The structure of shallow water fish assemblages in sandy beaches of a tropical bay in the southwestern Atlantic

The spatial and seasonal dynamics of shallow water fish assemblages and their relationship with physical-chemical variables were investigated on Guanabara Bay, one of the largest coastal bays in Brazil, southwestern Atlantic. Fishes were collected monthly between June 2016 and May 2018, in the shallow waters of six beaches distributed in four sectors along the major axis of the bay (adjacent and entrance - one site each, middle and inner - two sites each), using a beach seine net. A total of 79 fish species was recorded, mostly juveniles. Marine estuarine dependent, marine estuarine opportunist, solely estuarine and marine straggler were the most representative ecological guilds in the bay during the sampling period. Following the index of relative importance (IRI) categories, top dominant species (IRI ≥ 10%) within sector beaches were: the solely estuarine Atherinella brasiliensis from the inner to the entrance; marine estuarine dependent Eucinostomus argenteus in the middle; marine estuarine dependent Harengula clupeola and marine straggler Diplodus argenteus at the entrance, and marine stragglers Umbrina coroides, D. argenteus, and marine estuarine dependent Dactylopterus volitans in the adjacent sector beach. All analyses identified distinct fish assemblages that were distributed in two main areas: adjacent/entrance and middle/inner. Transparency and chlorophyll a were responsible for most of the spatial variability and played an important role in structuring fish assemblages. Water temperature was responsible for seasonal fluctuations indicating the distinct habitat use from different life cycle stages of fish species. This investigation suggests the importance of all sector beaches as nursery and growth areas due to the high representation of juvenile individuals.