Abstract
The coastal underwater environment is a complex habitat with high levels of noise, both natural and anthropogenic, turbid waters, and many underwater geographic features; however, marine mammals successfully navigate and locate food. Some do so without the use of active biosonar and the use of vision is also limited. It is clear, therefore, that these animals must rely heavily on ambient noise cues for orientation and object detection.This thesis investigates the natural (non-anthropogenic) ambient soundscape in the underwater coastal environment with particular focus on its potential use as an orientational reference and navigational aid to grey whales, Eschrichtius robustus. These whales are closely associated with shallow waters, migrating between Baja California, Mexico and Alaska without the apparent use of echolocation. Field work was conducted along the central coast of British Columbia, Canada where the ambient noise field was examined at various spatial scales and on the temporal scale with respect to its importance to foraging grey whales.
Results show that there is a difference in the frequency range and temporal process between the surf signatures in bays with varying substrate, shore slope, and exposure to wind and swell. On a shallow-sloping sandy shore, the surf had a frequency range of 0 - 10 kHz with a distinct centre frequency at 1.1 kHz. The wave-breaking events were between 5 and 7 seconds in duration and the surf could be observed at a maximum distance of approximately 300 - 400 m from shore. In contrast, on a steeper-sloping mixed-sediment shore, the surf was more broadband (1.2 – 20 kHz) and did not have a discernible centre frequency. The wave breaking events were also of shorter duration (3 seconds) and could be observed at distances greater than 600 m. On a relatively steep solid rocky shore, the surf was more narrowband (0 – 4 kHz) with a centre frequency at approximately 860 Hz. The wave-breaking events were also of short duration (3 – 4 seconds) and could only be observed at distances of around 60 m from shore. Being able to identify the sediment type from a distance would be beneficial for grey whales as they feed on a variety of prey species which are associated with their respective habitats (e.g. sheltered soft sediment bays, shallow offshore waters, surf zone and kelp beds on rocky substrate). Ambient noise maps of two feeding bays reveal that the main sources under relatively calm weather conditions and in the absence of shipping noise appear to be surf on shore and biological noise, and their contribution varies depending on the characteristics of the bay. Kelp beds are of particular interest to grey whales as their principal prey in this study area is typically associated with them. Experimental measurements and simulation results show that kelp beds create both acoustic bright spots (increase of 3 dB at 3 kHz) and shadows (attenuation of 0.12 dB/m with spectral filtering at 1.1, 2.3, 5.3, and 7.7 kHz) in the ambient soundscape. Thus it is plausible for greys to detect kelp beds through variations in the ambient sound field. Remote acoustic recordings over nearly a 24-hour period at a location in two feeding bays reveal that no discernible diurnal pattern due to biological activity was present in the ambient noise spectrum, although variation occurred due to changes in the weather conditions.
Due to the relationship between grey whales and coastal waters, these animals are highly susceptible to human disturbances. With poor understanding of the importance of the natural ambient soundscape to marine organisms, the increasing amount of anthropogenic noise in the oceans could have adverse effects. This work shows that acoustic cues are available in the ambient soundscape which could aid grey whales and other marine animals to navigate and locate their food; however, these variations between areas along the coast could easily be masked by industrial and vessel traffic noise. Similar acoustic experiments are scheduled to continue in the upcoming 2010 summer which will enhance our knowledge of the coastal underwater soundscape.
| Date of Award | 9 Mar 2011 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | W M Megill (Supervisor) & Philippe Blondel (Supervisor) |