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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.sea lions in rehabCalifornia 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 64 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:

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 64 UMEs documented since 1991. Causes of UMEs include:

*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:


Past and Current UMEs



No. Year Species Location Cause
64 2017 North Atlantic right whales Atlantic Undetermined
63 2017 Humpback whales Atlantic (ME-NC) Undetermined
62 2015 Guadalupe fur seals California Undetermined
61 2015 Large whales Western Gulf of Alaska Undetermined
60 2013 Bottlenose dolphins Mid-Atlantic Infectious Disease
59 2013 Bottlenose dolphins Florida (East Coast) Ecological Factors
58 2013 Manatees Florida Undetermined
57 2013 California sea lions California Ecological Factors
56 2012 Bottlenose dolphins Texas Undetermined
55 2011 Pinnipeds Alaska Undetermined
54 2011 Pinnipeds New England Infectious Disease
53 2011 Bottlenose dolphins South Carolina Undetermined
52 2011 Manatees Florida Ecological Factors
51 2010 Cetaceans Northern Gulf of Mexico Human Interaction
50 2010 Bottlenose dolphins Florida (St. Johns River) Undetermined
49 2010 Manatees Florida Ecological Factors
48 2009 Bottlenose dolphins Virginia Undetermined
47 2008 Harbor porpoises California Ecological Factors
46 2008 Bottlenose dolphins Florida (Indian River) Undetermined
45 2008 Common dolphins, Atlantic while-sided NC - NJ Undetermined
44 2008 Bottlenose dolphins Texas Undetermined
43 2007 Guadalupe fur seals Northwest Undetermined
42 2007 Large whales California Human Interaction
41 2007 Manatees Florida (SW) Biotoxin
40 2007 Cetaceans California Undetermined
39 2007 Bottlenose dolphins Texas and Louisiana Undetermined
38 2006 Manatees Florida (Everglades) Biotoxin
37 2006 Harbor porpoises Pacific Northwest Undetermined
36 2006 Pinnipeds North Atlantic Infectious Disease
35 2006 Humpback whales North Atlantic Undetermined
34 2006 Sea otters Alaska Undetermined
33 2005-2006 Bottlenose dolphins Florida (panhandle) Biotoxin
32 2005 Large whales North Atlantic Undetermined
31 2005 Harbor porpoises North Carolina Undetermined
30 2005-2006 Multi-species (manatees and bottlenose dolphins) Florida (west coast) Biotoxin
29 2004 Small cetaceans North Carolina Undetermined
28 2004 Small cetaceans Virginia Undetermined
27 2004 Bottlenose dolphins Florida (panhandle) Biotoxin
26 2003 Harbor seals and Minke whales Maine Undetermined
25 2003 Manatees Florida (west coast) Biotoxin
24 2003 Large whales (primarily humpback whales) Gulf of Maine Undetermined
23 2003 Sea otters California Ecological Factors
22 2002 Manatees Florida (west coast) Biotoxin
21 2002 Multispecies (common dolphins, California sea lion, sea otters) California Biotoxin
20 2001-2002 Hawaiian monk seals Hawaii (Northwest Hawaiian Islands) Ecological Factors
19 2001 Bottlenose dolphins Florida (Indian River) Undetermined
18 2000 Harbor seals California Infectious Disease
17 2000 California sea lions California Biotoxin
16 1999-2001 Gray whales California, Oregon, Washington Undetermined
15 1999-2000 Bottlenose dolphins Florida (panhandle) Biotoxin
14 1998 California sea lions California Biotoxin
13 1997 Harbor seals California Infectious Disease
12 1996 Bottlenose dolphins Mississippi Undetermined
11 1996 Manatees Florida (west coast) Biotoxin
10 1996 Right whales Florida, Georgia Human Interaction
9 1994 Bottlenose dolphins Texas Infectious Disease
8 1994 Common dolphins California Undetermined
7 1993 Pinnipeds Washington Human Interaction
6 1992-1993 Pinnipeds California Ecological Factors
5 1992 Bottlenose dolphins Texas Undetermined
4 1992 Phocids New England Infectious Disease
3 1991 Bottlenose dolphins Florida (Sarasota) Undetermined
2 1991 California sea lions California Infectious Disease
1 1991 Harbor seals New York Infectious Disease
Updated: August 25, 2017