Impact of sheep grazing on juvenile sea bass, Dientrarchus labrax L., in tidal salt marshes

摘要

The diet of young of the year sea bass, Dicentrarchus labrax L., from sheep graxed and ungrazed tidal salt marshes were compared qualitatively and quantitatively in Mont Saint-Michel Bay. In areas without grazing pressure, the vegetation gradient changes from a pioneer Puccinellia maritima dominated community at the tidal flat boundaries through a Atriplex portulacoides dominated community in the middle of the marsh to a mature Elymus pungens dominated community at the landward edge. The A. portulacoides community is highly productive and provides important quantities of litter which provides a habitat and good supply to substain high desities of the detrivorous amphipod Orchestia gammarellus. In the grazed areas, the vegetation is replaced by P.maritima communities, a low productive grass plant, and food availability and habitat suitability are reduced for O. gammarellus. Juvenile sea bass colonise the salt marsh at flood during 43% of the spring tides which inundate the salt marsh creeks. Theyforage inside the marsh and feed mainly on O. gammarellus in the ungrazed marshes. In graxed areas, this amphipod is replaced by other species and juvenile sea bass consume less food from the marsh. This illustrates a direct effect of a terrestrial herbivore on a coastal food web, and suggests that management of salt marsh is complex and promotion of one component of their biota could involve reductions in other species.