An unusual mortality event (UME) is defined as a sudden mass stranding that results in a die-off of a specific marine mammal population and requires urgent attention.
Not only do these events lead to a massive decline in marine mammal populations but they also signify changes in the health of our oceans. Investigating unusual mortality events is crucial to maintaining marine mammal populations, addressing changes in our ecosystems, and protecting human health.
Since 1991, there have been 65 documented cases of UMEs happening to: 57% cetaceans, 26% pinnipeds, 13% manatees and 4% sea otters. These UMEs have lead to the deaths of over 12,000 animals. Currently, there are nine active UME cases:
- Alaska Pinnipeds, since 2011 (undetermined)
- Texas Bottlenose Dolphin, since 2012 (undetermined)
- California Sea Lion, since 2013 (ecological factors)
- Florida East Coast Manatee, since 2013 (undetermined)
- California Guadalupe Fur Seal, since 2015 (ecological factors)
- Atlantic Humpback Whale, since 2017 (undetermined)
- North Atlantic Right Whale, since 2017 (undetermined)
- Atlantic Minke Whale, since 2018 (undetermined)
The History Behind Addressing Unusual Mortality Events
During the 1980s the United States experienced a large quantity of deaths occurring in various marine mammal species. In response to these mortalities NOAA Fisheries developed a working group of individuals specialized to address these events.
In 1992, congress passed the Marine Mammal Health and Stranding Response Act as an amendment of the Marine Mammal Protection Act. Within this amendment came a working group of scientists, conservationists, state and federal agencies who focus on marine mammal unusual mortality events. Additionally, two international representatives from Canada and Mexico are involved. Although they do not contribute to the voting process they do provide insight to discussions, sample analysis, and pertinent information for the investigation of UMEs. The primary task for this working group is to determine when the deaths of marine mammals are considered a UME.
What Classifies a Series of Mortalities as an Unusual Mortality Event?
The working group has developed seven criteria to determine if mortalities are part of a UME. If the series of death meets one of the following criteria then it will be considered a UME:
- An increase in the degree or a significant change in the nature of the mortality, morbidity, or strandings compared to previous reports
- Marine mammal mortality, morbidity, or strandings are occurring at a different time of year as compared to previous reports
- An increase in mortality, morbidity, or strandings at a specific area or geographic region, possibly increasing range overtime
- There is a variation in species, age and sex of the marine mammals that are experiencing mortality, morbidity, or strandings in a given area as compared to previous reports
- Marine mammals that are experiencing mortality, morbidity, or strandings experience an abnormal physical condition or pathological signs that deviate from normal strandings
- Mortality, morbidity, or strandings are associated with different behavioral patterns for live animals in the region, such as variations in swimming or diving behavior or sightings in habitats usually avoided
- Mortality, morbidity, or strandings coincides with an unusual decline in the population, species or stock
The Reasons for Unusual Mortality Events
The reason behind an unusual mortality event is difficult to determine and results in 49% of the cases being undetermined. Of those that are determined, biotoxins are the leading cause of unusual mortality events.
These toxins may be the result of harmful algae blooms that occur through the U.S. waters. The two most common biotoxins that result in mass die-offs are brevetoxin and domoic acid.
The second known cause of mortality events is a tie between ecological factors and infectious disease.
Ecological factors are defined as a change in the environment such as ocean warming, pH, salinity, and ocean currents. Infectious diseases are the result of bacteria, viruses, fungi or parasites.
Although many of these infectious agents are naturally occurring in the marine ecosystem, a weakened immune system or the introduction of various agents through runoff or changing climates can spread disease.
The last known cause of unusual mortality events is human interaction. Death by human interaction can result from direct or indirect contact with these marine mammals. Direct contact includes vessel strikes or by-catch in fisheries. Indirect contact includes ingestion of marine debris or entanglement in lost fishing gears.
“Marine Mammals.” Federal Register, 14 Dec. 2006, www.federalregister.gov/documents/2006/12/14/E6-21300/marine-mammals.
NOAA. “Frequent Questions – Unusual Mortality Events.” NOAA Fisheries, www.fisheries.noaa.gov/insight/frequent-questions-unusual-mortality-events.
NOAA. “Active and Closed Unusual Mortality Events.” NOAA Fisheries, www.fisheries.noaa.gov/national/marine-life-distress/active-and-closed-unusual-mortality-events.
NOAA. “Marine Mammal Unusual Mortality Events.” NOAA Fisheries, www.fisheries.noaa.gov/national/marine-life-distress/marine-mammal-unusual-mortality-events.