- Advances in mobile autonomous vehicles for oceanographic sensing provide new opportunities for passive acoustic monitoring of marine mammals. Acoustically equipped mobile autonomous platforms, including gliders, deep-water profiling floats, and drifting surface buoys can survey for a variety of marine mammal species over intermediate spatiotemporal scales. Additionally, such mobile platforms may provide an effective tool for population density estimation of marine mammals. This dissertation advances our understanding of how gliders, deep-water floats, and surface drifters can be used for passive acoustic monitoring and density estimation of two cetacean species, fin whales (Balaenoptera physalus), and Cuvier’s beaked whales (Ziphius cavirostris).
One glider and two drifting deep-water floats were simultaneously deployed in the vicinity of a deep-water cabled hydrophone array offshore of San Clemente Island, California, USA. The glider was able to follow a pre-defined track while float movement was somewhat unpredictable. Fin whale 20 Hz pulses were recorded by all recorders throughout the two-week deployment and presence at hourly and daily scales were comparable across all recorders. Performance of an automated template detector did not differ by recorder type. However, the glider data contained up to 78% fewer fin whale detections per hour compared to the floats or stationary hydrophones because of increased low-frequency flow noise present during glider descents. Flow noise was related to glider speed through water and dive state. Glider speeds through water of 25 cm/s or less are suggested to minimize flow noise.
The cabled hydrophone array was also used to estimate fin whale localizations and tracks concurrently with the glider survey. These tracks were used in a trial-based approach to estimate a detection function for six-minute snapshots containing fin whale 20 Hz pulses. Detection probability was strongly dependent on 40 Hz noise levels (flow noise) recorded on the glider. At the median noise level of 97 kHz dB re 1 μPa2/Hz, maximum detection range was nearly 40 km and the estimated effective survey was 870 km2. Density of fin whales was estimated as 2.4 whales per 1000 km2 (coefficient of variation, CV 0.55) using a group size estimate from the tracked whales and an externally derived vocal rate from tagged fin whales. The framework presented here could be applied to other baleen whale species to advance the use of autonomous gliders for density estimation of cetacean species.
A second two-week glider and float deployment was conducted concurrently with the deployment of a commonly used deep-water stationary recorder, the High-frequency Acoustic Recording Package (HARP) and an array of drifting near-surface recorders in the Catalina Basin, California, USA. Acoustic recordings were analyzed for the presence of multiple marine mammal species, including beaked whales, delphinids, and minke whales and were compared across the glider, float, and HARPs. Detections of beaked whale echolocation clicks were variable across recorders, likely due to differences in the recording limits of each system, the spatial distribution of the recorders, and the short detection radius of such a high-frequency, directional signal type. Delphinid whistles and clicks were prevalent across all recorders, and at levels that may have masked beaked whale vocalizations. Minke whale (Balaenoptera acutorostrata) boing sounds were detected almost identically across all recorder types, as was expected given the relatively long detection range of the boing call type.
Spatially explicit capture-recapture was used to estimate density of Cuvier’s beaked whales from the near-surface drifting array of acoustic recorders. A snapshot approach was used with presence or absence of echolocation clicks within a 1-minute snapshot acting as the sampling unit. Using external estimates of group size and echolocation probability in a 1-minute snapshot, the density of Cuvier’s beaked whales, from the two best models was estimated at 5.48 animals per 1000 km2 (CV 0.46). This estimate was similar to estimates calculated using trial-based and distance sampling approaches applied to the same data set. Simulation experiments were conducted to investigate potential bias in estimated density caused by the configuration of the drifting array. Bias from the array configuration was found to be negligible, increased array spacing (approximately doubling and tripling between-sensor spacing) decreased bias, and the drifting aspect of the recorders also decreased bias, compared to simulations with stationary sensors.
This work provides evidence that animal presence and absence at broad spatial scales such as hours and days are comparable across gliders, deep-water floats, and stationary recorders. The spatial advantage of mobile instruments is most pronounced for species with short acoustic detection ranges, such as beaked whales. Marine mammal density can be estimated from gliders and mobile drifters using either a trial-based or SECR approach examples presented here provide an exciting advance in marine mammal population monitoring.