The demersal fish assemblage of the coastal area adjacent to the Tagus estuary (Portugal): relationships with environmental conditions

Abstract

The subtidal fish assemblage of Tagus estuary coastal area was sampled in order to infer about its relationship to environmental conditions and its possible nursery function for marine species. Fish sampling took place between May 2001 and February 2002 with an otter trawl. In addition to fish sampling, a physical, chemical and biological characterization of the water column and bottom sediment was performed. Density and biomass were determined and ecological and feeding guilds assigned to each fish species. Correspondence analysis (CA) was used to determine the spatio-temporal structure of the assemblage and its correlation to environmental variables. A total of 36 fish species were identified. The fish assemblage was dominated by few species, both in terms of density and biomass. The main fish species were Callionymus lyra, Arnoglossus laterna, Diplodus bellotti, Echiichthys vipera and Raja undulata. Seasonally Trachurus trachurus, Sardina pilchardus and Engraulis encrasicolus were also abundant. Over 50% of the fish caught were juvenile stages. Benthic carnivorous species feeding on invertebrates dominated the assemblage. CA results indicated that distinct fish species occur in the northern and southern areas, although they distance only a few kilometres from each other. Depth and sediment type were the main structuring factors of the fish assemblage. Temperature and salinity showed reduced importance on overall assemblage structure although that may result from the particular climatic regime of the sampling year. The environmental preferences of the most important species are briefly discussed and the importance of integrating various spatial scales in multispecific studies of coastal fish assemblages emphasized. The presence of high densities of marine species juveniles indicates that this area may function as a spring–summer nursery for several species.