Invasion dynamics and management of the Atlantic blue crab (Callinectes sapidus) in a European salt marsh: Evidence from the Bay of Cádiz, Spain

Biological invasions are reshaping coastal ecosystems, yet invader performance in aquaculture-influenced salt marsh habitats remains poorly resolved. We conducted an 18-month survey (January 2023-July 2024) at two contrasting salt marsh sites in the Bay of Cádiz, Spain, to compare the invasive Atlantic blue crab (Callinectes sapidus) with the native European green crab (Carcinus maenas). Monthly fyke-net sampling was paired with concurrent prey assessments and water-column measurements. We captured 168 C. sapidus and 1086 C. maenas individuals. Mean abundance (individuals net^-1 24 h^-1) was higher for C. sapidus at the Fish site (1.83 ± 0.36) and for C. maenas at the Oyster site (16.28 ± 1.91). Biomass (ash free dry mass, g) was higher for C. sapidus at the Fish site (11.63 ± 2.08 vs 4.58 ± 1.15 at the Oyster site) and for C. maenas at the Oyster site (10.53 ± 1.22 vs 1.12 ± 0.43 at the Fish site). Body sizes mirrored these patterns (C. sapidus at Fish site: +14.5 mm carapace width, +5.9 mm carapace length relative to Oyster site; C. maenas at the Oyster site: +2.7 mm carapace width, +2.3 mm carapace length versus the Fish site; p < 0.05). Seasonal peaks differed (C. sapidus: late spring-early summer; C. maenas: winter-spring), with a winter non detection of C. sapidus. Linear mixed-effects models showed that C. sapidus abundance increased under warmer, macroinfauna-rich conditions, whereas C. maenas was more abundant in cooler, more turbid waters with higher availability of fish and hyperbenthic prey. These patterns suggest thermal-prey pathways in aquaculture-modified salt marshes and provide cues for targeted monitoring, removal, and future management.