Estrutura trófica de assembléia de peixes em praias do trecho médio dos rios Araguaia e Tocantins, estado do Tocantins, Brasil

Abstract

The current study aims to compare spatial and temporally the trophic structure of the ichthyofauna in beaches of the middle course of Araguaia and Tocantins Rivers. Sampling was conducted in four periods of the hydrological cycle (low, rising, high and receding) in 2000, and between 2004 and 2005 in the Araguaia and Tocantins Rivers respectively. The taxonomic composition and structure of fish assemblages was determined from presence/absence and relative abundance data. The diet was analyzed using the methods of frequency of occurrence and volumetric, combined in an Alimentary Index (IA). Food categories were defined and the species trophic structure was determined according to the specimens abundance and species proportion in each category per sample. A total of 3505 specimens of 112 species from the Araguaia River and 1942 of 55 species from the Tocantins River were collected. Characiformes and Characidae were the most representative order and family respectively in both rivers. The taxonomic composition was different among beaches sampled of both rivers, however, no significance differences were found along the hydrologic cycle in each river. Stomach contents of a subsampling of 41 species of Araguaia River and 20 of Tocantins River to determine the diet composition. Species were grouped in eight trophic categories, based in IA values. Insectivorous was the main category in both rivers. Araguaia and Tocantins Rivers present a similar trophic structure, when considered the specimens relative abundance per category; however, significance differences were found in species proportions per trophic category between rivers. In the Tocantins River there was predominance of insectivorous and herbivorous, 60% of the species were grouped into the category of insectivorous, while on the Araguaia River despite insectivorous are more representative, participation was very expressive of zooplanktivorous. Only in the Araguaia River, the trophic structure change along the hydrological cycle when considering the data of the species proportion per trophic categories. During the high period in the Araguaia river there was a predominance of herbivorous species, while in other periods, insectivorous was the most representative category. This high period was also presented the greatest richness of trophic guilds, the species being distributed more evenly among these categories. Species in these beach environments seeking ideal and safe site for food, since they generally have great feeding plasticity with opportunistic strictly behavior.