A variety of experiments have been done and observations have been made to determine how sound may affect marine mammals. Research suggests that increased background noise and specific sound sources might impact marine animals in several ways. The potential impacts include sounds that cause marine animals to alter their behavior, prevent marine animals from hearing important sounds (masking), cause hearing loss (temporary or permanent), or damage tissue. In at least a few well-documented cases there is a relationship between the use of mid-frequency sonar and the stranding of cetaceans, particularly beaked whales.

Behavioral responses to sound vary greatly. In order to understand how anthropogenic sounds may impact marine life, the animal's reaction to known sounds must first be measured. Observations of normal behavior, "control" or "baseline" data, provide the reference points for measuring any changes occurring during or after sound exposure. It is important to obtain baseline data that describe both the typical value of the measurements and the range of natural variability. (see How do you determine if a sound source might affect a marine animal?)

An animal's behavioral response depends on a number of factors, such as hearing sensitivity, tolerance to noise, exposure to the same noise in the past, behavior at the time of exposure, age, sex, and group composition. Some marine animal responses to sound are momentary inconsequential reactions, such as the turn of a head. Other responses are short-term and within the range of natural variation in these behaviors. In other cases, more significant changes in behavior have been observed. Some of the strongest reactions occur when the sounds are similar to those made by predators.

The following sections discuss the effects on marine animals that have been documented in the scientific literature. These effects include: behavioral responses, such as changes in movement, breathing patterns, and vocalizations; masking; and hearing loss.

Whales sometimes come ashore and are stranded. There have been incidents in which there was a relationship between the time and location of beaked whale strandings and the use of multiple, mid-frequency sonars in nearshore areas. Why these whales stranded is not understood (1). Additional research is needed to increase our knowledge of ocean noise and how marine mammals respond to it.

Additional Resources
• Cornell Lab of Ornithology, Bioacoustics Research Program - Effects of Human-made Sound on the Behavior of Whales
• Merrill, J. editor. 2004. Human-generated Ocean Sound and the Effects on Marine Life. Marine Technology Society Journal, Volume 37(4).
• National Research Council. 2003. Ocean Noise and Marine Mammals. The National Academies Press, Washington, D.C.
• National Research Council. 2005. Marine Mammal Populations and Ocean Noise: Determining when noise causes biologically significant effects. National Academies Press, Washington, D.C.
• NOAA Ocean Acoustics Program Marine Mammal Hearing and Acoustic Impacts Bibliography
• Office of Naval Research. 1998. Proceedings from the Workshop on the Effects of Anthropogenic Noise in the Marine Environment, 10-12 February 1998. Edited by Gisiner, R., Cudahy, E., Frisk, G.V., Gentry, R., Hofman, R., Popper, A.N. and Richardson, W.J. Available at: http://www.onr.navy.mil/sci_tech/personnel/341/docs/proceed.pdf