Feeding of juvenile pirarucu (Arapaima gigas, Arapaimidae) in their natural environment, lago Quatro Bocas, Araguaiana-MT, Brazil

The stomach content of samples of juvenile Arapaima gigas was analized to obtain information about feeding in natural environments. This species occurs in the Amazonian basin, predominantly in floodplain environment. This is the case of the valley of the middle rio Araguaia, where the lago Quatro Bocas is situated. Juveniles A. gigas prefered insects, microcrustaceans and gastropods, most of autochthonous origin. All the stomachs examined contained at least one food item.

The Arapaimidae family (Osteoglossiformes) includes two species, one (Arapaima gigas Schinz, 1822) found in the Amazonian basin and rivers of Guianas and another (Heterotis niloticus) in Africa (Ferraris et al., 2003). A. gigas, popularly known as pirarucu, is one of the largest among the South America freshwater fishes. They are commonly found in floodplain environments, where large lakes are formed, and they can also be observed in slow running rivers. The valley of the middle rio Araguaia is characterized by the occurrence of innumerable marginal lakes, which are suitable environments for the development of pirarucu. Among these, is the lago Quatro Bocas (15º23'20''S and 51º42'45''W), a 108 hectares lake, located in the left edge of the rio Araguaia, which it connectes in high water periods (Fig. 1).
According to Borges (1987), this system has a well accentuated dry season that culminates in September/October and a period of high water, between February and April. These environmental fluctuations promote a bigger availability of trophic niches, offering a larger amount and quality of food resources in some periods of the year.
In December 2002, samplings were initiated in the lago Quatro Bocas, according to four seazonal periods within one year. In March 2003, during high water level, 31 juveniles of A. gigas were captured. Juveniles were not captured again until December 2003. The objective of this study was to analyze the stomach contents of these specimens to get up-todate information about the feeding of juveniles of this species.
In the field, the specimens were fixed in 10% formalin and transferred to the laboratory of ichthyology of the Instituto  Hynes (1950) and Hyslop (1980). The volume of each food item was determined using graduated test tubes. For food items volume less than 0.1 ml a graduated glass plate was used where the value obtained in mm 3 was transformed in ml (Hellawell & Abel, 1971). Identification of the food items was aided by description given by Needham & Needham (1982), McCafferty & Provonsha (1983), and Borror & DeLong (1988). Analysis of the stomach contents of 31 individuals, ranging in standard length between 22 and 26.5 cm, showed that the diet was composed by 16 food items. Of these, six items stood out in FV and FO, respectively, including: organic detritus, 53% and 100%; insects remains, 21% and 100%; Conchostraca, 7% and 100%; Gastropoda, 7% and 100%; Hemiptera 4% and 74% and Coleoptera 3% and 74%. All individuals included in the sample contained some food item in their stomachs. A single individual, 25 cm long, had ingested small fish of the Cichlidae family in its stomach (Table 1). Queiroz & Sardinha (1999) reported that in juveniles of pirarucu with the size smaller than 50 cm, the main items found in the stomachs were microcrustaceans. According to these same authors, shrimps have been frequently found in individuals longer than 150 cm; the only items common to all ages are the aquatic insects, mainly, Coleoptera and Hemiptera .
These results demonstrate a high capacity for taking advantage of autochthonous resources available in the environment. Organic detritus, inorganic particles with portions of fine sand and vegetal remaining portions also demonstrated the ability of specimens to pick up food near the bottom. In the stomachs of juveniles of the A. gigas studied here small food items were predominant, basically invertebrates from diverse orders.
Fontenelle (1948) confirms that adult pirarucus are essentially carnivorous, showing a preference for the fish commonly known as cascudo (Loricariidae).
Consumption of insects by juvenile pirarucu observed in this study can be explained by the fact that the environment from which this fish was obtained was a marginal lake with relatively low anthropogenic activity and abundant vegetation on its edges. Wootton (1990) and Gerking (1994) reported that many fish species have flexible food habits, answering to variations in the availability or exploitation of the potential food. Andrian (1981) confirms that the food regimen of the species is not fixed and immutable, because it is prevailed by biotic and abiotic factors that are constantly changing. Few species are strictly carnivorous or herbivorous (Lagler et al., 1962). In the present work, the consumption of vegetal detritus and seeds was small when compared with items of animal origin. The studies carried out by Solar (1943, cit in Menezes, 1951 revealed that pirarucu is an omnivore fish which picks up food on benthic organisms and portions of plants that could be carried by water. About the origin of the food, Lowe Mc-Connell (1999) noticed a linear succession of dominant food sources in streams and rivers and that the allochthonous materials were more important for fish in headwater. The same author com-

P R O O F S
Feeding of juvenile pirarucu ments that, as the environments extend and get deeper, as in lakes, the autochthonous food sources become more important. From the analysis of food items found in the diet of A. gigas, it was characterized that the main food resources have autochthonous origin. The studied species revealed, in this initial phase of life, food preference for aquatic invertebrates; however, an ample trophic plasticity was also noticed. These results are relevant to the preservation and production of this species. The A. gigas is a species that can be considered to be in danger of extinction. Protection politics must take into account scientific information about this species obtained in its natural habitats.