"Mass mortality" refers to a situation where a large number of living beings die at the same time, often due to a common cause or event. Full definition
Prior to the earthquake, mass mortality events had dramatically altered the reef ecosystem over the course of 25 years. (earthtimes.org)
An exception is the northward spread of two protozoan parasites (Perkinsus marinus and Haplosporidium nelsoni) from the Gulf of Mexico to Delaware Bay and further north, where they have caused mass mortalities of Eastern oysters (Crassostrea virginica). (ipcc.ch)
Coral bleaching and slowing of coral calcification already are causing mass mortalities, increased coral disease, and reduced reef carbonate accretion, thus disrupting coral reef ecosystem health [40], [124]. (journals.plos.org)