O presente estudo investigou a biologia reprodutiva do bagre amarelo Cathorops spixii (Agassiz, 1829) na Baía de Pinheiros, litoral do Paraná, Brasil. Os peixes foram coletados mensalmente no período de junho de 2003 a maio de 2004, com rede de arrasto de fundo. De cada exemplar foram tomados os dados de comprimento total, peso total, sexo e peso da gônada. As seis fases de desenvolvimento ovocitário (ovogênese) e os cinco estádios de desenvolvimento ovariano (escala de maturidade) foram determinadas por análises microscópicas dos ovários. Através dos valores individuais e media mensais do Índice gônado-somático (IGS) tratado para sexos separados, da freqüência relativa dos estádios de desenvolvimento ovariano e das análises histológicas durante o período de estudo foi possível determinar que o período reprodutivo de C. spixii dá-se entre os meses de setembro e novembro. Fêmeas apresentando ovários desovados ou em recuperação foram registradas nos meses de novembro a abril, com altos percentuais no período de dezembro a março. A desova foi caracterizada como sendo do tipo total.

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The construction of dams causes changes in river variables, as a result of direct changes in their hydrological and biogeochemical cycles. One of the most notable changes is the flow regulation, which reduces seasonal events and the hydrostatic pressure exerted by freshwater, increasing the saltwater wedge intrusion into the system. Changing the salinity regime causes modifications in downstream ecosystems as well as in the distribution and composition of the fish fauna. In Brazil, the São Francisco River stands out, which has a system of cascading dams, built between the 70’s and 90’s. Because of these changes caused in the natural course of the river, this study aimed to analyze the patterns of composition and occurrence of the ichthyofauna at the mouth of the São Francisco River and relate them to the physical and chemical variables of the region. In order to evaluate the patterns of composition and occurrence of the fish fauna at the mouth of the São Francisco River, monthly trawls were conducted along the bank and physical and chemical variables were analyzed in the river channel over a period of one year. The relationship between abundance and species richness with environmental variables was verified using Generalized Linear Models. A total of 101,958 fish belonging to 87 taxa were caught, with emphasis on marine fish, both in number of individuals (99.92%) and in biomass (99.31%). A spatial gradient was detected, in which sites 1 and 2 were under marine influence, sites 3 and 4 represented the transition between the environments and site 5 was under the influence of brackish and freshwater. In general, the effect of the São Francisco River dams on the fish fauna was observed, with a predominance of fauna with more estuarine and less freshwater characteristics.

The length–weight relationships (LWR) are important tools for ichthyology, they are useful to demonstrate the environmental influence on growth. In this study we estimate the LWR of the Atherinella brasiliensis in the shallow areas of the São Francisco river estuary and compare the growth of this specie in different environments of the Brazilian coast. Specimens of Brazilian silverside (A. brasiliensis, n = 3483), between 2.1 and 13.6 cm in length, were caught in the shallow areas of the São Francisco River estuary between May 2017 and April 2018. The length-weight ratio of the species was represented by the equation y=0.0047x3.0913. Frequency distribution indicates that the species is present in the shallow areas of the São Francisco River estuary in all its ontogenetic stages and apparently reproduces throughout the year with constant input of recruits into the population. Von Bertalanffy’s growth model and longevity were adjusted to reach asymptotic length (L∞) of 14.7 cm, growth coefficient (K) of 0.91 year-1 and longevity (A95) of 3.25 years. The analysis of variance revealed differences for the maximum total length. However, the differences found between the present study and data available in the literature appear to be caused by environmental and methodological differences.