The Marajó Bay is located south of Marajó Island and receives all the discharge from the Tocantins River and part of that from the Amazon. The seasonality of these discharges causes a variation in the salinity of the estuary, influencing in a determinant way the composition of the ichthyofauna of the region. Two seasons are distinguished in the Amazon River regime: (1) Dry season (Summer - June to December), when the discharge of the Tocantins and Amazon Rivers decreases and the marine waters penetrate more prominently into the Amazon estuary; (2) Rainy season (Winter - January to May), when the discharge of these rivers increases, causing the sea to retreat up to 160 km from the Amazon estuary. This work was carried out in three stretches of the Marajó Bay: (I) Tucunduba creek (Bélem-Pa), margin of the Guamá River and innermost portion of the bay; (II) Paracuri creek (Icoaraci), intermediate portion; (III) Anuerá creek (Vigia-Pa), margin of the Guajará-Miri River, outermost portion of the bay. Ninety-eight species or groups of species were identified, and their composition varied according to the period of the year. The most abundant species in the dry season were: Anchoviella elongata (33.30%), Cathorops spixii (12.99%), hyphessobrycon cf minor (9.53%), Rhinosardinia amazonica (8.63%), being the Engraulidae family, the one with the highest expression (24.13%). In the rainy season the most abundant species were: Anchoviella elongata (14.99%), Plagioscion squamosissimus (10.88%), Brachyplatystoma vaillantii (8.04%) and Arius couma (5.85%), the Engraulidae family remained the most frequent in the catches (33.81 %).
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