For just over ten years, the coasts and bays of Menorca have hosted a visitor who seems to have arrived to stay and who opens a new challenge for the conservation of local flora and fauna. It’s about the blue crab (callinectes sapidus), a giant crustacean native to the coasts of the western Atlantic, which has been fascinating biologists around the globe for years due to its ability to adapt and voracity. This predator combines aggressive behavior with high fecundity: a single female can release up to two million larvae in a reproductive season – which usually occurs during the warmer months -, so it is capable of quickly colonizing new environments, as has happened in the Mediterranean, where it has found warm waters and suitable habitats to establish itself as an invasive species.
The blue crab, also known as the blue crab, has a life cycle that includes migrations between fresh and salt waters, so areas of Menorca such as the Es Grau lagoon, Mongofre or the brackish waters of the Son Bou torrent are ideal for its reproduction. According to experts, this crustacean is a ferocious predator and scavenger that competes for food and shelter with native species, which makes it “an environmental challenge” that alters ecological balances and demands management strategies based on scientific knowledge.
“The arrival and proliferation of the blue crab in Spain, and particularly in Menorca, is explained by a combination of natural and anthropogenic factors. It was documented for the first time in the Mediterranean on the Turkish coast between 1935 and 1945 and, since then, its expansion towards the northwest began and intensified greatly in the last decade,” explains scientist Aina Blanco Magadán in dialogue with elDiario.es. who together with Dr. Raul Triay Portella have been developing the research project “Habitat suitability and resource exploitation of the Blue Crab in Menorca” for several years.
According to Blanco, in Spain, the presence of this crustacean with blue limbs and considerable dimensions was recorded starting in 2012, especially in the Ebro Delta, where it has shown rapid growth. In the case of Menorca, “it has the potential to become a key point for the expansion of the blue crab in the Mediterranean, since the marine currents that connect areas such as the Ligurian Sea, Corsica and Sardinia, together with areas of larval accumulation such as the Gulf of Lion, could facilitate the dispersal of this species from the island to other territories.”
But how is it possible that a crustacean native to the Gulf of Mexico reaches the Mediterranean and settles on its coasts, overwhelming local species? For researchers Aina Blanco and Triay, the keys could be two: on the one hand, the reproduction characteristics of this animal and, on the other, marine currents. “We must consider that the larval period of this species is 6-8 weeks, a period in which they travel long distances within the Mediterranean Sea basin transported by sea currents, guided by salinity differentials in the water, settling after this period in humid areas such as deltas, lagoons, lagoons and torrents.”
In that same sense, experts consider that understanding how and, above all, where these crustaceans reproduce is key to controlling their reproduction. In the case of Menorca, the investigation is ongoing. “We are still in the initial stages to fully understand its dynamics in Menorca, but without a doubt controlling the areas where the females could lay their eggs is presented as a key possibility to limit their reproduction and prevent the island from acting as a center of spread to other parts of the Mediterranean,” they point out.
“It devours everything, leaves nothing and reproduces very quickly”
While scientists and researchers throughout the Mediterranean basin gauge the impact and scope that this invasive species can have on marine, coastal, and island ecosystems, some territories are already experiencing its serious consequences. For several years, groups of Tunisian fishermen from Djerba, Gabès and the island of Kerkennah have been denouncing to the Food and Agriculture Organization of the United Nations (FAO) the degree of devastation that the blue crab generates in their nets. “It devours everything, leaves nothing and reproduces very quickly,” say associations of artisanal fishermen on the North African coast in a report published by Guardian in 2019.
However, some fishermen have managed to turn catastrophe into opportunity. According to the British media, one of the ways to control the plague that Tunisia has adopted in recent years has been to encourage the massive capture of this species and transform the local market by directing it towards the export of this crustacean, highly valued in North America and some. Asian countries.
“These creatures first appeared in 2014 and wreaked havoc on local species and destroyed fishing nets. The fishermen asked the Government for help and a scheme was introduced to provide them with cages and subsidize 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 tons of blue crab worth around 3 million euros. Much of the catch is frozen and sent to Asia, and is now so valuable that fishermen are careful not to overfish the crab and deplete the stock,” comments the British newspaper.
Impact on Menorca
Regardless of its market possibilities and outside the economic question, all experts agree that the arrival of this crustacean represents a significant threat to the ecosystems of Menorca, although specific knowledge about its impact on the island remains limited. “Although we still do not have detailed data on the abundance of their populations or their specific diet in Menorcan habitats, our project, among other objectives, seeks to determine the specific diet of the blue crab in Menorca, since this lack of “This information makes it difficult to accurately evaluate how it is affecting the local flora and fauna, although experiences in other parts of the Mediterranean alert us to its destructive potential,” explains Aina Blanco.
The expert recognizes that, despite the incipient nature of the research, some emblematic species of Menorca such as the green crab (carcinus aestuarii), already in decline, and the critically endangered European eel (anguilla anguilla), could be especially at risk. against the new invading arthropod. “The blue crab could displace the green crab through direct competition and prey on juvenile stages of the eel, further altering the delicate ecological balance. Furthermore, the quality of Menorca’s coastal habitats, rich in wetlands and shallow bays, could offer ideal conditions to maintain stable blue crab populations. However, this aspect requires a more detailed analysis,” the scientist emphasizes.
The blue crab could displace the green crab through direct competition and prey on juvenile stages of the eel, further altering the delicate ecological balance.
Aina Blanco
— Researcher
Despite the relative success of the strategy of mass harvesting of blue crab and its eventual contribution to some local economies, there is no scientific evidence to support the idea that only fishing for more crab will reduce the population and, therefore, it will be possible to maintain environmental balance and protect native species. This is, partially, one of the thesis put forward by Paula Tummon FLynn, a member of the investigation on the blue crab “Biological Invasions” published this year by Harvard University. According to Flynn and his team, this practice could generate what is known as the “Hydra Effect”, that is, the response of the invasive species to overfishing is to release even more larvae. “Intensive extraction of individuals can trigger a compensatory response in the species. For example, reducing population density could reduce competition for resources, favoring the survival and growth of juveniles,” the study states.
In that same sense, researchers Blanco and Triay consider the challenge of the blue crab in Menorca. “It is essential to reflect on this background before implementing intensive fishing as a management measure. Any strategy must be accompanied by systematic monitoring of populations and an analysis of its long-term effects. Without this evaluation, we run the risk of applying measures that may be counterproductive, exacerbating the proliferation of blue crab instead of controlling it,” they explain. And they add: “the context of the island offers a unique opportunity to approach it as an open-air laboratory. The humid areas of Menorca, where the crab can proliferate, are few, small and accessible, which makes them ideal places to carry out experiments that allow us to discern which management strategy best adapts to our local reality.”
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