The feeding ecology of the Brazilian silverside, Atherinella brasiliensis, in a sub-tropical estuary of Brazil was investigated through the gut analysis of 1431 individuals. We described dietary composition and analysed seasonal, estuarine habitat, and body size variations in the diet; trophic level; feeding diversity; and gut fullness indices. Results reveal that A. brasiliensis is a typical, generalistic and opportunistic predator that makes use of a wide array of prey types (at least 89 different types), with zooplankton (mainly calanoids), diatoms, terrestrial insects, and plant detritus making up the bulk of the overall diet. The exotic calanoid Temora turbinata ranked as the primary prey. A wide feeding diversity (mean H′ = 2.26), low trophic level (mean TROPH = 2.57), and high gut replenishment were persistent across seasons and habitats. Diet composition varied largely and significantly with respect to habitat, season, and body size. A closer assessment showed that habitat and season had a stronger effect on diet than fish size.

Ninety-nine length-weight relationships (LWR) were estimated for 70 bentho-demersal fish species captured in four Brazilian estuaries between latitudes 0°S and 25°S. LWR are published for the first time for 13 species. The allometry coefficient (b) of the LWR (W = aTL b ) showed a median of 3.147. Positive allometry was dominant (64 of the 99 species x estuary cases), followed by isometry (28) and negative allometry (7). Twenty-two species were caught in two or more estuaries. In most cases (29 of 33 comparisons), the values of both regression parameters differed significantly among estuaries.

We described the fish assemblage in the estuary of the Guaraguaçu River (one of the largest tributaries of the Paranaguá Bay Estuary, located within Brazil's Atlantic Forest Biosphere Reserve) from June 2005 to May 2006, and assessed the seasonal and spatial effects of abiotic environmental attributes on the fish assemblage structure. Despite some oscillations in salinity, the upper and lower estuaries had year-round persistent oligohaline and polyhaline conditions, respectively. Despite high species richness (55 species), the Guaraguaçu River Estuary fish community contains a few dominant taxa; 11% of the richness accounts for [60% of its density and biomass. The most abundant species (in terms of both biomass and density) was Atherinella brasiliensis. Species whose densities were most strongly associated with the upper estuary were Centropomus parallelus, Ctenogobius schufeldti, Eucinostomus melanopterus, Platanichthys platana, Trinectes paulistanus, and Eugerres brasilianus. Those whose densities were most strongly associated with the lower estuary were A. brasiliensis, Sphoeroides greeleyi, Eucinostomus argenteus, Sphoeroides testudineus, Diapterus rhombeus, and Harengula clupeola. Throughout the year, canonical correspondence analysis identified: (1) the pattern of horizontal stratification of salinity along the river as being the most important variable for explaining most of the fish fauna structure; and (2) a strong relationship between the fish fauna and the salinity gradient along the estuary. Analysis of similarity further confirmed that each estuarine zone supports a year-round persistent and relatively homogeneous fish species assemblage. Total mean density and biomass remained constant over time in each estuarine habitat, but density shifted in the most abundant species, which appears related to recruitment patterns. Such species and abundance persistence likely occurs because seasonal rainfall-induced changes in river discharge are not sufficient to significantly shift runoff and salinity and thus fish assemblage structure (species composition, density and biomass) along the estuary. Such a lack of seasonal fish fauna movement as a response to changes in river flow contrasts with other estuarine systems around the world.

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This work provides information about the fish assemblage structure along the estuarine gradient of Baía da Babitonga, south Brazil. The seasonal and spatial dynamics of fish and their relationship with physical–chemical variables were investigated. A total of 70,085 fish of 70 taxa were collected. Late larva and early juveniles of Engraulidae, Eucinostomus spp. and Mugil spp. dominated in abundance, representing 62% of all fish captured. Permutational multivariate analysis of variance identified distinct fish assemblages within the bay and during the year. The marine straggler species Harengula clupeola, Oligoplites saliens and Trachinotus carolinus and the estuarine migrant Anchoa tricolor were characteristic of the outer-most bay area, while the estuarine resident and migrant species Atherinella brasiliensis, Anchoa januaria, Sphoeroides greeleyi and Citharichthys spilopterus, and the marine migrant Diapterus rhombeus were characteristic of the inner portion of the estuary. The seasonal changes in community structure observed were mainly related to the greater abundance of T. carolinus in the warm/wet season, Micropogonias furnieri in the transition season and Oligoplites saliens in the cold/dry season. Depth, followed by salinity, explained the greater part of the variability in the abundance of dominant species and was found to be important in shaping the assemblages. Nevertheless, the amount of variation unexplained by the measured abiotic variables was relatively high (73%), suggesting the effect of additional regulatory factors. Many fish species use the shallow waters of the bay in transitory or permanent ways, and knowledge about their relationship with the environment is necessary for the success of conservation strategies for this ecosystem.