Browsing by Subject "Passive acoustic monitoring"

Beaked whales (family Ziphiidae) and sperm whales (Physeter macrocephalus) are apex marine predators found throughout the world’s deep oceans. These species are challenging to observe, and little is known about fundamental aspects of their ecology, including their spatiotemporal distributions and habitat use. Passive acoustic monitoring (PAM), can be used to detect their echolocation clicks during foraging dives, thereby providing an indication of species presence. My dissertation investigates the distribution, seasonal occurrence, and diel variability in acoustic detections of beaked whales and sperm whales in the western North Atlantic Ocean, using multi-year passive acoustic recordings collected along the continental slope between Florida and Nova Scotia. First, I describe spatiotemporal patterns in detections of beaked whale echolocation clicks from five beaked whale species and one signal type of unknown origin. At least two beaked whale click types were detected at each recording site, and detections occurred year-round, with site-specific variation in relative species occurrence. Notably, Cuvier’s beaked whales (Ziphius cavirostris) were regularly detected in a region where they have not been commonly observed, and potential habitat partitioning among Cuvier’s and Gervais’ (Mesoplodon europaeus) beaked whales was apparent within their overlapping ranges. To examine the potential effects of using duty-cycled recording schedules on the detection of beaked whale clicks, I performed a subsampling experiment, and found that short, frequent listening periods were most effective for assessing daily presence of beaked whales. Furthermore, subsampling at low duty cycles led to consistently greater underestimation of Mesoplodon species than either Cuvier’s beaked whales or northern bottlenose whales (Hyperoodon ampullatus), leading to a potential bias in estimation of relative species occurrence. Next, I examine the occurrence of sperm whale echolocation clicks, which were recorded commonly between southern New England and North Carolina, but infrequently off the coast of Florida. In the northern half of the study region, I observed distinct seasonal patterns in the daily prevalence of sperm whale clicks, with a winter peak in occurrence off Cape Hatteras, North Carolina, followed by an increase later in the spring at sites further north, suggesting a shift in sperm whale concentrations which may relate to enhanced productivity occurring at higher latitudes in the spring. Finally, I explore the variability in daily detection rates of beaked whales and sperm whales in relation to dynamic oceanographic conditions off the Mid-Atlantic coast. Detection rates did not appear to correlate with temporal environmental variability, and persistent habitat features may be more important in predicting the occurrence of these species. Together, my dissertation provides substantial baseline information on the spatiotemporal occurrence of beaked and sperm whales in the western North Atlantic Ocean, highlighting the diversity within this guild of deep-diving odontocetes and demonstrating the use of PAM to provide species-specific insight into their ecology.

Passive acoustic monitoring is being used more frequently to examine the occurrence, distribution, and habitat use of cetaceans. Long-term recordings from passive acoustic recorders allow the examination of diel, seasonal, and inter-annual variation in the occurrence of vocalizing marine mammals. With the increased use of passive acoustics as a tool for studying marine mammals, the ability to classify calls to the species level is becoming more important. While studies have found distinctive vocalizations in some cetacean species, further work is required in order to differentiate the vocalizations of delphinid species. I sought to classify odontocete vocalizations to species and to describe temporal variation and depth-related differences in the occurrence of cetacean vocal events detected in archival passive acoustic recordings in Onslow Bay, North Carolina. To determine if odontocete species in offshore waters of North Carolina could be distinguished by their whistles and clicks, I used a towed hydrophone array to make acoustic recordings of species encountered during concurrent visual and acoustic surveys between 2007 and 2010. I recorded whistles from four species (Atlantic spotted dolphins, bottlenose dolphins, rough-toothed dolphins, and short-finned pilot whales) and clicks from five species (Risso's dolphins in addition to the four species listed above). After running a classification and regression tree (CART) analysis on 22 measured variables from the contours of four species' whistles, I generated an optimal classification tree that had a correct classification rate of 74.2%. My results indicate that species-specificity exists in the four species' whistles. My examination of the spectral structure of clicks showed that only Risso's dolphins produced clicks with distinctive spectral banding patterns, although I found that other click parameters, particularly peak and center frequency, might be useful in differentiating the other species. I then used the distinctive banding pattern that I observed in Risso's dolphin clicks to identify this species in recordings made by archival passive acoustic recorders that were deployed at various times and locations between 2007 and 2010. I used these recordings to determine how vocal events varied temporally and spatially in Onslow Bay. My analysis of vocal events observed in these recordings showed that Risso's dolphins, sperm whales, and other delphinids are present in Onslow Bay throughout the year; Kogia spp. occur sporadically; and fin and minke whales produce calls that can be detected only between late fall and early spring. I also detected low-frequency downsweeps and two types of low-frequency pulse trains produced by unknown species. After looking at the occurrence of fin whale 20-Hz pulses in relation to downsweeps, I suggest that the downsweeps are produced by sei whales due to the lack of overlap in occurrence. When I looked for diel patterns in the odontocete vocal events, I found a nocturnal increase in the occurrence of clicks from Risso's dolphins and sperm whales, but no diel variation in Kogia clicks. I also found that unidentified delphinids showed either an increase in click events at dawn or at night, depending on the time of year and recording location. Finally, my analysis of acoustic data from five recorders deployed in three different depth ranges revealed that there was greater unidentified delphinid and sperm whale vocal activity on recorders located in deep waters, suggesting a greater diversity and density of animals in deeper waters of Onslow Bay. Together, the results of my dissertation demonstrate the value of passive acoustic monitoring in understanding the distribution and temporal trends in cetacean occurrence, and highlight the importance of classifying sounds to the species level in order to better understand the temporal and spatial patterns found.