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

The diet of young of the year sea bass, Dicentrarchus labrax L., from sheep grazed and ungrazed tidal salt marshes were compared qualitatively and qua ntitatively in Mont Saint-Michel Bay. In areas without grazing pressure, th e vegetation gradient changes from a pioneer Puccinellia maritima dominated community at the tidal hat boundaries through a Atriplex portulacoides dom inated community in the middle of the marsh to a mature Elymus pungens domi nated community at the landward edge. The A. portulacoides community is hig hly productive and provides important quantities of litter which provides a habitat and good supply to substain high densities of the detrivorous amph ipod Orchestia gammarellus. In the grazed areas, the vegetation is replaced by P. maritima communities, a low productive grass plant, and food availab ility 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. They forage inside the marsh and feed mainl y on O. gammarellus in the ungrazed marshes. In grazed 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 an d promotion of one component of their biota could involve reductions in oth er species. (C) 2000 Elsevier Science Ltd. All rights reserved.