Environmental drivers of juvenile Atlantic blue crabs (Callinectes sapidus) abundance in northern Adriatic

The Atlantic blue crab ( Callinectes sapidus) is a highly invasive species posing significant ecological and economic threats to coastal lagoons in the Mediterranean Sea. Due to its recent spread in the Mediterranean, management is hindered by a critical knowledge gap concerning spatiotemporal variation in abundance of juveniles stages that use lagoons as nursery habitat. This study addresses this gap by providing the first field surveys of juvenile blue crab abundance and its environmental drivers in the northwestern Adriatic Sea. Four coastal lagoons in Northern Italy (Goro, Fattibello, Comacchio, Pialassa Baiona) were sampled in spring and autumn 2024 using a beach seine net designed to capture small crabs, with a total of 343 crabs (5–145 mm CW) collected. Carapace width-weight relationships indicated negative allometric growth. Zero-inflated Poisson models and Random Forest analysis identified refuge availability, salinity, and lagoon-sea connectivity as the key predictors of juvenile abundance. Abundance was highest in areas with refuges (e.g., Phragmites sp. marsh grass areas, artificial substrates) and in lower salinity waters, consistent with the characteristics of nursery habitats identified in blue crab’s native habitats. The Goro lagoon, featuring high connectivity with the sea and refuge availability, was identified as a primary spring recruitment site, evidenced by a peak in small juveniles (<14 mm CW). These results highlight environmental factors that may create "invasion hotspots" providing crucial information for targeted management and control strategies in invaded Mediterranean ecosystems. • Knowledge gap on the abundance of juvenile blue crabs in the Mediterranean is filled • Refuges, salinity and lagoon connectivity are key predictors of blue crab abundance • Highest abundance of juvenile crabs is found in refuges and in areas of low salinity • Goro lagoon is identified as a spring recruitment site • The results help to target the management of invasion hotspots