Determining how assembly rules (e.g. limiting similarity, environmental filtering and neutrality) shape community structure along environmental gradients and across spatial scales is still controversial. The study of functional relationships between coexisting species may help to disentangle among these assembly rules. Here, we compared pairwise functional dissimilarities between fish species to their corresponding pairwise co-occurrences. Fish assemblages (n  835) were sampled monthly in Patos-Mirim system (Brazil) using both bottom trawling and beach seining. Species occurrences were recorded and functional traits related to locomotion and food acquisition were measured on several individuals from each species. The region studied was divided in two areas corresponding to each side of a floodgate located in São Gonçalo Channel: a freshwater channel up the floodgate and, down the floodgate, the Patos Lagoon estuary. The relationship between functional dissimilarity and co-occurrence between species pairs was assessed using quantile regressions for each month and at different spatial scales. Overall, quantile regression coefficients between functional dissimilarities (either based on locomotion types or feeding habits) and co-occurrence values were negative, suggesting that co-occurrence increases with functional redundancy regardless of spatial scale. Our results support the assumption that environmental filters have more influence than biotic interactions on the structure of fish assemblages even locally.

Taim Hydrological System is a unique subtropical wetland in southern Brazil harboring an exceptional biological diversity. In 1978, an ecological reserve was established to protect part of this area from increasing anthropogenic impacts in its surroundings. Fishes have been poorly investigated in this system. Based on a multi-gear sampling study encompassing several years (2001)(2002)(2003)(2004)(2005), and on comparisons with previous fish inventories, we provide an up-to-date species list of fishes occurring in the Taim Wetland. In addition, we made the first preliminary description of fish dominance patterns found in the main lakes of the system (Flores, Nicola, Jacaré and Mangueira). Checklist comparison resulted in 62 fish species distributed in 24 families, with Characidae (19 species) and Cichlidae (7) showing the highest species richness. Six species are cited for the first time in the reserve: Trachelyopterus lucenai (Auchenipteridae), Hoplosternum littorale (Callichthyidae), Rineloricaria cadeae (Loricariidae), Eigenmannia trilineata (Sternopygidae), Odontesthes mirinensis and O. perugiae (Atherinopsidae). Apparently, the black catfish T. lucenai invaded the system in the last decade and became one of the dominant species in the pelagic waters. Although differences in gears hindered direct comparisons, differences in species composition and dominance patterns between shallow margins and pelagic waters of lakes seem to occur in the lakes. A more diverse assemblage dominated by small fishes (< 5cm) occurred in the protected, vegetated lakes margins. In contrast, an assemblage with lower species richness comprised of larger fishes (> 15 cm) seemed to dominate in pelagic waters.O Sistema Hidrológico do Taim é um banhando subtropical localizado no sul do Brasil que abriga uma diversidade biológica excepcional. Em 1978, uma reserva ecológica foi estabelecida para proteger parte dessa área do crescente impacto de ações antrópicas nos seus arredores. Os peixes tem sido muito pouco investigados nessa região. Baseado em coletas com vários amostradores durante vários anos (2001)(2002)(2003)(2004)(2005), e em comparações com estudos prévios, esse trabalho fornece uma lista de espécies atualizada dos peixes ocorrendo no Taim. Também é apresentada a primeira descrição preliminar dos padrões de dominância nos principais lagos que compõem o sistema (Flores, Nicola, Jacaré e Mangueira). A análise comparada indicou a presença de 62 espécies de peixes distribuídas em 24 famílias, sendo Characidae (19 espécies) e Cichlidae (7) as famílias com o maior número de espécies. Seis espécies são registradas pela primeira vez na reserva: Trachelyopterus lucenai (Auchenipteridae), Hoplosternum littorale (Callichthyidae), Rineloricaria cadeae (Loricariidae), Eigenmannia trilineata (Sternopygidae), Odontesthes mirinensis e O. perugiae (Atherinopsidae). Aparentemente, o penharol T. lucenai invadiu o sistema na última década e tornou-se uma das espécies dominantes nas zonas pelágicas das lagoas. Embora diferenças nos amostradores não p...

The communication between the Patos and Mirim lagoon systems occurs via a natural channel called São Gonçalo. In 1977, a dam was built in this channel to prevent entrance of saline waters from Patos Lagoon estuary into the Mirim Lagoon. Our results showed an abrupt discontinuity in salinity and fish species distribution along the study sites. Sites below the dam showed salinity values higher than zero, whereas sites above had zero salinity values across all sampling periods. Marine and estuarine fishes (e.g., mullets Mugil platanus, M. curema, silversides Odontesthes argentinensis, Atherinella brasiliensis, sardine Brevoortia pectinata, and white croaker Micropogonias furnieri) were not captured above the dam. If these juvenile fishes could enter the Mirim lagoon in greater numbers, they probably would enhance catches in the artisanal fishery. We hypothesized that the São Gonçalo dam acts as a barrier hindering the entrance of salinity water and fishes inside the Mirim Lagoon.A comunicação entre o sistema lagunar Patos-Mirim ocorre através de um canal natural denominado São Gonçalo. Em 1977, foi construída uma eclusa com o objetivo de evitar a entrada de água salgada, proveniente do estuário da Lagoa dos Patos, para o interior da Lagoa Mirim. Os resultados mostraram uma descontinuidade abrupta na distribuição da salinidade e dos peixes ao longo da área de estudo. Estações de coleta abaixo da barragem tiveram valores de salinidade acima de zero, enquanto os valores foram iguais a zero nas estações acima da barragem durante o período amostrado. Peixes marinhos e estuarinos (p.ex., tainhas Mugil platanus, M. curema, peixes-rei Odontesthes argentinensis, Atherinella brasiliensis, sardinha Brevoortia pectinata) não foram capturados acima da barragem. Caso entrassem em abundância na Mirim tais espécies poderiam contribuir positivamente para os desembarques da pesca artesanal nessa região. Dessa forma, é sugerida a hipótese de que a barragem do São Gonçalo atua como uma barreira impedindo a entrada de águas salobras e peixes marinhos e estuarinos para o interior da Lagoa Mirim.