Impacts of the blue crab invasion on Manila clam aquaculture in Po Delta coastal lagoons (Northern Adriatic Sea, Italy)

The blue crab Callinectes sapidus Rathbun, 1896 (family Portunidae) is a native species of Western Atlantic coasts, from Canada to Argentina. Due to its ecological plasticity, it has shown high invasive capabilities outside its native range of distribution and it is nowadays widely distributed in the Mediterranean Sea. Since spring 2023, the number of records and the reported abundance of blue crabs along Italian coasts have been dramatically increasing, raising concerns for the possible impacts deriving from its invasion, particularly in the Northern Adriatic Sea coastal lagoons, a major shellfish production area of Italy. The present paper reports a first assessment of i) blue crab occurrence and spread in two coastal lagoons of the Veneto Po Delta, mainly exploited for Manila clam aquaculture, and ii) the related impacts of blue crab on Manila clam production. Results from two surveys conducted in Summer (July) and Autumn (October) 2023 in Sacca del Canarin and Sacca degli Scardovari showed i) very high occurrence of the blue crab in both lagoons, with higher concentration in Sacca del Canarin, and numerous ovigerous females, especially in Summer, ii) high Manila clam loss up to 100%, with clam shells showing signs of blue crab predation up to 56% and seed absence in natural recruitment areas. The results highlight the need and urgency to establish a structured monitoring plan on a larger scale, both spatial and temporal, to better assess the extent of blue crab invasion, its impacts on ecosystems and on shellfish farming and fishing activities. This is undoubtedly necessary to define the best strategies for blue crab containment and management.