Soares, J. Organization of the ichthyologic demersal communities on the main and secondary channels of the Ria Formosa

The organization of the demersal ictiological communities of the Ria Formosa was made, based on monthly sampling of 12 sampling sites with a beam trawl from March to August of 2001. The multivariate analysis of the fish communities using the Bray Curtis coefficient, revealed seasonal and spatial structural patterns of several species. These patterns also seem to occur at abundance and biomass levels and at the values of the indexes used. The Shannon (H’), the Margalef (Dmg) and the Equitability (E) indexes were relatively high for the whole lagunar system, as well as for the majority of the sampling sites. These indexes, the abundance and the biomass peaked in May and in June. Species Relative Importance Index (IRI%) shows a pattern of spatial organization similar to the observed with the multivariate analysis. Factors like the type of channel, anthropogenic influence, type of substrate and its coverage by sea vegetation like Zostera spp. and Ulva lactuca, seem to strongly influence the structure of the communities of the Ria. However, these factors appear to play a secondary role compared with the abiotic variable temperature. The fish assemblage of this sampling period is composed of 63 species, belonging to 22 taxonomic families. Regarding the occurrence regime, Resident species are the most abundant and have generated the largest biomass, followed by the Seasonal species in terms of abundance, and by the Semi-Resident ones when it comes to biomass. Gobius niger Linnaeus, 1758 (Resident) was the most important species and the most widespread. Halobatrachus didactylus (Schneider, 1801) (Semi-Resident) was close by, but only in biomass and occurrence frequency (Foc). Besides the high importance of the seasonal and commercial important species Diplodus vulgaris (E. Geofrey St.-Hilaire, 1817) and Spondyliosoma cantharus (Linnaeus, 1758), it was interesting to observe the close association of these species with the presence of vegetation and with the dawning of Summer. Hippocampus guttulatus (Leach, 1814) is the fifth most abundant species of the Ria Formosa and achieves second place in occurrence frequency. Atherina spp. was a somewhat important gender in some sampling sites, which indicates that the sampling equipment used captured a significant amount of the site’s pelagic populations.