Factors affecting seasonal variations in demersal fish assemblages at an ecocline in a tropical-subtropical estuary

Seasonal changes of fish species composition in terms of biomass, density and number of species in three areas of the main channel of the Paranagua 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(-1) and 34 kg ha(-1), 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 Paranagua 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 < 0.01) was the most important environmental variable, responsible for structuring patterns of fishes assemblages in the west-east axis of Paranagua Estuary. During the end of the dry season, both salinity (P < 0.01) and dissolved oxygen (P < 0.05) 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 ecocline-determining environmental factors. This behaviour is suggested as a strategy to avoid competition and predation during the rainy season in the middle estuary. The Paranagua 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. (C) 2008 The Authors Journal compilation (C) 2008 The Fisheries Society of the British Isles