Mass die-offs of hundreds — sometimes hundreds of thousands — of fish, birds and other animals appear to be increasing in both frequency and in the numbers of individuals involved, according to a new study.

The report by researchers at Yale University and four other schools found that disease and human-related activities were the top killers in more than 700 “mass mortality events” since 1940 that were studied. The report included looks at lethal episodes that affected 2,407 different animal populations.

The research didn’t include the 1999 mass die-off of lobsters in Long Island Sound, but the apparent causes of that deadly event were similar to those cited by the report in many of the other cases of mass deaths.

The 1999 die-off almost wiped out lobsters in most areas of the Sound, and the population still hasn’t recovered. Scientists believe increasing water temperatures from global warming made cold-water-loving lobsters more vulnerable to disease, manmade pollution and pesticides, all factors listed as critical elements in mass mortality cases in the new report.

Samuel Fey, the Yale biologist who co-authored the research study, said the apparently growing numbers involved in such mass die-offs are one of the chief concerns to come out of the study, published in the Jan. 12 issue of the Proceedings of the National Academy of Sciences.

“That was the single biggest surprise for us,” Fey said. “It was a big red flag for us.”

Scientists from Dartmouth College, the University of San Diego, the University of California, and Southern Illinois University also participated in the research project.

Fey said he and the other researchers had expected to see a decline in the average numbers of fishes, birds and mammals killed in mass mortality events. They initially assumed that overall averages would drop because so many more die-offs have been studied and reported on in recent years than in past decades.

Instead, they discovered the numbers of individual members of species dying in such mass events seem to have been continually rising for birds, fish and marine invertebrates, such as sea urchins and lobsters.

There is some good news included in the study’s results: those mass die-off numbers for individual mammal species appear to be holding steady, while the death tallies for amphibians and reptiles seems to be declining.

Fey noted that far more mass mortality cases have occurred across the globe since 1940 than were covered by the new report. The study was limited to only those events that triggered papers published in scientific journals.

He said more comprehensive studies are needed to get an accurate picture of what’s happening with mass die-offs. Fey said only about 9 percent of scientific papers on mass mortality events reviewed for the study mentioned how a particular die-off affected a species’ overall population.

Fey, 30, has a doctorate in ecology and evolutionary biology from Dartmouth College and was raised in Farmington. He is currently a postdoctoral fellow in the Yale department of ecology and evolutionary biology.

Mass mortality events, where large numbers of an individual species die over a relatively short period, are different from extinctions. But major die-offs can virtually wipe out a species in a specific locale.

Fey said one example was the 1983 event that resulted in an estimated 99 percent of a particular species of sea urchin (Diadema antillarum), being eradicated from the Caribbean Sea. Experts believe that event was caused by a water-born disease.

In 1988, a virus outbreak in Siberia among Baikal seals, the only freshwater seal species in the world, killed tens of thousands of the seals in Lake Baikal, Fey said. Experts believe there are fewer than 85,000 of those seals remaining, according to the Seal Conservation Society.

The new study found that disease was listed as the cause in 26 percent of mass die-offs. Human activity, primarily pollution, was found to be the primary trigger for 19 percent of big mortality events, while biotoxicity, such as harmful algae blooms, was cited as the third leading cause at 15.6 percent.

Even biotoxicity events can be related to human activities, such as fertilizers that are washed into streams and rivers and estuaries like Long Island Sound, where they can lead to dangerous algae blooms.

Fey said the study demonstrated that there are “so many connections” between different causes of die-offs. The researchers concluded that mass mortality cases are often triggered by “multiple interacting stressors” working together, such as disease, starvation and changes in the environment.

“I see a clear connection between our study and climate change,” Fey said. Increases in average global temperatures, and more rapid variations in temperatures and weather, are all involved in how species cope in the environment, Fey said.

Fey said he believes the new study of mass mortality events “lays the foundation” for a better understanding of how die-offs relate to the health of entire species and the environment as a whole.

Fey said he hopes the findings of the report will lead to better research programs to document mass die-offs that are certain to occur “in our uncertain future.”