Composição e estrutura trófica das assembléias de peixes em um trecho do médio rio teles Pires, Mato Grosso, Brasil

The Teles Pires River is located in a historically impacted region in the Tapajós River basin, an extended clear water river in Amazonian basin. Fishes are the most conspicuous vertebrates in its waters and have suffered by anthropic impacts. However, studies on the fish fauna in this region remain rare, and knowledge on the composition and determination of the trophic structure is essential to understand the fish assemblages. Thus, the current work aims to determine the composition of the trophic structure, as well as to analyze the attribute of the food web in the fish assemblages, comparing temporal and spatially, along the hydrological cycle in a stretch of rapids in the middle Teles Pires River, Mato Grosso State, Brazil. Individuals were collected using a set of gillnets of different mesh sizes (from 24 to 120mm), in four field trips from July 2008 to May 2009, including the four different hydrological periods. A total of 1385 individuals from 90 species, 16 families and four orders were captured. The order Characiformes and the family Characidae were predominant in biomass and number of individuals. The stomach contents were analyzed from 990 individuals to determine their diet, trophic structure (richness, biomass and number of individuals) of the assemblage and to analyze the attributes of the food web (number of species in the web, total number of links, linkage density and connectance). Trophic categories were defined by the calculation of the Alimentary Index, combining values of frequency of occurrence and volume used to the diet analysis. Fish species were categorized in seven trophic groups. Herbivorous, insectivorous and piscivorous were the most representative in biomass and number of individuals, which consumed autochthonous resources. Analysis on the attributes of the food web showed a low trophic complexity, and temporal and spatial variations were found on the composition and trophic structure of fish assemblages in biomass, number of individuals and attributes of food web. We suppose these variations may be related especially to the changes on the food resources availability, which were abundant in rising and high seasons in downstream of Cachorro rapids, where values of richness, biomass, number of individuals and attributes of food web were considerable higher.O rio Teles Pires é um dos formadores do rio Tapajós, um dos maiores rios de águas claras da bacia Amazônica, e corre por uma região que historicamente tem sofrido fortes impactos ambientais de origem antrópica. Os peixes, como vertebrados mais conspícuos em suas águas, vêm sofrendo as conseqüências destes impactos. Contudo, o conhecimento sobre esta fauna ainda é insipiente. Assim, a determinação da composição e da estrutura trófica das assembléias de peixes é de fundamental importância. O presente trabalho teve como objetivo determinar a composição e a estrutura trófica, além de analisar atributos de teia alimentar das assembléias de peixes, comparando-as, temporal e espacialmente, ao longo de um ciclo hidrológico em um trecho de corredeiras no médio rio Teles Pires, Mato Grosso. Para isso, os exemplares foram obtidos com uso de malhadeiras em quatro coletas, entre julho de 2008 e maio de 2009, abrangendo os diferentes períodos hidrológicos. Foram capturados 1385 exemplares pertencentes a 90 espécies, distribuídas em 16 famílias e quatro ordens. Houve predominância, em termos de biomassa e número de exemplares da ordem Characiformes e da família Characidae. Do total de exemplares coletados, 990 tiveram os estômagos analisados para se determinar a estrutura trófica (riqueza, biomassa e número de exemplares) e para as análises dos atributos de teia alimentar (número de espécies na teia, número total de elos tróficos, densidade de elos tróficos e conectividade). Os grupos tróficos foram determinados pelo cálculo do Índice Alimentar, combinando os métodos de freqüência de ocorrência e volume relativo dos principais tipos de alimento consumido por cada espécie. As espécies foram classificadas em sete grupos tróficos, dos quais, herbívoros, insetívoros e piscívoros foram os mais representativos quanto à biomassa e número de exemplares, consumindo predominantemente recursos de origem autóctone. A análise dos atributos de teia alimentar mostrou baixa complexidade trófica. Verificou-se que existe variação temporal e espacial significativa na composição e na estrutura trófica quanto ao número de exemplares e proporção de espécies por grupo trófico e em alguns atributos de teia alimentar. Supomos que estas variações podem estar especialmente relacionadas a mudanças na disponibilidade de recursos, que teria maior abundância nos períodos de enchente e cheia e nos locais a jusante da corredeira Cachorro, onde valores de biomassa, número de indivíduos e alguns atributos de teia foram considerados altos

Diet of two syntopic species of Crenuchidae (Ostariophysi: Characiformes) in an Amazonian rocky stream

Abstract This study assessed the diet of two poorly known syntopic fish species of the family Crenuchidae, Characidium aff. declivirostre and Leptocharacidium omospilus, in a Presidente Figueiredo´ rocky stream, Amazonas, Brazil. The stomach contents were analyzed and their Frequency of Occurrence (FO %) and Relative Volume (Vol %) were combined in a Feeding Index (IAi). We examined 20 individuals of C. aff. declivirostre and 23 of L. omospilus. The Morisita-Horn Index was used to estimate the overlap between the diets of these species. Immature insects were the most valuable items consumed by both fish species. The diet of C. aff. declivirostre was mainly composed of larvae and pupae of Chironomidae, while L. omospilus predominantly consumed larvae of Hydroptilidae, Hydropyschidae and Pyralidae. Thus, both species were classified as autochthonous insectivorous. Characidium aff. declivirostre was considered a more specialized species, probably reflecting lower feeding plasticity or the use of more restricted microhabitats compared to L. omospilus. When the food items were analyzed at the family taxonomic level, the diet overlap between these species was considered moderate (Morisita-Horn Index = 0.4). However, a more thorough analysis, at the genus level, indicates a very low diet overlap. Therefore, we conclude that the feeding segregation between C. aff. declivirostre and L. omospilus may favor their co-existence, despite their high phylogenetic closeness