The present study describes the seasonal changes of the fish species composition in three areas of the main channel of the Caete´River estuary, Brazil. The fish faunas of each habitat differed in density, biomass and species composition. Mean fish density and biomass for the Caete´River estuary channel was 0Á25 individuals m À2 and 0Á9 g m À2 respectively. Analysis of catch data showed that the number of species, total density and total biomass differed significantly between areas and seasons. For the most important species, the mean density of Cathorops spixii, Aspredinichthys filamentosus, Aspredo sp. 2, Pimelodus blochii, Pseudauchnipterus nodosus and Macrodon ancylodon, differed significantly between seasons while the mean density of Stellifer rastrifer, Stellifer microps, Aspredo aspredo, Aspredo sp. 1 and Cynoscion acoupa did not. The mean biomass of these species, with exception of S. microps and Aspredo sp. 1, also differed significantly between seasons. In the Caete´estuary seasonal salinity fluctuations appeared to be the main factor that structured the fish assemblage in the entire estuarine system. At least 85% of the species captured by the artisanal and subsistence fisheries in the Bragantine region required estuarine conditions to complete their life cycle. # 2005 The Fisheries Society of the British Isles

Seasonal changes of fish species composition in terms of biomass, density and number of species in three areas of the main channel of the Paranagu´a Estuary (axis east-west) are described in relation to seasonal fluctuations in salinity, water temperature and dissolved oxygen in the main channel. Two hundred and thirty-four samples were collected monthly, between July 2000 and June 2001, in the main channel. Seventy-nine species of 29 families were captured with a total estimated mean density and biomass of 1513 individuals ha À1 and 34 kg ha À1 , respectively. The number of species and total mean density differed significantly among areas and seasons, but the total mean biomass differed only significantly throughout the ecocline (areas) of the Paranagu´a Estuary. For the most abundant species, the mean densities of Stellifer rastrifer, Aspistor luniscutis, Menticirrhus americanus, Sphoeroides testudineus, Cynoscion leiarchus and Symphurus tesselatus (with the exception of Cathorops spixii and Genidens genidens) differed significantly among seasons. The mean biomass of these species, with the exception of G. genidens, S. rastrifer, A. luniscutis and S. testudineus, also differed significantly for the factor seasons. Area was a significant factor for the eight most abundant species (density and biomass), except S. testudineus (density), G. genidens, C. leiarchus and S. tesselatus (biomass). The season v. area interaction term was significant for C. leiarchus (density). Most of these differences occurred during the rainy season when fishes concentrated principally in the middle of the estuary, where the salinity remained stable. It is suggested that the salinity stability in the middle of the estuary is the main reason why the most estuarine resident fish species move downstream and remain there, regardless of the increased freshwater runoff. Moreover, canonical correspondent analysis output detected that during the late rainy season, the variable dissolved oxygen (P < 0Á01) was the most important environmental variable, responsible for structuring patterns of fishes assemblages in the west-east axis of Paranagu´a Estuary. During the end of the ‡Author to whom correspondence should be addressed. Tel. and fax: þ55 8121268225; dry season, both salinity (P < 0Á01) and dissolved oxygen (P < 0Á05) were responsible for this ecological feature in the estuary. Finally, it was possible to detect that juveniles and adults of some important species respond differently to seasonal fluctuations of the ecoclinedetermining environmental factors. This behaviour is suggested as a strategy to avoid competition and predation during the rainy season in the middle estuary. The Paranagu´a Estuary did not fit with the pre-existing models described in the tropical and subtropical estuarine fish literature since its main channel fish assemblages remained within its bounds even during the rainy season.

The present study investigated the assemblage of fish species in an intertidal mangrove forest during high tide in a macrotide region. We describe the seasonal changes in the fish assemblage composition in relation to biomass, density, and species number in tidal creeks of the Furo do Meio, Caeté Estuary, Brazil. A total of 29 107 individuals of 49 species in 26 families were caught using a block net. Their total weight was 526 kg (total density 0.11 ind. m -2 and total biomass 2.1 g m -2). Analysis of the catch data showed that the number of species varied significantly between creeks, and that total fish biomass differed significantly between seasons. The densities and biomass of the 2 most important species, Cathorops pleurops and Colomesus psittacus, were significantly different between seasons. The densities and biomass of C. pleurops, Pterengraulis atherinoides, Pseudauchnipterus nodosus, and Stellifer naso showed significant temporal differences. Significant differences between creeks were observed in the density and biomass of Anchovia clupeoides and Rhinosardinia amazonica. The abundance-biomass comparison (ABC) plots for the fish fauna in the creeks of the Furo do Meio showed that the dominant species increased in number and weight at the beginning of the rainy season. As a result of increased rainfall in March and April, salinity declined to values between 6 and 8 psu. At that time, the dominant species made up more than 60% of the total biomass and density and Hill's index of diversity (N1) declined, whereas the number of species (N0) and evenness (E2) did not change. After April, rainfall decreased, and density and biomass returned to levels similar to those before the rainy season. The number of species and the density and biomass in the mangrove tidal creeks are compared with published data for other tropical and subtropical estuaries. Migration trends were inferred from the results of the seasonal fluctuations of density and biomass of the most important species in the Furo do Meio, and are compared with data from other studies in the main channel of the Caeté Estuary.