Unusual Mortality Event

A formal US finding that a marine mammal population is dying off unexpectedly and in significant numbers, which triggers an official investigation and response.

Under the US Marine Mammal Protection Act, an Unusual Mortality Event is a stranding that is unexpected, involves a significant die-off of a marine mammal population, and demands an immediate response. Declaring one is a formal step, not a description: it opens an investigation into what is killing the animals, and a dedicated contingency fund helps the stranding network respond.

The decision rests with an expert working group that NOAA Fisheries set up in 1991 and that was formalised by amendments to the act in 1992. Its members come from universities, conservation groups and state and federal agencies. An event is judged unusual if it meets one or more of seven criteria, and the group decides within 24 hours of its first consultation.

NOAA treats these events as indicators of ocean health as well as of the species involved. The North Atlantic right whale has been under one since 2017. The dead, seriously injured and sick whales counted in it make up more than 20 percent of the population, and NOAA names entanglement in fishing gear and vessel strikes as the leading causes.

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