CASE. Blue crab, a powerful invader in Corsica

It arose in the 1990s in the village of Corsica, blue crab was not a concern at first fishermen. On the other hand, since 2020, its expansion is in such a way that it replaces local species up to nullifying the work of professionals. If nothing is done quickly to confront this species that lives in the Americas, the situation in the lagoons risks becoming catastrophic both ecologically and economically.

Pliers powerful enough to crack oyster shells, cut nets and fishing nets.

READ ALSO: The relevance of actions on the ground in the conditions of the invasion of the blue crab in Corsica

A blue carapace surrounded by orange spines, the width of which can reach twenty-three centimeters. Eight pairs of teeth ending in a pair of spikes.

The blue crab is a real disaster, a hyperactive and aggressive master hunter. It is spreading exponentially in the brackish waters of the island, especially in Palu and Bigulia ponds, where it destroys fishing gear and gradually destroys fish and crustacean populations.

It is even considered dangerous to humans because of the power it manages to develop in its claws.

Urgency to act

This exotic animal creates a real problem because it disrupts the biodiversity of the places where it lives. A true Attila of the Lagoon, it hunts brilliantly and adapts perfectly to the areas it colonizes, gradually causing native species to disappear.

In its native habitat, which stretches from Canada to Brazil on the Atlantic coast, it is caught to be eaten. “Its Latin name is Callinectes Sapidus, which means “A good, tasty swimmer,” explains Marie Garrido, project manager at the Corsican Environment Authority, which is responsible for the blue crab in particular. “The individuals we catch in Corsica are larger than those found in America. They can reach 24 centimeters in width and weigh more than 950 grams. His swimming abilities allow him to crawl about fifteen kilometers a day. »

Little by little, he takes over the territory, destroying everything in his path and making the work of fishermen impossible. Corsica offers it the optimal conditions for its development, prompting swift action to counter its evolution before it is too late.

Jean-Louis Guaitella is trying to find a solution for his colleagues to continue exploiting the Bigulia pond.  - Angela Riccardi

“The risk is that a common species will eventually disappear in favor of another that is probably not sustainable,” continues the project manager. Indeed, studies of invasive species tend to show that once an optimal rate of spread is reached, a decline to eventual extinction begins. A phenomenon that remains theoretical and may take about twenty years. However, it is important to act quickly to try to control the number of blue crabs on the island and the damage they cause. Now it is accepted that it will be impossible to eradicate it.

READ ALSO: VIDEO. Corsican blue crab: an invader to appreciate on plates

Observations conducted in the summer of 2022 show that now more than ever, action is needed. And then next spring. Corsica was lucky enough to benefit from the experience of fishermen from Occitania, where in three years the situation deteriorated so much that almost all the professionals dropped anchor for good.

In Corsica, observations made last summer by fishermen, in particular at Bigulia Pond, lead to the opinion that from June 2023 we will find ourselves in a disaster scenario if nothing is done.

A blue crab cuts a fillet with powerful claws.  - Angela Riccardi

Decision on cohabitation

“Today we can consider taking action by practicing targeted fishing in sensitive sectors. In particular, where there are professional fishermen or protected species. These operations must be conducted during key periods of the blue crab’s life cycle.” continues Marie Garrido.

The beginning of a decision and a plan of action that would allow simultaneous scientific work, studying the species and, in particular, its movements, methods of reproduction, its evolution vis-à-vis the environment of origin. All this in close cooperation with local fishermen.

With the help of the appropriate tackle, the crab can be caught, but due to the lack of knowledge on the part of the population, it is almost impossible to sell it.  - Angela Riccardi

For them today, the blue crab is a real plague that destroys their working tools. They can catch crabs, but with appropriate tackle. Machines that are already specified by the FAO (Food and Agriculture Organization of the United Nations), but require investments that today’s professionals are not ready to undertake. First, because they have suffered significant operational losses since the beginning of this invasion and will have a hard time finding the funds they need. Then because, even stocked, they have no confidence that they will be able to sell these crabs.

Ideally, research should be conducted prior to the creation of a large-scale on-site action plan. Unfortunately, time is running out and the point of no return is approaching faster than expected.

Once the crab is entangled, it must be broken to untangle it.  - Angela Riccardi

“It is a priority for us to be able to install strong nets at the entrance and exit of the ponds during the period when they are closed to prevent the movement and capture of animals, says Jean-Louis Guaitella, a professional fisherman in Bigoulia and president of the rate commission of the Regional Committee of Marine Fisheries and Marine Economy (CRPMEM) of Corsica. We need to prevent female smolts from leaving the pond and laying their eggs in saltier water, which is more suitable for larval survival and dispersal.”

With the help of one claw, this crab is able to split an oyster shell.  - Angela Riccardi

A device that can be effective quickly, but also worries scientists about the impact it could have on other species that need to be saved. Today, the debate revolves around funding training, equipment and how to support professionals so they don’t leave the lagoons.

Because if they can no longer live off the produce of their nets, fishermen will inevitably leave the most affected lagoons and leave the sector.

Other areas of research are focused on the development of predators of this powerful crustacean. Especially sea bream and octopus.

READ ALSO: Faced with blue crab infestation, appeal to hobby fishermen

Unfortunately, the presence of the latter in the Mediterranean is increasingly rare.

Eels and green crabs are gradually replaced by blue crabs.  - Angela Riccardi

If at the moment the concern is mainly focused on the Palu and Bigulia ponds, the crab remains very present in the other lagoons of the island. Diana and Urbina seem to be less affected by the higher salinity, but in Sicily callinectes sapidus was able to adapt and live in saltier waters. Some other lagoon areas of Corsica have also been badly affected, such as Balistra, but these are areas that are not fished professionally or are not fished very much. The blue crab is also found in protected areas, where it is difficult to implement means of action and observation.

Therefore, it is necessary, first of all, to preserve the environment and the fishing traditions of the Corsican lagoons, which are exploited.

In the second phase, it will also be necessary to study the species in detail in order to better control it and reduce its spread in the Mediterranean.

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