguia, H. 2006. Acoustical dead zones and the spatial aggregation of whale strandings. Journal of Theoretical Biology 238: 764-770.

Mass strandings of cetaceans almost always involve toothed whale species. In North America, the majority of these strandings are made up of sperm whales, long-finned pilot whales, short-finned pilot whales, false killer whales, and white-sided and white-beaked dolphins.

In some single stranding events, the cetacean is clearly sick or injured; however, in all mass and some single strandings, the animals appear relatively healthy. There are many hypotheses to explain strandings of apparently healthy animals. They include disorientation of animals that use magnetic cues for navigation due to anomalies in the earth’s magnetic field; failure of the whales’ echolocation due to features of the land, such as sandy substrates or gently sloping topography; and behavioral tendencies to follow a leader; among others.

One interesting note is that there is a strong geographical clustering of stranding sites; only two hypotheses can explain this clustering effect: geomagnetic anomalies and echolocation distortion.

Several of the species, such as sperm whales and pilot whales, that mass strand are typically found in deep water. Strandings involving these species must involve two steps. First, the animals must move closer to shore, perhaps to forage, and then they must somehow become disoriented.

This study modeled acoustic propagation under different environmental conditions to determine the locations of acoustic "dead zones," where echolocation cues may overestimate the distance to shore.

Four sites, the Massachusetts coast and three sites in New Zealand, were chosen for this analysis because of the large numbers of documented strandings and the varied shape of the coastlines. These dead zones were mapped out and historic whale stranding sites were overlaid on the maps. In three out of the four study sites, there was a significant correlation between observed and predicted stranding locations.

There were many locations that were predicted to attract strandings where no strandings were reported; however, a high proportion of these sites lack the gently sloping beaches that are thought to be a prerequisite for strandings. In Massachusetts, there were also several sites where strandings were predicted and not reported, but these areas have rocky substrates, which generally do not attract strandings.

The model can be improved in the future by including the bottom topography and substrate type. Although it appears that several factors are likely to be included in stranding locations, acoustic distortion, or “dead zones,” do seem to play a role in cetacean strandings. 

The influence of sociality on population structure in bottlenose dolphins
Source: Lusseau, D., Wilson, B., Hammond, P.S., Grellier, K. Durban, J.W., Parsons, K.M., Barton, T.R., and Thompson, P.M. 2006. Quantifying the influence of sociality on population structure in bottlenose dolphins. Journal of Animal Ecology 75: 14-24.

Social animals organize themselves in units that can be different depending on factors such as the presence of preferred associates, prey availability, habitat complexity, and others. These units may have varying foraging success depending on environmental conditions and may affect the dynamics of the population, such as genetic structure, spread of disease, and information transfer.

This study used network and association analyses, as well as estimates of association rates to define the social structure in bottlenose dolphins in the Moray Firth, Scotland.

Bottlenose dolphins along the Scotland coast appeared to be separated into two social units, which had limited interactions. The two units appeared to be related to the ranging patterns of the individuals, and could be described as inner and outer Moray Firth social communities.

In spite of the lack of interaction between units, their home ranges largely overlapped, which may be related to the abundance of prey. There may be enough prey in this area to support the two social units without much competition. Alternatively, the two units may have foraging specializations, as have been described in other bottlenose dolphin populations, which allow the animals to avoid direct competition.

There was some exchange of individuals between the two social units, which may lead to genetic or information transfer between units. Both social units contained calves, meaning that there were females in both groups, but the sex of many individuals was unknown.

The association rate of these dolphins was relatively low, which means that although there were some associations that lasted for years, the overall structure was dominated by casual acquaintances lasting only short periods. 

Dive depths and feeding behavior of bottlenose dolphins
Source: Hastie, G.D., Wilson, B., and Thompson, P.M. 2006. Diving deep in a foraging hotspot: acoustic insights into bottlenose dolphin dive depths and feeding behavior. Marine Biology 148: 1181-1188.

In aquatic ecosystems, prey is distributed in three dimensional space and marine mammals have evolved diving abilities to exploit subsurface resources. Diving, however, has an energetic cost that must be offset by the benefit of prey that is available at depth.

In this study, bottlenose dolphins were studied in the Moray Firth, Scotland, where they have a strong preference for foraging and feeding in deep, narrow coastal channels. Feeding could only be directly witnessed when it occurred at or near the surface.

In 93% of feeding observations, the dolphins produced low frequency calls, termed "brays." Stranded dolphins in this region have been found to have benthic (bottom-dwelling) prey in their stomachs, indicating that they can feed at great depths.

This study used a vertical hydrophone array to record bottlenose dolphin echolocation clicks in order to estimate dive depth and feeding activity.

The results indicated that echolocating bottlenose dolphins consistently dove close to the sea floor. Feeding, "bray," calls were predominantly made at 20 to 30 m depth, but echolocation sounds recorded just prior to these feeding calls were in deeper water. The highest number of clicks were recorded within the top 10 m of the water and decreased to a maximum depth of 58.5 m. These results indicate that dolphins used the entire water column, and although they fed at depth, the majority of their time was spent at the surface. 

Wind turbine underwater noise and marine mammals
Source: Madsen, P.T., Wahlberg, M., Tougaard, J., Lucke, K., and Tyack, P. 2006. Wind turbine underwater noise and marine mammals: implications of current knowledge and data needs. Marine Ecology Progress Series 309: 279-295.

In recent years, there has been increased focus on the exploitation of renewable resources and alternative sources of energy. There are many offshore wind farms that are being planned for northern Europe and North America. Offshore wind farms have so far been constructed in shallow waters (less than 20 m), but there is a potential to place them in 20 to 100 m in the future. There are concerns about the impact of these wind farms on the marine environment; one of these potential effects is the production of low frequency underwater noise during construction and operation.

This review focused on four species of marine mammals: harbor porpoise, harbor seals, bottlenose dolphins, and right whales, which are all commonly found in shallow water.

Baleen whales, such as the right whale, produce powerful low frequency (10 Hz to 10 kHz), long duration sounds. Toothed whales, such as the harbor porpoise and bottlenose dolphin, produce short, high frequency sounds for echolocation and navigation, as well as frequency modulated whistles for communication; toothed whale sounds range in frequency from 1 to 150 kHz. Seals vocalize in the frequency range from 50 Hz to 60 kHz.

The effects of the construction and operation of offshore wind farms could potentially vary significantly for different marine mammals, based on the frequency range of their communication. In addition, simple sound propagation models will not be completely accurate to assess potential impacts, because they do not account for the complexities of shallow-water habitats.

Construction of wind farms may include activities such as pile driving, trenching, and dredging, all of which will produce underwater noise of varying intensities and frequencies.

Pile driving is considered to be the most potentially detrimental activity associated with the construction, because it produces a very high source level and broad bandwidth sound. Few studies have looked at the effect of pile driving on marine mammals. A study on the effects of pile driving on ringed seals in Alaska did not find dramatic behavioral reactions at received levels of at least 150 decibels. However, pile driving in the western Baltic found a significant, though temporary, effect on the haul-out behavior of harbor seals.

The response to pile driving on toothed whales has been documented during the construction of two wind farms in Denmark. In one, the abundance of echolocating harbor porpoise was found to significantly decrease during pile driving activities. There was a return to normal click rates within a few days of the completion of the pile driving. In the second case, similar results were observed, although the return to normal click rate was much faster.

There are no published results of the effect of pile driving on baleen whales, although there have been studies of the effect of underwater air gun sounds on bowhead whales. In this study, the bowheads demonstrated an avoidance response at received levels of 122 decibels at ranges of 3.5 to 7.5 km.

No studies have been conducted to measure the response of marine mammals to noise from the operation of offshore wind farms, but there was one study that used simulated wind turbine noise playbacks to both harbor seals and harbor porpoise. Surfacings of porpoise and seals were fewer than expected in the range of 0 to 60 m from the sound source for porpoises and up to 200 m for seals.

The highest intensity recorded for pile driving was 200 decibels at a range of 100 m, with a maximum duration of 10 ms and a mean frequency of 380 Hz. Because of the short duration of the sound, it is unlikely that it will "mask" sounds that marine mammals are producing or receiving. Although the potential behavioral impact on marine mammals is still poorly understood, the noise from construction, especially pile driving, is likely to have a much more significant impact on the animals than noise from the operation of the wind farms.

The authors recommend that operational guidelines for wind farms include maximum noise levels that are less than 110 decibels at 100 m. Mitigation during pile driving is also recommended. Techniques may include acoustic deterrence devices to ensure that marine mammals are far enough away from the source sounds to reduce the risk of permanent hearing damage or measures, such as bubble curtains, to reduce the sound level in the water. 

   
(larger copies of these photos are available in the members-only photo galleries)

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