The uncontrolled expansion of blue crabs as an invasive species in the Mediterranean
UA/DICYT The American blue crab (Callinectes sapidus) is an invasive voracious alien species, with no known predators and with high fecundity and survival rates, that has spread throughout the Mediterranean. Since it appeared in the Ebro Delta in 2012, this crab native to the American Atlantic has expanded by sea, rivers and wetlands all over the region of Valencia. Its recurrence and continuous presence is a fact in our coasts.
University of Alicante Marine Research Centre (CIMAR) scientist Carmen Barberá reports that collaboration between the scientific community and management bodies to define control measures has been currently enabled; however, its expansion is faster than research activity and the efforts made by administrations. This crab has potential high fertility and survival rates, which could also be the reason why it has been established in this area successfully.
The traditional fishing fleet is one of the most affected sectors, as the crab accidentally destroys the nets when they are caught. The study was carried out as a result of a request made by the Guardamar Fishermen's Association, as they fish king prawns and cuttlefish with gill nets in the area of Guardamar and Santa Pola during the summer. According to the researcher, one of the measures to control its expansion could be to get a permit to fish with more specific fishing baskets, as happens in the American Atlantic, where the crab comes from. In this sense, they are working with fishermen to try out different types of fishing baskets.
The problem is similar to other geographical points. In La Albufera in Valencia and in Mar Menor the problem is even worse all year round because they also fish in winter.
The University of Alicante-Santa Pola Town Council Marine Research Centre (CIMAR) actively collaborates in the follow-up of the distribution of this invasive species in the Santa Pola Salt Lake Nature Reserve, where they were caught for the first time in 2014. The study on the blue crab is included in the line of research on Follow-up of alien species.
At the moment the study has focused on getting to know the migrations made by the crab from the sea to fresh water and vice versa for reproduction purposes, as the scientist stated. Its behavioural pattern is known in the natural distribution area in coastal and estuarine areas: females move to the sea to lay their eggs during the summer. Eggs require some salinity for the larvae to hatch. In the early Autumn, females migrate with juveniles to freshwater streams. Barberá points out that one of the objectives is to define very well where they are at all times.
In this regard, the CIMAR counts on the collaboration of the Guardamar Fishermen Association, whose traditional fleet accidentally catches crabs at the Segura River mouth, a usual fishing ground for king prawns with gill nets. This crustacean is thought to be one of the crab's favourite prey when they migrate to the sea.
The researcher states that it is difficult to justify how this species has been expanded from the Ebro Delta towards the south, as water currents do not go that way. We have to think more about a human factor, such as the ballast water of ships. In terms of ecological impact, it can be higher in wetlands and salt lakes, as they are more closed ecosystems where any change affects more intensely. However, the socio-economic problem is greater in the sea since it affects fishermen by destroying nets and making them waste days of work.
Aim of study
The aim of the study is to get to know this species’ ecology in these new geographical areas and environments that they occupy as an alien species, where — dangerously — they may not have limiting factors to the growth of their populations, as it can be competitors, predators, temperature, etcetera. To this end, a simultaneous sampling plan has been designed in different connected ecosystems, such as the fresh waters of El Hondo in Elche, wetland, the brackish and hyper-salted water channels of Salt Lake Nature Reserve and the estuarine-marine waters of the Segura River mouth in Guardamar. The results of the study will be published very soon, while researchers continue to work on it. Isabel Esteso, currently working on her Master’s final project on the blue crab, is one of the researchers involved. In her study, among others, she collects data such as the measurement of water temperature where the crabs are found, as well as oxygen, salinity and pH levels.
Researchers know exactly where the crab communities are located as they have been sampling from the Segura River mouth (in Guardamar) to El Hondo in Elche and it has shown up everywhere, as confirmed by the CIMAR scientist.
In order to answer some questions (such as what they eat, who eats them, what the growth limits of their populations are, when and how they are captured more efficiently, how profitable their commercialisation is and whether this would serve as a control measure), CIMAR has requested a national project — Ecology and Impact of the Atlantic Blue Crab in the Spanish Mediterranean Coastal Lagoons and Adjacent Waters (ECESIS) — to the Ministry of Science, Innovation and Universities, a decision that they expect to be published in January 2019.
This national project has also been participated by the Polytechnic University of Valencia and the Spanish Oceanographic Institute (Murcia Oceanographic Centre), and it will be coordinated by researcher Silvia Falco Giaccaglia, from the Polytechnic University of Valencia. Principal researchers are Elena Guijarro García, from the Spanish Oceanographic Institute, Oceanographic Centre of San Pedro del Pinatar, in Murcia); Alfonso Ramos Esplá, from CIMAR-UA; and Francisca Giménez, from the UA. Also, the scientists have the support of the fishing sector to study the ecological and socio-economic impact of the crab proliferation in different wetland ecosystems and marine environments in the regions of Valencia and Murcia. The project is included in a line of R&D Projects on Knowledge Generation and R&D+i Research Challenges, awarded by the State Research Agency of the Spanish Ministry of Science, Innovation and Universities, within the framework of 2018 Calls for Application.
There are many factors to consider. We must not forget that, according to records in CIESM Atlas of Exotic Species in the Mediterranean, it is an invasive species already established in the Mediterranean and any wrong action could further its expansion.
The blue crab was first detected in the province of Alicante, both in Guardamar and Santa Pola in 2014. Two years earlier, in 2012, it was seen in the Eastern Mediterranean, in the Ebro Delta. It has been known in the area of Tunisia, in Turkey, since the 40's of the 20th century. However, this crab is originally from American North Atlantic. In the U.S.A, it is just another fishing species, although there they fish with special fishing gear, as explained by Carmen Barberá. How they came to the Mediterranean is still unknown. Researchers think that the most admissible theory is that they arrived in the ballast waters of ships. However, it is also known that they experimented with their farming in Turkey. The fact is that right now it is all over the Mediterranean and Strait of Gibraltar.
The blue crab is known for being extremely aggressive, with hard short claws and strong jaws, being able to even eat another crab. Researchers have come to the conclusion, by analysing the content in their stomachs, that these crabs eat each other. On the other hand, due to their accelerated metabolism, involving rapid digestion, the analysis of the content is difficult. According to researchers Carmen and Isabel, everything they have digested before the bait is difficult to detect in their stomachs. The scientists also stated that they still have not known whether they are feeding in fresh water or in the sea; it seems that they are feed on king prawns mainly.
Some of these crabs can be more than twenty centimetres in length, from end to end of their legs, and weigh more than 450 grams as an adult. They are hidden in daylight and fish at night. Scientists have not noticed interaction with other target species, but this cannot be analysed easily as fishermen avoid areas where more crabs are found.
They have been found in the sea during the summer and then they migrate towards fresh water in gullies, rivers, the salt lake reserves from Santa Pola to El Hondo in Elche.
While in the area of Alicante ten crabs have, at best, been extracted once caught in the nets, in the Ebro Delta, fishermen are pulling out (using the trawling fishing method) ten kilos of crabs about two hundred grams each every day.
In the area of the Albufera, with brackish water, they stay there in winter, and they move to the marine area during the summer. In Murcia, the crabs are in the Mar Menor, which is hypersaline. In this area, the fishermen are also having a lot of problems, as they fish for king prawns and the nets are very fine. The crabs are located at the river mouth and if they get caught, they break the nets. In the North Atlantic, it is known for sure that they have their predators, which we still have not been able to prove this here.
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