Drones have photographed lemon sharks feeding on invasive jaguar cichlids in Sueste Bay, part of the Fernando de Noronha archipelago off the north-east coast of Brazil. It is thought to be the first time this predator–prey interaction has been recorded.
Researchers believe the unusual behaviour was driven by heavy rainfall that briefly linked freshwater habitats, where the fish live, to the sea, where the sharks hunt.
Published in Environmental Biology of Fishes, the findings shed light on how sharks respond to sudden arrivals of non-native fish.
Native to Central America, the jaguar cichlid (Parachromis managuensis) is a freshwater species introduced to the Brazilian archipelago, most likely as a source of protein for human and animal consumption.
In periods of heavy rainfall, the freshwater reservoir where the jaguar cichlids live begins to swell, eventually spilling over – carrying the fish with it – into the salty water of the lower-lying Sueste Bay.
Although the cichlid species can tolerate mildly salty water, it becomes stressed at higher salinities; its heart rate increases and its normal behaviour is disrupted. In Sueste Bay, salinity can reach well above healthy levels for the fish.
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“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,” says Bianca Rangel, first author of the study and a postdoctoral researcher at the University of São Paulo.
The effects of the salinity change on the fish were clear, say the researchers, who observed the jaguar cichlids swimming erratically.
The behaviour didn’t go unnoticed by the resident lemon sharks (Negaprion brevirostris), a medium-sized, yellow-brown shark that uses the bay’s warm, shallow waters as a feeding area, as well as a breeding ground and nursery.
Using drones, the team recorded both lemon shark pups and adults feeding on the disoriented fish.
The researchers emphasise that this behaviour appears to be opportunistic. Freshwater fish do not typically form part of lemon sharks’ natural diet, and it is not yet clear whether jaguar cichlids routinely spill into the bay or whether this event was unusual.
“This year there was another overflow, but no one observed this interaction,” says Rangel. “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,” adds Rangel.
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The ecological impact of the jaguar cichlid on the archipelago is still unknown, but invasive freshwater predators elsewhere are known to disrupt native communities.
The researchers believe that shark predation alone will not eliminate the invasive fish. However, sharks may help curb their numbers when individuals wash into the bay after storms.
Top image: Lemon sharks hunting jaguar cichlids. Credit: Bianca Rangel/IB-USP
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