Voracious and almost without predators, the blue crab was first sighted in the Ebro Delta on Spain’s Mediterranean coast in 2012, and since then the population has expanded exponentially, wiping out native species and forcing the fishing industry to adapt and find new markets.

The blue crab (Callinectes sapidus) is a native of the east coast of the United States. It probably found its way into the Mediterranean via ships’ ballast tanks, says Carmen Barberá, a researcher at the Marine Research Centre at the University of Alicante, who is a specialist in invasive species.

When ships are not fully laden they maintain their trim by filling ballast tanks with water, which they later discharge at their destination, dumping marine life from one side of the world to the other. “The fact that the crabs are concentrated in ports such as Castellón and Barcelona is further evidence of this,” says Barberá.

In their natural habitat the crabs become inactive when the seawater temperature drops in winter. “The Mediterranean temperature suits them, even more so after the warm winter we’ve had,” says Barberá. “There are also lots of wetlands on the Mediterranean coast and the females need both fresh and saltwater to complete the reproductive cycle. The delta offers the perfect environment for the crab to flourish.”

According to Francesc Vidal, director of the Ebro Delta nature reserve, the crabs are now being found further inland up river.

The females can produce up to eight million eggs as many as 18 times over two years. Gestation takes 30-50 days and the young grow rapidly, devouring everything – including their own kind – with their large claws and powerful jaws.

The creature’s only predator is the octopus, but only a big octopus could win a fight with a blue crab’s deadly pincers, Barberá believes. There are also many molluscs in the delta, both wild and farmed, that the crabs feed on. From the delta the crab has spread along the coast.

Further south, in the saltwater lagoon of the Mar Menor in Murcia, the blue crab has driven out the native green species, which the blue variety has probably eaten.

A male blue crab catching a frog in the Ebro Delta, Spain. Photograph: Jean-Claude Carton/Biosphoto

“We are still researching what it eats in winter when it’s in freshwater but at sea it seems to live mainly on prawns,” says Barberá.

“It devours everything, and the catch of local crabs and cockles has fallen as a result,” says Joan Balagué, head of the fisherman’s association in Sant Carles de la Ràpita on the Ebro Delta. It has reached a point where blue crab is often the only catch. They destroy conventional nets and are fished with cages.

Barberá and Balagué both agree that eradication is not an option and that intensive fishing is the only way to control the species, one of several that have invaded the region in recent years.

“Here in Alicante we have made a study of invasive species in the Mediterranean,” say Barberá. “Various species from the Red Sea were introduced via the Suez canal but some species are also entering through the Straits of Gibraltar because of climate change, not only because the water is warmer but because the currents are changing.”

The blue crab has also found a home for itself on the southern shore of the Mediterranean, particularly off the coast of Tunisia.

The creatures first appeared there in 2014 and wreaked havoc among the local species, as well as destroying fishing nets. The fishermen appealed to the government for help and a scheme was introduced to provide them with cages and to subsidise the value of the catch until they could develop a market.

According to the ministry of agriculture, in the first seven months of last year, Tunisia produced 1,450 tonnes of blue crab worth around €3m (£2.5m). Much of the catch is frozen and shipped to Asia, and it is now so valuable that fishermen are careful not to overfish the crab and deplete the stock.

Aerial view of Buda island in the Ebro Delta nature reserve, Tarragona. Photograph: Joan Gil/Alamy Stock Photo

In the Ebro Delta they are catching about one tonne of crabs every day, according to Balagué. In 2018 they netted 53 tonnes with a value of €136,000. In January of this year alone they landed 12 tonnes, more than the total catch for 2017.

“Even though we’re catching close to a tonne a day, the price is stable and in fact higher than last year,” he says. The crab is becoming more popular, especially in paella and other rice dishes – the signature dishes of the delta and Valencia where most of the crabs are found.

So, is the blue crab a curse or a blessing? “The crab is here, conditions for its success are very good,” says Balagué. “It has become another resource and there’s a market for it so it’s profitable for fishermen. As fishermen we have to adapt and make the most of what we can get.”