The strange behavior of fish

It was detected by submariner Gregg Furstenwerth. It was a fish from the Florida Keys (United States), with unusual behavior. The bream was spinning and swirling in inverted circles. Puzzled, he recorded the behavior with his underwater camera. Soon, he would learn of many other similar cases. What causes the strange behavior of fish?

The sightings occurred near the lower Keys, a tropical archipelago of islands south of mainland Florida. Then three reports appeared in Miami and others. To date, this has been detected in at least 44 species, including the smalltooth sawfish. It is critically endangered. Many fish have been found dead, although there is no official count.

The fish's strange behavior is causing unexplained deaths.
The fish’s strange behavior is causing unexplained deaths.

Possible culprits

It is unknown what causes fish to get sick. A joint investigation was initiated between several universities, institutes and state organizations. “We want to know what it is,” says Alison Robertson, a marine scientist. “We hope to identify it and be able to provide solutions.”

It’s very strange to see such a long-term event affecting so many different species. Possible causes include harmful algal blooms. These can produce neurotoxins that affect the behavior of fish. Low oxygen and parasites are other factors.

Of particular concern is the smalltooth sawfish, listed under the U.S. Endangered Species Act since 2003. It is one of five species of sawfish in the world and the only one in U.S. waters. By early March, 21 sawfish had turned up dead. There are many others in danger. “From the sawfish’s point of view, it is obviously very worrying that so many large animals are dying,” the researchers say.

The sawfish is particularly threatened.
The sawfish is particularly threatened.


The small-toothed sawfish that can reach 3.6 meters in length. It historically existed on both sides of the Atlantic. But only two populations remain in the Bahamas and the US, the latter being larger. This means that the species’ recovery is largely dependent on the US population.

The strange behavior of the fish is a clear threat. “We don’t know yet if it’s potentially catastrophic,” says Robertson; “My hope for spreading the news is simply that it reaches more people.”

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