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Criteria for Determining an Marine Mammal Unusual Mortality Event


Unusual Mortality Events

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Sea Lions in Rehab
Credit: Pacific Marine Mammal Center

2013 Bottlenose Dolphins in the Mid-Atlantic

2013 Bottlenose Dolphins in Florida

2013 California Sea Lions

2011-2012 Bottlenose Dolphins in Texas

2011 Northern Alaska Pinnipeds

2011 Northeast Pinnipeds

2010-2014 Northern Gulf of Mexico Cetaceans

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Criteria for Marine Mammal Unusual Mortality Events

History of the Working Group

Event Fund


Working Group Charter [pdf]

dolphin stranding
Atlantic white-sided dolphins
(Lagenorhynchus acutus)
Photo: Don Lewis of Wellfleet Audubon

The Working Group on Marine Mammal Unusual Mortality Events (Working Group) has developed a set of criteria to be utilized in determining an unusual mortality event. A single criterion or combination of criteria may indicate the occurrence of an unusual mortality event. The criteria are:

  1. A marked increase in the magnitude or a marked change in the nature of morbidity, mortality or strandings when compared with prior records.

  2. A temporal change in morbidity, mortality or strandings is occurring.

  3. A spatial change in morbidity, mortality or strandings is occurring.

  4. The species, age, or sex composition of the affected animals is different than that of animals that are normally affected.

  5. Affected animals exhibit similar or unusual pathologic findings, behavior patterns, clinical signs, or general physical condition (e.g., blubber thickness).

  6. Potentially significant morbidity, mortality or stranding is observed in species, stocks or populations that are particularly vulnerable (e.g., listed as depleted, threatened or endangered or declining). For example, stranding of three or four right whales may be cause for great concern whereas stranding of a similar number of fin whales may not.

  7. Morbidity is observed concurrent with or as part of an unexplained continual decline of a marine mammal population, stock, or species.

For more information, please see the Federal Register notice (71 FR 75234), published on December 14, 2006, that announced these revised criteria developed by the Working Group.
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