Carvalho, G.F. Characterization of the ichthyofauna of the shallow channels and tidal pools of the Ria Formosa’s salt marshes.

The aim of this study was to identify the ichthyofauna of the shallow channels and tidal pools of the Ria Formosa’s salt marshes, and to that end monthly samples were taken using a Riley push net in 6 sites between May 2001 and April 2002. The sites selected aimed to represent the diverse microhabitats found in the study area, namely the types of substrata (seagrass or unvegetated soft substratum) and the extent of a channel’s isolation. The habitats studied were of considerable importance for the ichthyofauna. Forty-nine species (a further 2 during preliminary samples) were caught, of which several were of protected status and/or of commercial importance. The Shannon diversity index was relatively low across the whole study (H’=0.97; E=0.17), principally due to the great abundance of a particular species Pomatoschistus microps. Multivariate analysis using the Bray-Curtis coefficient demonstrated the high similarity between all studied months. Those sites which were most isolated and which did not have a substantial seagrass bed (Pn2 and Pn3) were predominant in terms of abundance and biomass, while those sites with a permanent covering of vegetation (Pn1, Pn4 and Pn6) had higher levels of species richness, Shannon’s diversity and evenness. The greater availability of food and the lower risk of predation at the Pn2 and Pn3 sites were probably responsible for their high densities. Significant similarities were observed between communities at sites with identical substrata, with some species revealing a preference for a particular type of substratum (e.g. Syngnathus abaster, Syngnathus typhle - seagrass beds; Pomatoschistus microps - unvegetated soft substratum). The Pn5 site clearly revealed this differentiation between communities of vegetated and non-vegetated substrata. While it initially had no vegetal covering, a substantial seagrass bed developed during October and the winter months, which led to a change in the structure of the fish community: an increase in species associated with submerged aquatic vegetation and a decrease in species associated with the unvegetated soft substratum. This change, as well as the similarities between communities at sites of identical substrata, was also identifiable during the multivariate analysis (Bray-Curtis) and in the comparison of Shannon diversities obtained at the various sites. The habitats studied revealed an important role as nurseries for various species, some of commercial value (e.g. Dicentarchus labrax, Spondyliosoma cantharus, Diplodus vulgaris).