Definition: In this task, you will be presented with a passage and have to answer a question based on your understanding of the events inferred from the passage. Try to find the best answer that is most likely to fill in "_". Note that the URLs in the text have been replaced with [Link].
Input: (CNN) The National Oceanic and Atmospheric Administration (NOAA) launched an investigation Thursday into a large number of humpback whale deaths from Maine to North Carolina. The agency declared the deaths an unusual mortality event, the first one observed in humpback whales in nearly a decade. 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." The last unusual mortality event declared for humpback whales was in 2006. Two other events were declared in 2005 and 2003, said Deborah Fauquier, veterinary medical officer for NOAA Fisheries Office of Protected Resources. The cause of those UMEs was undetermined.A large number of humpback whales have been dying along the east coast of the United StatesIt's the first such mass die-off that NOAA has observed in humpback whales in nearly a decade 
 Question:Since the marine mammal UME program was created in 1991, there have been 63 formally recognized _ in the United States involving a variety of species.

UME