Summary of goals
The aim of this project was to develop and implement methods for estimating cetacean density from fixed passive acoustic sensors. Obtaining spatially and temporally referenced estimates of absolute density for cetacean species is fundamental to understanding their basic biology, and also to monitoring and mitigating the effect of man-made impacts on their populations. Passive acoustics is increasingly being recognized as a useful and practical modality for gathering information about cetaceans, and there is a growing body of research on the use of towed acoustic arrays to estimate density, either alone or in conjunction with more traditional visual surveys. Fixed hydrophones have enormous potential for two reasons. First, they can provide extended temporal coverage for a much lower cost than a towed array. Applications include ocean observatories, monitoring sensitive areas, etc. Second, there already exist extensive bottom-mounted fixed arrays that can potentially be used to monitor cetaceans. However, fixed hydrophones present additional challenges relative to their towed counterparts. Issues include animal movement, lack of random placement of detectors and inability under some circumstances to localize the animal vocalizations. In this research, we addressed these and other issues by extending the most widely used methodology to estimate density in visual surveys, distance sampling, to deal with a wide variety of fixed array configurations.
Summary of approach
Our approach was to proceed via a series of linked case studies, starting with situations where we believed it was feasible to produce robust estimates of absolute density, and proceeding incrementally to situations where our inferences would be less reliable. Our case study data came from two US Navy instrumented testing ranges: the Pacific Missile Range Facility (PMRF, located off the east coast of Kaua’i, Hawai’i) and the Atlantic Undersea test and Evaluation Center (AUTEC, located in the Bahamas). The dense arrays of fixed hydrophones on these ranges provide a unique resource for developing and evaluating our methods, under “best case” conditions, and also for sub-sampling to evaluate the performance of methods under conditions closer to those that could be achieved in other places. We leveraged on the the very significant efforts that had already been expended in both acoustic data collection on these ranges, and also in the development of methods for data processing and analysis. Our case studies were: (1) humpback whales at PMRF (Note: this case study was halted due to technicalities); (2) sperm whales at AUTEC; (3) beaked whales at AUTEC; and (4) sperm whales using single hydrophones at AUTEC. In addition we have developed computer simulation capabilities for testing our methods. By having developed a solid, statistically-based foundation for the use of fixed passive acoustic data to estimate cetacean density, we hope this research will have a substantial and lasting impact in ocean science.