In the present paper were studied the diet, the body composition, and the gross energy value of the stomach contents, fillet and liver, as well as some fruits and seeds that are part of the natural diet of the matrinxã, linking these parameters with the river's annual water level flutuations. The matrinxã was observed to feed on seeds, fruits, flowers, remains of vegetables and insects, presenting an omnivorous feeding habit. Empty stomaches were frequent (68.5%) in the different hydrologic periods. The proximal analysis and gross energy value of the fillet and liver of the analyzed fish did not present significant differences between the sexes (P>0.05). However, differences between the periods of low and high waters were observed. Median proximal composition and gross energy values of the stomach contents, expressed as dry matter revealed a low content of protein (8.9%) and ash (1.3%), and high value of lipids (70.2%) and energy (775 Kcal EB/100g) during high water. During the low water period, protein (24.9%) and ash values (6.3%) were high, and lipids (13.2%) and energy values (416.9 Kcal EB 100g) were low. Abundant deposits of body fat were verified the whole year, decreasing during the spawning period in January.", 'en
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