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Marine Mammal Unusual Mortality Events

To understand the health of marine mammal populations, scientists study unusual mortality events. Understanding and investigating these events is important because they can serve as indicators of ocean health.

California sea lion at marine mammal center fort macarthur California sea lion in rehabilitation - Photo: Marine Mammal Care Center

Under the Marine Mammal Protection Act, an unusual mortality event (UME) is defined as "a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response."

A blue and white Unusual Mortality Event logo showing illustrations of a dolphin, sea lion, and manatee

Understanding and investigating marine mammal UMEs is crucial because they can be indicators of ocean health, giving insight into larger environmental issues which may also have implications for human health. To fill this role, NOAA Fisheries established the Working Group on Marine Mammal Unusual Mortality Events in 1991. The working group was formalized in 1992 with amendments to the MMPA amendments.

The working group members are comprised of experts from scientific and academic institutions, conservation organizations, state and federal agencies who work closely with stranding networks and have a wide variety of experience in biology, toxicology, pathology, ecology, and epidemiology. Core members are appointed for three-year terms and have voting privileges. International participants and federal government representatives contribute significantly throughout the UME investigations but do not have voting rights.

An UME is determined to be "unusual" by the working group if it meets one or more of the seven criteria. All possible information is reviewed and, within 24 hours of the initial consultation, it is decided whether an event is considered an UME. The Marine Mammal Unusual Mortality Event Contingency Fund is used to help the Marine Mammal Stranding Network investigate and respond to UMEs. You can donate to the UME Contingency Fund through Pay.gov and help cover costs incurred by the Marine Mammal Stranding Network while investigating UMEs.


A bar chart that breaks down Unusual Mortality Events by cause (1991-2024). Gray represents pending UMEs, orange is human interactions, green is ecological factors, red is biotoxins, and blue is infectious disease



A pie chart showing causes of Unusual Mortality Events (1991-2024). Infectious Disease = 15%, Biotoxin = 18%, Ecological Factors = 16%, Human Interactions = 6%, and Undetermined = 46%


A pie chart showing the species involved in Unusual Mortality Events. Cetaceans = 55%, Pinnipeds = 27%, Manatees = 14%, and Sea Otters = 4%
A pie chart showing Unusual Mortality Events by region. Atlantic = 38%, Gulf of Mexico = 29%, Pacific Coast = 26%, Alaska = 6%, and Hawaii = 1%


Understanding Marine Mammal Unusual Mortality Events

Scientists study and investigate unusual mortality events (UME) to understand the health of marine mammal populations. UMEs can serve as indicators of ocean health.

marine mammal unusual mortality events logo