Environmental assessments are made to determine the potential impacts to marine mammals from Naval activities. The problem that the Navy confronts in preparing these assessments is a basic lack of scientific understanding of marine mammal population dynamics and behavioral ecology. To date, marine mammal assessment has relied on visual surveys from surface vessels and from the air. The difficulty and expense of these marine mammal visual surveys and the low numbers of sightings that they generate has impeded their usefulness for Navy environmental assessment. Acoustic techniques have the potential to provide an efficient and accurate method for assessing marine mammal populations in areas of naval interest such as within the Southern California Off-Shore Range (SCORE), a region where naval operations are frequently conducted and where marine mammals are seasonally abundant.  

The objective of this project was to develop methods for acoustic monitoring of marine mammals. Researchers compared four methods for estimating marine mammal seasonality and relative abundance—aerial surveys (visual), ship-based transect surveys (visual), sonobuoy-based mobile acoustic surveys, and continuous fixed-site acoustic surveys. Simultaneous application of these techniques allowed for comparisons to determine the combination of methods most suitable for long-term marine mammal monitoring in the SCORE range.

Southern California Bight bathymetry map shown with 200 m contours; the five stars depict the acoustic recording packages (ARPs) locations on Cortez and Tanner Banks

Aerial survey flights were conducted quarterly from 2001-2003 to assess whale abundance during each season. These surveys covered a series of lines ranging in length from 25 to 200 km, with a total survey area of approximately 12,600 km². Ship-based surveys consisting of both acoustic and visual techniques were conducted at SCORE six times each year. Ship-based visual surveys consisted of teams of observers working daylight hours to individually record sightings and group sizes. Biopsy, photo-identification, and detailed behavioral information were collected at select times during these surveys. Data from sonobuoys were used to estimate marine mammal calls, differentiate calls from individuals, and localize individual members. Fixed acoustic recording systems provided a continuous year-round survey for marine mammal presence. The project examined data from these systems and deployed seafloor broadband hydrophone recorders within the SCORE region where coverage was not available from existing hydrophones.

For the Navy to comply with legislation protecting marine mammal habitat, data on marine mammal locations and seasonal densities within areas of frequent naval operations must be collected continuously. Acoustic methods were shown in this project to have great potential for improving marine mammal population assessments, as they produce significant numbers of detections and can be conducted with lower cost and less seasonal bias than visual methods. Such techniques are a first step in understanding the impact of sound on marine mammal behavior. SCORE personnel are now using these outcomes to operate a real-time system for marine mammal detection and classification, manage a database of seasonal marine mammal presence within SCORE, and run a predictive model for marine mammal presence under varying environmental conditions. (Project Completion - 2005)