Mass Die-Offs of Fish and Birds Rise Over 70 Years

It turns out that mass die-offs may be rising among birds and fish. Scientists have analyzed 727 mass die-offs of nearly 2,500 animal species from the past 70 years and found out that these events are increasing.

These mass mortality events occur when a large percentage of a population dies in a short amount of time. While die-offs are rare and fall short of extinction, they can pack a devastating punch for local populations; in fact, they can kill up to 90 percent of a population in one shot, which makes the animals more susceptible to other risk factors later.

Now, scientists have taken a closer look at incidents of mass kills over time. They focused on the period from 1940 to the present, examining events documented in scientific literature. Although they acknowledge that some of the increases seen can probably be attributed to better recording, the researchers still noted that there was still an increase in mass die-offs after accounting for reporting bias.

"The catastrophic nature of sudden, mass die-offs of animal populations inherently captures human attention," said Stephanie Carlson, one of the researchers, in a news release. "In our studies, we have come across mass kills of federal fish species during the summer drought season as small streams dry up. The majority of studies we reviewed were of fish. When oxygen levels are depressed in the water column, the impact will affect a variety of species."

Overall, disease accounted for 26 percent of the mass die-offs. Direct effects tied to humans caused 19 percent of the mass kills, and climate and weather extremes contributed to about 25 percent. In addition, the scientists found that the number of mass mortality events has been increasing by about one event per year over the 70 years the study covered.

The findings reveal that it's crucial to monitor not only environmental changes, but the biological response to it. The researchers highlighted ways to improve documentation of such events in the future, including citizen science.

The findings are published in the journal Proceedings of the National Academy of Sciences.

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