Scientists studying a type of catfish colloquially known as ‘monsters of the river’ insist their disturbing eating habits are no reason to fear them.
Rhône fish was 2.7m long
The creatures, silures in French, are now an established part of French river life and pose no danger to other fish populations except when they feast on migrating species at dams and fish ladders, experts say.
Pictures of silures caught by fishermen are a regular feature of local newspapers, with specimens of 2m long and an estimated 80kg not uncommon.
The French record is for a 2.7m fish, weighing an estimated 120kg, caught in the Rhône.
Fishermen’s tales proven
Rémi Masson, a scientist and underwater photographer, said: “The length is certain but it is more difficult to be exact with the weight because they are long, heavy and slippery.”
He was the first to photograph and film a group of silures, confirming what had until then been dismissed as fishermen’s tales.
It is believed the fish were part of the French river system until the end of the last Ice Age.
A small population existed in the Rhine river on the French/German border, but in the 1960s specimens from Russia were let loose in France, allegedly by fishermen, and they have now spread everywhere.
Cannibal fish are getting bigger
Mr Masson dismisses calls for the fish to be eliminated as an invasive species.
“It will be virtually impossible as they are now in every major river in France.
“They are still spreading into some tributaries of the big rivers, which might be why there are more reports of them, but in rivers like the Rhône the population is actually falling, although the average size of the fish is growing.
“This is because they are cannibals and self-regulate the population naturally by large fish eating smaller ones.”
Self-regulate numbers by eating smaller fish
The groups of fish photographed and filmed by Mr Masson helped scientists discover more about the creatures, which normally lead quite solitary lives.
At the centre of the swarm is always a giant silure of over 2m.
“We discovered that the aim of the smaller fish was to rub against the larger fish, and each other, sending chemical messages through the skin, probably to say ‘don’t eat me!’,” said Mr Masson.
It explains, he says, why there are no fish smaller than a metre in the swarm.
“Until they reach that size, they are prey to the bigger ones.”
Not dangerous to humans but will protect young
Another discovery was that they use sound to communicate.
“We do not understand the language, but there are two different sounds – a sort of ‘uh, uh, uh’ and clicks, like dolphins.”
The only time silures are aggressive towards humans is for a couple of weeks between June and July when males guard nests of eggs, sometimes in vegetation along the side of banks. If swimmers approach, the male can bite, but their teeth are small and in rows like a kitchen grater, so rarely penetrate the skin.
“Big bites can leave a mark, and people get a fright, but the fish is not dangerous to humans,” said Mr Masson.
“There have only been a couple of recorded cases.”
Carnivores that adapt to any prey
Silures are carnivores and very adaptive to their environment, changing behaviour to catch different prey.
Near Albi, on the Tarn river in southern France, there is a notorious spot where silures wait in the shallows for pigeons to drink, then launch themselves out of the water to grab and swallow them in one gulp.
They have also been credited with limiting Louisiana crawfish, another invasive species, which was overwhelming native French crawfish.
Scientists are most concerned about groups of silures that have learnt to wait in ambush by dams and fish ladders for returning migrant salmon, eels and lampreys.
“The dams slow the migrating fish and concentrate them, and the silures pounce. But you cannot say they are the problem – it is the dam which is the problem,” said Mr Masson.
Call for more studies
Another scientist who has spent time studying silures, Thomas Trancart, told The Connexion that in some places 80% of migrating lampreys were eaten by silures.
Salmon and shad were affected in similar numbers.
Dr Trancart, who works for the Muséum National d’Histoire Naturelle in Paris, said: “There is a paradox because silures are the biggest fish in our rivers, but have had very few scientific studies devoted to them.
“For example, there are no surveys of the density of silures or their biomass in France.
“From the number seen in some rivers, it seems it is still an expanding species.”
You can eat silures
In America, eating giant catfish very similar to silures is a long-established tradition.
However, attempts to persuade French people to do the same have not been successful. Mr Masson said: “I have never eaten it myself, but there are top Michelin chefs who use it a lot.
“They say it has a firm flesh, which cooks well. It does not have much taste but adapts to a wide range of sauces. And it is meant to barbecue well, with the smoke giving an extra boost to the flavour.”
Limit eating river fish due to pollution
Pollution fears put many off. Some parts of the Rhône, in particular, are heavily contaminated with chemicals.
As a result, the government recommends river fish, especially large fish such as silures, pike and carp, are only eaten twice a month. However, Mr Masson believes these fears are often over-rated.
“In general, the fish are free of contaminants, but the French government prefers blanket bans and strict advice, which puts people off.”
In spite of silures’ odd appearance and voracious feeding habits, Mr Masson says he has a deep affection for them.
“They are very curious animals. If you dive underwater and just sit quietly, holding your breath, any in the vicinity will come over and have a look.
“It is always impressive to see such large animals close up.”