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Marine Mammal Unusual Mortality Events

What is an "unusual mortality event"?
An unusual mortality event (UME) is defined under the Marine Mammal Protection Act as:

"a stranding that is unexpected; involves a significant die-off of any marine mammal population; and demands immediate response."

In recent years, increased efforts to examine carcasses and live stranded animals have improved the knowledge of mortality rates and causes, allowing a better understanding of population threats and stressors and the ability to determine when a situation is "unusual."

Understanding and investigating marine mammal UMEs is important because they can serve as indicators of ocean health, giving insight into larger environmental issues which may also have implications for human health and welfare.California sea lions in rehabilitation post-UME.
Credit: Pacific Marine Mammal Center

How many unusual mortality events (UMEs) have occurred involving marine mammals?
The marine mammal UME program was established in 1991. From 1991 to the present, there have been 63 formally recognized UMEs in the U.S., involving a variety of species and dozens to hundreds of individual marine mammals per event.

The table below indicates the years, species, and cause (determined or suspected) for each UME.

Where do unusual mortality events generally occur?
UMEs have occurred in and along the U.S. coasts of:

• Atlantic
• Gulf of Mexico
• Pacific coasts of the continental U.S.
• Alaska
• Hawaii

The states with the most declared UMEs are:

1. California
2. Florida

What species are most common?
The most common species involved in UMEs are:

1. Bottlenose dolphins
2. California sea lions
3. manatees

What causes unusual mortality events?
Causes have been determined for 32 of the 63 UMEs documented since 1991. Causes of UMEs include:

• infections
• biotoxins (poisonous substances produced by a living organism)*
• human interactions, and
• malnutrition

*Since 1996, UMEs associated with biotoxins from harmful algal blooms have become more prevalent. The majority of recent UMEs have been attributed to toxicity from:

• domoic acid [pdf]
• brevetoxin [pdf]

Past and Current UMEs