A mass mortality event (MME), also known as a mass die-off (or simply die-off), is an incident that kills a vast number of individuals of a single species in a short period of time. The event may put a species at risk of extinction or upset an ecosystem. This is distinct from the mass die-off associated with short lived and synchronous emergent insect taxa which is a regular and non-catastrophic occurrence. Causes of MME's include disease and human-related activities such as pollution. Climatic extremes and other environmental influences such as oxygen stress in aquatic environments play a role, as does starvation. In many MME's there are multiple stressors. An analysis of such events from 1940 to 2012 found that these events have become more common for birds, fish and marine invertebrates, but have declined for amphibians and reptiles and not changed for mammals.
Known mass mortality events
Migratory birds (1904), Minnesota and Iowa In March 1904, 1.5 million migrating birds died in Minnesota and Iowa during a strong snowstorm. According to The Guardian, this was the largest avian mortality event on record in the region. Records of MMEs have been kept since the 1880s. MMEs of this size are rare, however, and few before or since have been as big as the 1904 event. According to the records, MMEs "are always associated with extreme weather events such as a drop in temperature, snowstorm or hailstorm".
George River caribou (1984), Canada In 1984, about 10,000 caribou of the George River caribou herd—one of Canada's migratory woodland caribou herds—drowned during their bi-annual crossing of the Caniapiscau River when the James Bay Hydro Project flooded the region.
Harbour seals (1988), North Sea In 1988, the deaths of 20,000 harbour seals in the North Sea were found to be caused by phocine distemper virus.
Sea lions (1998), New Zealand Ten years later, two strains of bacteria were implicated in the deaths of approximately 1,600 New Zealand sea lions.
Fur seals (2007), Prince Edward Islands On Marion Island in 2007, some 250–300 adult male subantarctic fur seals died in a two-week period. It was suggested, though not proven, that this gender-biased mortality was caused by Streptococcus sanguinis, a bacterium carried by the house mouse, an alien species accidentally introduced to the island in the 1800s.
Muskoxen (2003), Canada In 2003, a rain-on-snow event encased the ground in ice, resulting in the starvation of 20,000 muskoxen on Banks Island in the Canadian Arctic.
Birds (2010), Arkansas Shortly before midnight on New Year's Eve 2010, between 3,000 and 5,000 red-winged blackbirds fell from the sky in Beebe, Arkansas. Most died upon hitting the ground, but some were living but dazed. Laboratory tests were performed and the Arkansas Livestock and Poultry Commission, the National Wildlife Health Center in Madison, Wisconsin, and the University of Georgia's wildlife disease study group procured specimens of the dead birds. In addition to the blackbirds, a few grackles and starlings also fell from the sky in the same incident. A test report from the state poultry lab concluded that the birds had died from blunt trauma, with an unlicensed fireworks discharge being the likely cause.
Seabirds and marine life (2010–2013), Gulf of Mexico The months-long Deepwater Horizon oil spill that began in April 2010 in the coastal waters of the Gulf of Mexico resulted in about 600,000 to 800,000 bird mortalities. Dolphins and other species of marine life continued to die in record numbers into 2013.
Birds (2011), Arkansas The Beebe, Arkansas bird deaths were repeated again on New Year's Eve of the following year, 2011, with the reported number of dead birds being 5,000. On 3 January 2011, more than five hundred starlings, red-winged blackbirds, and sparrows fell dead in Pointe Coupee Parish, Louisiana. On 5 January, "hundreds" of dead turtle doves were found at Faenza, Italy. According to Italian news agencies, a huge number of the birds were found to have blue stains on their beaks that may have been caused by paint or hypoxia. Over the weekend of 8–9 January, "over a hundred" dead birds were found clustered together on a California highway, while "thousands of dead gizzard shad" (a species of fish) turned up in the harbors of Chicago.
Fish (2011), Brazil Between 28 December 2010 and 3 January 2011, 100 tons of dead fish washed ashore on the Brazilian coast. On 3 January, an estimated two million dead fish were found floating in the Chesapeake Bay in Maryland. On 7 March, millions of small fish, including anchovies, sardines, and mackerel, were found dead in the area of King Harbor at Redondo Beach, California. An investigation by the authorities within the area concluded that the sardines had become trapped within the harbor and depleted the ambient oxygen, which resulted in the deaths. The authorities stated that the event was "unusual, but not unexplainable".
Cows (2011), Wisconsin On 14 January, approximately two hundred cows were found dead in a field in Stockton, Wisconsin. The owner of the cattle has told deputies that he suspected the animals died of infectious bovine rhinotracheitis (IBR), or bovine virus diarrhea (BVD). Authorities in Wisconsin sent samples from the carcasses to labs in Madison in order to determine cause of death.
Fish (2013), Pakistan On 5 August, around 100 tonnes of dead fish, primarily mullets, were found floating in Karachi Port Trust waters, including Karachi harbour, Manora channel, and Chinna Creek.
Saiga antelope (2015), Kazakhstan In 2015, some 200,000 saiga antelope died within a period of one week in a 20 km2 (7.7 sq mi) area of the Betpak-Dala desert region of Kazakhstan. They had gathered in large groups for their annual calving. It was determined that warm and humid temperatures had caused Pasteurella multocida, a strain of bacteria that normally lives harmlessly in their tonsils, to cross into their bloodstream and cause hemorrhagic septicemia. This event wiped out 60% of the population of this critically endangered species. Mass mortality events are not uncommon for saiga. In 1981, 70,000 died; in 1988 there were 200,000 deaths; and more recently, in 2010, 12,000 died.
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