Whales and other cetaceans (such as dolphins and porpoises) are considered stranded when they are found on the shore (dead or alive), when they are found dead at sea (e.g., if the body is floating in the water), or when found alive at sea but unable to return to their natural habitat (e.g., if trapped in shallow water or injured). As defined in the U.S. Marine Mammal Protection Act, an unusual mortality event (UME) is “a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response.” A UME can occur over multiple years and across regions and may affect a single species or various species. Individuals or groups of animals can strand, depending on the species and situation. For more detailed information and scientific citations, please see the full FAQ document linked below.

Strandings are often caused by 1) injuries due to vessel collisions, entanglement or ingestion of active and derelict fishing gear and marine debris, or other human interactions; 2) infectious and non-infectious diseases; 3) malnutrition; 4) unusual weather events or oceanographic conditions; or 5) some combination of these or other factors. Climate change has altered the migration and distribution of whale species and their prey and, in some cases, increased their interaction with vessels and other anthropogenic activities. Climate change also has the potential to contribute to changed patterns of pathogen emergence, distribution, abundance, and transmission, all of which can lead to increased strandings. For more detailed information and scientific citations, please see the full FAQ document linked below.

Generally, existing evidence suggests that the main anthropogenic causes of death for baleen whales are vessel strikes and entanglement in fishing gear. However, the cause varies by species. While we lack information for many whale mortalities (e.g., those that aren’t detected or necropsied, or for which cause of death cannot be determined; see What are the biases and limitations of stranding data?), data suggest that the greatest risk to humpback whales is vessel collisions (50% of necropsies showed evidence of vessel interaction), whereas 65% of North Atlantic right whales that have been killed or injured were entangled in fishing gear. Recent increases in the number of baleen whale deaths in the Northwest Atlantic region relate to a combination of natural and anthropogenic factors. These may include shifting prey and whale distributions inshore, infectious disease, and changes in the locations and amount of shipping activity leading to increased interactions with vessels. For more detailed information and scientific citations, please see the full FAQ document linked below.

Strandings can provide important information on disease, health, and causes of death. Stranding data are especially important for learning about susceptible and rare populations/species. Stranding and entanglement networks fill a valuable bio-surveillance role, as they are often the first to detect threats to marine mammal populations. For more detailed information and scientific citations, please see the full FAQ document linked below.

Detecting strandings and determining their causes is challenging. Many factors influence stranding occurrences and rates. This means it is difficult to understand whether observed changes in the timing, location, or rate of strandings reflect changes in actual mortality in a population. Availability of stranding data depends on the strandings being detected by people (e.g., some animals never wash ashore), knowledge of how to report the stranding, and stranding network partners that are able to further investigate an event when it is detected and reported. For more detailed information and scientific citations, please see the full FAQ document linked below.

Similar to an autopsy in humans, a necropsy is the examination of a deceased animal to identify cause of death and collect other information on the health of the individual, the species, and the marine environment. In the United States, whale necropsies are conducted by organizations that are members of the National Marine Mammal Stranding Response Network. All reported strandings are documented, but it is not possible to conduct necropsies in many cases due to human safety, the state of carcass decomposition, stranding location, or other reasons. Necropsies can provide valuable information but cause of death cannot always be determined. Complicating factors include the level of decomposition, inability to collect or process samples due to limited accessibility or resources, inconclusive test results, and difficulty identifying multiple factors that may contribute to the cause of death. For more detailed information and scientific citations, please see the full FAQ document linked below.

Necropsy costs are typically covered by the individual Stranding Network organization that conducts the necropsy (e.g., non-profit organization, academic institution, local, tribal, state or federal government agencies). However, such organizations may be eligible for monetary support through the competitive John H. Prescott Marine Mammal Rescue Assistance Grant Program through the federal government. The federal government distributes several million dollars per year to support recovery and data collection of marine mammals, including necropsies. In 2023, over $4 million was distributed through the Prescott Grant Program (the total amount varies annually as appropriated by Congress). As of 2023, Congress limited these grants to a maximum of $150,000 per organization per 12-month period (the average award in 2023 was $81,425). There is a supplementary federal fund distributed during Unusual Mortality Events (UMEs), when necropsies may be conducted more frequently or thoroughly. There are also non-federal funding sources, such as states, non-federal grants, and private individual or organization donors. For more detailed information and scientific citations, please see the full FAQ document linked below.