Date of Award

Winter 2021

Project Type

Dissertation

Program or Major

Earth Sciences

Degree Name

Doctor of Philosophy

First Advisor

Jennifer L Miksis-Olds

Second Advisor

Larry Mayer

Third Advisor

Kim Lowell

Abstract

Sound can propagate great distances underwater and is an important mode for marine life to obtain information. Human activities in the ocean such as global shipping, coastal construction, gas and oil exploration, and mapping navigation routes intentionally and unintentionally emit sound into the ocean, potentially interacting with marine life. Therefore, it is essential that the effects of anthropogenic noise on marine life and the ambient marine acoustic environment be understood. Most of the work, to date, has focused on the impact of low-frequency (<1 kHz) sources such as shipping noise, which is ubiquitous in the ocean, and mid-frequency (1-10 kHz) sources such as naval sonar, to which many marine mammals have shown to be sensitive. The effect of these sources can be as salient as a mass stranding event or as benign as an animal swimming away from a source of noise with no other effect. Less work has focused on higher frequency sources (>10 kHz), including ocean-mapping sonar systems. However, most marine mammals, namely toothed whales (odontocetes), are capable of hearing mapping-sonar signals. The exposure of marine mammals to anthropogenic sound sources in the open ocean is regulated by the National Marine Fisheries Services through the Marine Mammal Protection Act (MMC 2015), the Endangered Species Act (DoI 2003), and the National Environmental Policy Act. Without a better understanding of the interaction of mapping sonar with marine mammals, the current guidelines imposed for marine mammal protection may not be protective enough, or alternatively, may be too conservative. To gain a better understanding of the potential effect of mapping sonar and marine mammals, a scenario was examined that is possible to occur and has a high potential for a biologically meaningful interaction between mapping sonar and a sensitive marine mammal species: a 12-kHz multibeam echosounder (MBES) mapping survey and beaked whale foraging. This represents a possible interaction since 1) the relatively low frequency of the 12 kHz MBES propagates further in the ocean environment than other mapping sonar frequencies (>30 kHz), and 2) beaked whales commonly reside in the deep-water environments where mapping with such a system would occur. Due to 1) the overlap of the frequency of this mapping system with beaked whales hearing, and 2) the life-sustaining nature of the behavior under consideration, this interaction has the potential to be biologically meaningful. To understand the effect of deep-water multibeam mapping activity on beaked whale foraging, the temporal and spatial foraging behavior of beaked whales was assessed during two three-day ocean mapping surveys over the Southern California Antisubmarine Warfare Range hydrophone array (SOAR, featuring 89 bottom-mounted receivers over a 1800 km2 area) utilizing a 12-kHz deep-water multibeam echosounder. Echolocation clicks recorded on the hydrophone receivers from foraging Cuvier’s beaked whales were used as a proxy to assess their foraging behavior. In addition, a soundscape analysis was conducted using the acoustic data from the hydrophone array to provide context for the behavior study findings, as well as provide a more general perspective on the contribution of the deep-water mapping activity to the marine acoustic environment. In the first phase of this work, passive acoustic monitoring data was used to identify foraging events of beaked whales. Four characteristics of the foraging events were used as proxies for foraging behavior and were subsequently compared Before, During, and After two deep-water ocean mapping surveys. These included 1) the number of foraging events (Group Vocal Periods, or GVPs), 2) the number of clicks per GVP, 3) GVP duration and 4) click rate per GVP. The findings of this effort revealed that only the number of GVPs increased during the deep-water mapping surveys, largely driven by the observations in just one of the survey years. This temporal analysis showed no impact on beaked whale foraging except for an increase in foraging effort during mapping activity. In addition, this finding was a stark contrast to foraging behavior of beaked whales during MFAS activity, during which the number of foraging events decreased. In the second phase of this work, an approach --the Global-Local-Comparison Approach (GLC)--was developed and tested that uses existing disparate spatial statistics and statistical hypothesis testing to assess whether a change in spatial behavior has occurred. Using three-prongs of assessment—global, local, and comparison—the approach provided knowledge about 1) the general distribution of observations over the entire area of study (i.e., clustered, random, dispersed), 2) identification of local hot and cold spots of activity, and 3) order-of-magnitude differences across distinct analysis periods, respectively. The approach was demonstrated on synthetic data and empirical case studies of marine mammal behavior to determine its effectiveness and limitations in assessing change in spatial observations across analysis periods. The results revealed that the approach was effective at identifying visually identifiable spatial changes, with robust statistical support. The GLC Approach was then used to assess spatial change in beaked whale foraging behavior before, during, and after ocean mapping activity using the spatial data from the foraging events used in the first phase of work. The analysis revealed that for one of the years of study there was no obvious change in foraging behavior globally, locally, or in magnitude in response to the mapping activity, whereas a local change in beaked whale foraging effort was identified during the second mapping survey year. There were obvious differences in the spatial use of the array by foraging animals between the two years outside of the survey work, which in addition to the differences in results between the two years of study, provided little support that the local change identified was necessarily a response to the mapping activity. The final phase of research was to characterize the contribution of one of the two ocean mapping surveys to the marine soundscape utilizing the acoustic data from the SOAR array, with a particular emphasis on understanding the contribution of the 12 kHz deep-water MBES. A comprehensive, multi-analysis approach focused on amplitude and frequency features of the changing soundscape across a nine-hydrophone subset of the array and across four analysis periods with respect to the survey activity: No Activity, Vessel Only, Vessel and MBES, and Mixed Acoustics was conducted. The analyses revealed that the contribution of the deep-water MBES to the acoustic environment was very stereotyped: contributing most substantially to the loudest sound levels in the soundscape, particularly in the 12.5 kHz decidecade band. These results aligned well with the physical characteristics of the system, i.e., nominal frequency, duty-cycle, transmission geometry, etc., suggesting these parameters can be reliably used to identify this source in subsequent soundscape studies. The assessment revealed that the MBES was the most consistent loud source throughout the survey period, but was intermittently present. There were other loud acoustic sources detected throughout the survey period, most frequently other vessels and biological activity. Several of the metrics used were weighted based on the hearing sensitivity of a mid-frequency cetacean, chosen specifically to provide context for what a Cuvier’s beaked whale may have heard if in the area where the survey was conducted. The most important finding related to this aspect of the work was that the survey activity, particularly the MBES sound, did not contribute uniformly in space, time, or frequency to the SOAR soundscape of the mapping survey: it had a very local and transient effect. In summary, at the resolution of the SOAR hydrophone array, this empirical work assessing beaked whale foraging during deep-water MBES mapping activity demonstrated: 1) no adverse changes in Cuvier’s beaked whale foraging behavior, and 2) no clear response to the deep-water MBES mapping activity. Deep-water MBES mapping activity contributed substantially to the change in sound levels at a finite scale around the survey vessel. This led to a temporally intermittent impact on the soundscape at a given location. Within these spatio-temporal bounds, deep-water MBES mapping activity has the potential to be detected by a Cuvier’s beaked whale due to its spectral overlap with the frequencies of best hearing sensitivity of this species, as well as its loudness. However, no adverse effects on Cuvier’s beaked whale foraging were observed here.

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