Factors influencing the abundance of European green crab Carcinus maenas: combined effects of temperature, habitat and predator release

The European green crab (Carcinus maenas) ranks among the top 100 most harmful invasive species worldwide. Due to its destructive impact on coastal habitats, it reduces the resilience of vulnerable fish populations. The Atlantic cod (Gadus morhua) is a relevant case because the species depends on complex habitats like eelgrass meadows, mussel reefs and artificial habitats as juvenile nursery areas. This study investigated factors driving Carcinus maenas abundance across different habitats under varying temperature conditions and local cod abundances. Underwater cameras were deployed across five years in several marine areas. We found that Carcinus maenas preferred restored boulder reefs and sandy seabed, while the abundances were lower at both natural and restored cobble reefs. This pattern was evident across several water temperatures, with peaking abundances between 12.5 °C and 16.6 °C, and plateauing abundances at higher temperatures. Importantly, the highest abundances were observed mainly in the absence of cod, while higher cod abundances were associated with lower Carcinus maenas abundances. This is likely caused by direct and indirect top-down effects of cod on mortality and activity. Given global warming and overexploitation of key predators, such as Atlantic cod, the habitat degrading effects of C. maenas will likely increase in the future, with detrimental effects for many coastal ecosystems. However, this scenario may be mitigated by rebuilding high abundances of key predators and ensuring top-down regulation of C. maenas populations. This requires that fisheries management accounts for interactive ecosystem effects, occurring between fish populations and marine habitats, at several levels of organization.