Visual observations of marine mammals made during large-scale surveys or associated with a specific research project can provide detailed information on the behavior, movement, and abundance of these animals in the wild. Scientists can compare baseline measurements obtained during “control” conditions to those obtained under experimental conditions, when sound is being transmitted. Changes in the animals’ behavior and/or abundance can be used to infer how the animals are affected by sound. Visual observations are most appropriate for cetacean and sirenian species, as these mammals must surface to breathe. Pinnipeds do come to the surface for air, but they are difficult to observe from a boat at sea.
Visual observation studies involve trained observers that watch for marine animals of interest. The observers can be shipboard, in airplanes, or on land. For shipboard studies each observer is responsible for surveying a particular area of water on each side of the observation vessel. Binoculars are typically used to extend the visual range of observers, and night vision goggles can be used during hours of darkness.
Once animals are spotted many things can occur. First, the number of animals present is estimated. Then, depending upon the goals of the project, animals can be photographed for photo-identification purposes, behavioral data can be recorded, and/or an individual animal can be followed for a period of time. When one animal is observed for a period of time, this is referred to as a focal follow. Whether one or many animals are being observed, typical behavioral observations include behavioral state (feeding, socializing, resting, etc.), presence or absence of calves/pups, respiration rate, dive duration, and spacing or movement of animals.
Visual observations can also be obtained from aerial surveys. During aerial surveys a small plane flies close to the water while trained observers count the number of animals that are spotted. Aerial surveys are useful in obtaining abundance estimates or observing shifts in distribution over a large area in a short amount of time. For example, aerial surveys were used to compare the number of humpback whales and their distribution within 40 km of the ATOC (Acoustic Thermometry of Ocean Climate) source in two years when the source was transmitting to a year when the source was off.
Often, aerial surveys are conducted in conjunction with shipboard studies. The aerial team can direct the surface ship to animals for detailed studies, reducing the time and money devoted to long shipboard searches.
- ATOC Transmission
- How is sound used to measure temperature in the ocean?
- Gordon, J., Thompson, D. and Tyack, P. 2003, “Proceedings of the workshop: The use of controlled exposure experiments to investigate the effects of noise on marine mammals: scientific, methodological, and practical considerations.” European Cetacean Society Newsletter No. 41- Special Issue Feb. 2003. 24pg.
- Mobley, J.R., Jr. 2004, “Results of 2004 Aerial Surveys of Humpback Whales North of Kauai.” Annual Report Submitted to: North Pacific Acoustic Laboratory (NPAL) Program Scripps Institution of Oceanography. (Link)
- Mobley, J. R., Jr. 2005, “Assessing responses of humpback whales to NPAL transmissions: Results of 2001-2003 aerial surveys north of Kauai.” Journal of the Acoustical Society of America 117: 1666-1673.