The blue crab (Callinectes sapidus), a species native to the Western Atlantic coasts, has been present in Europe since the early 20^th century. During more than a hundred years, it was not considered a severe environmental problem, but in recent decades it has expanded exponentially across Europe and North Africa, often reaching very high abundances in occupied areas. However, there was barely any solid evidence of the environmental impacts of this invasion in the scientific literature. A study, led by the Doñana Biological Station – CSIC and the Ebro Delta Natural Park and published in the journal Marine Pollution Bulletin, has just confirmed the worst-case scenarios.

The scientific team described the blue crab expansion in the Ebro Delta by compiling data on the presence of the species, from both direct observations and interviews with fishermen. They also analysed long-term (at least one decade) information on the abundances of the blue crab and other species, from monitoring programmes, traditional fisheries in coastal lagoons, and fisheries statistics. "All these analyses allow us to assess possible impacts of the blue crab on different fish and crustacean species", explains Miguel Clavero, a researcher at the Doñana Biological Station – CSIC.

The blue crab was first detected in 2012 in La Tancada, one of the biggest lagoons in the Ebro Delta. After five years, around 2017, the blue crab started a rapid expansion and its abundance increased exponentially, although it seems to have slowed down since 2020. Nowadays, the blue crab is present in every aquatic ecosystem in the Ebro Delta, within its two bays, the surrounding marine waters and along the Ebro River up to the Xerta Dam, an insurmountable barrier situated some 30 km upstream from the Delta plain. 

"The irruption of the blue crab has caused important declines of different species, including some globally threatened species, such as the Iberian toothcarp and the European eel", points out Clavero. There was a continuous decline in the abundance of the Iberian toothcarp (Aphanius iberus) while that of the blue crab increased. In 2020, this species reached its minimum values along the whole series. The European eel (Anguilla anguilla) has also dropped to its historically lowest abundances. This species endured a strong historical decline with a drastic decrease since the blue crab's inrush. The negative impact of the new invasion was prominent on other fish species. However, the mummichog (Fundulus heteroclitus) is the only species whose abundance has increased along with the blue crab's population. This is not surprising, having in mind that the mummichog is native to America and shares native habitat with the blue crab.

The Mediterranean green crab (Carcinus aestuarii), which was the dominant crab in the marine and saline waters in the Ebro Delta until the blue crab's arrival, suffered the most important decline. "After the irruption of the blue crab, the Mediterranean green crab has almost disappeared in the Ebro Delta. It is remarkable that the green crab abundance started to diminish when that of the blue crab had just started to increase", explains the researcher. Analysis of monthly fish market landings shows an astonishing temporal coincidence between the disappearance of the green crab and the first commercial captures of the blue crab.

The blue crab has become a keystone species in the Delta Ebro and has rapidly and severely changed the characteristics of aquatic communities. Impacts such as those described in this study are likely to affect other species not studied in this work, including threatened fish and molluscs and species with commercial value. It is predictable that blue crab will have an important transforming role in other Iberian coastal wetlands, such as the Albufera in Valencia, the lower Guadalquivir River, or the Ria Formosa. More research will be needed to quantify the impacts of this species and find tools to prevent or mitigate its impacts.

Nowadays, there are no effective management measures to control or limit the expansion of invasive populations of blue crab. As a preventive measure, intensive trapping strategies could be implemented in places with special interest for conservation (e.g. the most important Iberian toothcarp nuclei) or economics (e.g. intensive bivalve farms). 

Reference:

Miguel Clavero, Nati Franch, Rubén Bernardo-Madrid, Verónica López, Pere Abelló, Josep Maria Queral, Giorgio Mancinelli. Severe, rapid and widespread impacts of an Atlantic blue crab invasion. Marine Pollution Bulletin. https://doi.org/10.1016/j.marpolbul.2022.113479

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