Mass Die-Offs of Fish Are Increasing

Written by on January 16, 2015 in Fish, Marine Life

Researchers at UC Berkeley, the University of San Diego and Yale University completed a review of mass die-off incidents documented in scientific literature and found an increase in such events for marine invertebrates, fish, and birds over the last 70 years.

Fish mass mortality events have been increasing over the last 70 years. Photo credit: Ana Perugini via Wikimedia Commons.

Fish mass mortality events have been increasing over the last 70 years. Photo credit: Ana Perugini via Wikimedia Commons.

Mass die-offs are declared when a large percentage of a population dies in a short time frame. While they don’t result in extinction, they can kill more than 90 percent of a population.

“This is the first attempt to quantify patterns in the frequency, magnitude and cause of such mass kill events,” study senior author Stephanie Carlson, an associate professor at the University of California, Berkeley’s Department of Environmental Science, Policy and Management, said in a news release.

The researchers reviewed 727 mass die-offs, covering nearly 2,500 animal species. Some studies dated back to the 1800s, but the analysis focused primarily on documents from 1940 to the present. During that time, the number of mass die-offs of reptiles and amphibians decreased, while remaining unchanged for marine mammals. They note that some of the increase could be due to an increase in the reporting of mass mortality events. However, even after accounting for reporting bias, there was still a clear increase.

Disease was the primary factor, accounting for 26 percent of the mass die-offs. Nineteen percent were a result of effects tied directly to humans, such as environmental contamination. Processes directly influenced by climate, such as weather extremes, thermal stress, oxygen stress, or starvation, contributed to 25 percent of mass mortality events. Not surprisingly, the most severe events were those with multiple causes.

The study found that the number of mass die-offs increased by about one every year over the last 70 years.

“While this might not seem like much, one additional mass mortality event per year over 70 years translates into a considerable increase in the number of these events being reported each year,” explained study co-lead author Adam Siepielski, an assistant professor of biology at the University of San Diego. “Going from one event to 70 each year is a substantial increase, especially given the increased magnitudes of mass mortality events for some of these organisms.”

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Copyright © 2015 by Marine Science Today, a publication of Marine Science Today LLC.

About the Author

About the Author: Emily Tripp is the Publisher and Editor of She holds marine science and biology degrees from the University of Miami's Rosenstiel School of Marine and Atmospheric Science and a Master of Advanced Studies degree in Marine Biodiversity and Conservation from Scripps Institution of Oceanography. When she's not writing about marine science, she's probably running around outside or playing with her dog. .


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