Effets directs et indirects de la pêche sur des groupes trophiques de poissons démersaux, définis par une approche écomorphologique

Functional groups dynamics in marine communities help understanding how perturbations impact the functioning of an ecosystem. It is notably expected that functional groups respond differently to fishing pressure owing to their direct exposure to fishing and interactions between groups (indirect effects). A method was developed, based on an ecomorphological approach focusing on predation functions, to define size- and taxonomy-driven functional groups in demersal fish communities from the Celtic Sea and the North Sea. Eleven functional traits were used to define 12 functional groups in Celtic Sea (based on measurements on 930 individuals from 33 species) and seven groups in North Sea (710 individuals from 22 species). In the Celtic Sea, the functional groups better explained diet than size groups whereas diet in North Sea appeared rather size-driven. Time-series of functional group biomasses were assessed using ground-fish surveys data. Fishing seems to unequally affect different ecological functions in both communities. Furthermore it was shown that (i) fishing effects propagate from high to low trophic levels (top-down effects) in North Sea and (ii) ecological compensation may occur within functional groups in Celtic Sea. The functioning of both communities appeared impacted by fishing pressure. Nevertheless, the trophic functions appeared more affected by fishing in the North Sea than in the Celtic Sea.