Having a great knowledge about trophic ecology of populations is extremely necessary to explore them as feeding resource. This study evaluated some aspects of Cathorops spixii and Cathorops agassizii trophic ecology on Salinopolis Bay, Para, Brazil. The data sampling was made during the March to April of 2011, using bottom fish nets of 200 meters of length and nets ranging from 25 to 100mm between opposites knots. The fishing nets were placed in 33 areas, where 240 specimens were collected, 102 of C. spixii and 138 of C. agassizii. The specie C. agassizii showed a total length ranging from 8 to 21cm and their weight ranged from 4g to 88g. The total length of C. spixii varied from 10 to 25cm, while their minimum weight was 10g and 200g was the maximum value. On the specimens analyzed, was possible to identify the presence of 17 feeding items, 13 classified as autochthonous and 4 as allochthonous. Using the Feeding Index (IAi) we verified the highest relevance in the autochthonous items, and furthermore bivalve and superior plant as the main items in both species diet. The results of IAi tested by ANOSIM, evinced that there is no significant difference between C. spixii and C. agassizii diet, neither between both places. The results of Repletion Index found the absentness of difference among the river tidal and the bay. The niche breadth showed a high level of specialization in both species. Using niche overlap we verified a high overlap in this two species.
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