The annual gathering of around 1,000 giant trevally swimming in slow circles in South Africa’s Mtentu Estuary has long puzzled people around the world. The phenomenon even had Sir David Attenborough scratching his head in his 2013 documentary Africa.
Now, researchers think they might have got to the bottom of the mysterious behaviour. They have published their hypothesis in the African Journal of Marine Science.
Lead author Russell Dixon had heard many different theories about the unusual aggregation. "This phenomenon caught global attention, and all sorts of offbeat theories emerged – like an energy vortex in the river or a somewhat religious fish pilgrimage,” says Dixon, who was a researcher at Rhodes University and the South African Institute for Aquatic Biodiversity’s National Research Foundation at the time of the project.
“Early evidence showed breeding and feeding to be unlikely reasons. But not many people had likely explanations,” he adds.
So, he decided to try and solve the mystery. To do this, he worked with colleagues at his institutions as well as the Oceanographic Research Institute.
The team tagged 10 giant trevally (Caranx ignobilis) with acoustic tags (these huge fish are also known as giant kingfish in South Africa, although, confusingly, other countries use the term 'kingfish' for a different species altogether).
"After mooring receivers in the estuary and the sea to 'listen' for our tagged fish, we then left them in peace,” says Dixon. “We returned months and then years later to download the data and learn what they had been up to.”
When he reviewed the data, Dixon was “blown away”. Comparing the movement of the fish with sea temperatures showed a clear pattern, which implied the fish were cold.
“On days when cold coastal upwelling caused the sea temperature to plummet, we consistently detected almost all of these fish in the upper reaches of the estuary, where the warm surface freshwater sits above the colder (and heavier) seawater,” Dixon says. “On days with a warm sea, they were commonly absent from the estuary,” he adds.
When the waters in their usual habitat get too chilly for the fish, they swim upstream to bask in the estuary’s warmer waters.
Is that case closed? Not quite. “Some researchers think that [giant trevally] may enter the surface freshwater to get rid of saltwater parasites,” Dixon says. “We don’t dismiss that as a possibility.”
“In fact, there are still a few questions that we don't have perfect answers for,” Dixon adds, “like why they leave the upper estuary every night. Or why the Mtentu is the only estuary that gets to be the special gathering spot in the first place.”
Finding answers to what might be going on is important because – for whatever reason – the Mtentu Estuary is clearly important for these giant kingfish. If they lose this habitat, it could have a negative ripple effect.
“While the [giant trevally] are aggregated here, they are extremely vulnerable to various human-induced threats,” says Dixon. “Illegal gillnetting was historically not observed here, but it has more recently been reported.”
Protecting the fish could also benefit the economy. “If non-intrusive 'Kingfish watching' tours could become commercialised and led by local guides, the financial incentives could benefit the low-income community and thereby promote greater local custodianship,” he says.
Dixon believes studying the peculiar phenomenon is important for another reason – to encourage people to fall in love with nature. He says: “Hopefully it also inspires us all to be better custodians of it!”
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Top image: Mtentu Estuary for the air. Credit: Bruce Mann
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