A shift in natural behavior by an established predator could help control an invasive fish species found in Brazilian waters.
Research published in the journal Environmental Biology of Fishes described the intriguing activity of lemon sharks: They are now hunting a problematic freshwater fish.
According to an article from FAPESP shared by Phys.org, this shark is often found in Sueste Bay, a sea inlet where fresh water from a nearby mangrove swamp meets salt water in the archipelago of Fernando de Noronha.
“This is a breeding, nursery, and feeding area for lemon sharks,” said Bianca Rangel, the first author of the study, per FAPESP.
“The night before our observation, there were heavy rains, causing the Xaréu reservoir, where the fish live, to overflow into the mangrove swamp, which in turn also overflowed and created a connection with the bay.”
In March 2024, researchers spotted the shark hunting and feeding on the jaguar cichlid, an invasive fish species in the area. It was probably originally introduced as a protein source for humans, but it has since become problematic for the environment.
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The jaguar cichlid is a freshwater fish, but in saltwater, it becomes stressed and begins to swim erratically. The shark took advantage of this to catch the fish more easily.
“This year there was another overflow, but no one observed this interaction,” Rangel said. “We don’t know if there were fewer jaguar cichlids or if they simply disappeared.
“Since there’s no monitoring in the mangrove swamp and the reservoir, we have no way of knowing if the animal is still present. If freshwater fish entering the bay becomes commonplace, it’s possible that sharks will learn that after the rains, food will be available.”
The researchers said that it is unlikely that the shark will completely eliminate the invasive species, but it may be able to reduce the fish’s population.
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This is a positive development, since one of the reasons that invasive species become problematic is that they have no natural predators in their new environments.
A predator beginning to prey on this species can help keep it from getting too far out of hand.
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