The term stranding refers to an aquatic animal observed in an inappropriate location, for example, an offshore species found inshore. Most often, stranded animals are found on a beach or in shallow water. Along the U.S. coast alone, about 1,000 cetaceans and 2,500 pinnipeds strand annually. When three or more animals strand together in time and place, it is called a mass stranding. Communal animals, such as small cetaceans like pilot whales are the most common species found stranded. Mass strandings may include one or several species.
There are many identified causes of strandings, including disease, parasite infestation, ship strike, pollution exposure, starvation, extreme weather events, and tidal changes. However, determining the exact cause of a marine animal stranding or death is often difficult. On average, the cause of death can be determined in only about half of all stranding cases.
Much attention has recently been focused on anthropogenic sources of sound in the ocean and their potentially harmful effects on marine animals. To accurately assess whether or not sound has an effect on marine animals, it is necessary to understand the characteristics of sound and potential impacts that have been determined by scientific research.
Different sounds vary in their characteristics, such as intensity and frequency, which influence their potential for harmful effects. The potential impacts of anthropogenic sounds in the ocean include sounds that may cause marine animals to alter their behavior, prevent them from hearing other important sounds (masking), cause hearing loss (temporary or permanent), or damage tissue. In a few events, a relationship in time and space between the use of naval sonar and the stranding of cetaceans, particularly beaked whales was identified; however, the mechanism by which the sonar might have caused the strandings is still not determined.
In five well-documented cases, there is sufficient information about the military exercises and the times and locations of the strandings to determine that multi-ship exercises with sonar contributed to the strandings. These events occurred in Greece (1996), Bahamas (2000), Madeira, Portugal (May 2000), and the Canary Islands (2002 and 2004). The necropsies that were performed found similar injuries, but none of the animals were found to have acoustic trauma. There are currently few peer-reviewed scientific publications that describe and discuss these strandings. The majority of authoritative information on these strandings can be found in official investigation reports of the events. For a full discussion of these events and the potential effects of sonar that have been published in the scientific literature, please see the DOSITS section on marine mammal strandings.
Much more scientific research is needed to understand why a relationship in time and location may exist between cetacean mass strandings and the use of naval sonar. At present, no definitive answer has been provided by the available research.
It is important to put the potential of impacts to marine animals from naval sonar in perspective. In addition to anthropogenic sound, marine mammals face threats from many different human activities, including fishing, habitat destruction, ship strikes, and whaling. Of these threats, the most significant is fisheries bycatch, which causes more marine mammal deaths than any other human activity. Globally, it is estimated that more than 650,000 marine mammals are killed each year by being accidentally caught in fishing nets. This can be compared with the less than two dozen strandings considered to be related to naval sonar over the last two decades.