Europe’s biggest aquarium faces legal action over the deaths of 30 hammerhead sharks, which a marine conservation group alleges were mistreated.

Sea Shepherd France announced at the weekend that it would file a lawsuit on Monday against the Nausicaá aquarium in the French port of Boulogne, near Calais. The last of the 30 sharks, which were introduced into the aquarium in 2011 and 2018, died on Thursday.

Sea Shepherd accuses the aquarium of “serious mistreatment” of the sharks and “breaches of environmental law”.

Philippe Vallette, the manager of Nausicaá, rejected the allegations and the claim that they died because they were kept in captivity. He said the hammerheads died from “a fungus, latent in the organism of sharks, which spreads when they are in a weakened state".

Mr Vallette said hammerheads were prone to fungal infection even in the wild. “A female has 600 to 700 young during her life and the survival rate of a hammerhead shark is one for every two or three hundred.”

But shark experts say hammerheads die earlier when placed in captivity. Nicolas Ziani, an ichthyologist, said their life expectancy decreased from 30 to about 12 years.

Hammerheads are an endangered species, notably because of the practice of ‘shark-finning’ in which their fins are removed. Unable to swim properly, they sink to the sea bottom and die. Fishing of hammerheads is regulated under the CITES international treaty.