Ecologia trófica de quatro espécies de peixes de um rio no semiárido, Pentecoste - Ceará

Abstract

The semiarid rivers present two patterns: the intermittent and the irregular one, with a single flowing off period per year, during the rainy season. In these rivers, the natural hydrological disturb acts as an important factor driving the organization of the system, in which feeding interactions are complex and modified by seasonality and system productiveness. Fishes often feed on autochthonous or allochthonous resources, with their diets varying according to the with seasonal variation of food availability. The hypothesis of this the present study is that in the semiarid river herbivorous/onivorous fish species present trophic resources partitioning. The major aim of this study the present investigation is was to determine the trophic resources partitioning for four species of fishes at in the Curu River. Field work collections were carried out throughout the rainy season, at the Curu River, in Pentecoste city town, in Ceará state. The stomach content of 197 individuals of the species Astyanax bimaculatus (27), Astyanax fasciatus (79), Poecilia vivipara (90) e Hypostomus jaguribensis (1) was analyzed through the volumetric method in order to obtain for the frequency of each feeding item, so that the Index of Alimentary Importance (IAi) could have been measured. The morphometric data were used to obtain the intestinal quotient (IQ), the niche breadth estimated for the Levins’ index and the trophic overlap for the Pianka’s index. Patterns of trophic niche overlap were calculated and statistically tested against a null model using the randomization algorithms RA3 and RA4. A. bimaculatus and A. fasciatus fed on 13 and 15 items, respectively. Both species presented feeding preferences for the filamentous algae Spirogyra sp., encompassing 89.34% and 96.86% of their diets, respectively. Poecilia vivipara’s diet comprised 60 items, mainly made up of algae (Bacillariophyta - 28.82%, Chlorophyta - 23.17% and Cyanophyta – 5.2%). Amongst the “green algae”, Spirogyra sp. (12.89%) and the Closterium sp. (8.85%) were the chief principal items eaten. Aquatic insects (39.18%) made up the second most important item. A total of 42 items compounded the diet of H. jaguribensis, which detritus (27,72%), the algae Spyrogira sp. (23.42%) and Compsopogon sp. (19%) were the foremost most important items. The diatom group comprised the most abundant item, with 21 taxa and 16.38% of IAi. According to the intestinal quotient, A. bimaculatus e A. fasciatus showed a typical carnivorous/omnivorous intestine, whereas P. vivipara showed a herbivorous one. The analysis of variance shows that the intestinal quotient varied among species (F = 3.50; d.f = 2; p < 0.05). The niche breadth was greater for P. vivipara (1.32), followed by A. bimaculatus (0.78) and A. fasciatus (0.426). The higher niche overlap occurred within the Astyanax species (0.98). The null model analysis carried out for the total and clustered grouped items indicated that the observed values were always higher than the expected ones for the RA3 and RA4 matrices. In addition, the mean observed variance was higher than the simulated one. The four studied fish species fed mainly on autochthonous items, chiefly on algae. The null models demonstrated that there is a resource partitioning amongst these herbivorous/omnivorous fish species, where competition seems not to be the structuring agent.