This study describes assemblages of fish larvae in the Caeté Estuary located in Northern Brazil. The composition, temporal and spatial abundance patterns and developmental stages of fish larvae were examined along a salinity gradient of the river and in three creeks of the adjacent mangrove forest to ascertain the importance of these habitats as a spawning and nursery ground. Relationships between larval densities with environmental variables and tidal, lunar, stratum and diel effects were examined to gain insight into the possible causal mechanisms for the observed abundance patterns. Ichthyoplankton samples were collected monthly from July 1996 to October 1997 using a ringtrawl at the surface of the river and a trap net in the tidal creeks.

A total of 227036 larvae, representing 32 families and 78 species were collected in the entire system, with the Sciaenidae, Engraulidae and Eleotridae comprising 87 % of all teleosts sampled. Results showed that the fish assemblage in the Caeté Estuary is constituted of many rare species and a few species in large numbers. The most abundant larval taxa were the eleotrid Guavina guavina (35.6 %) and the engraulid Anchovia clupeoides (20.3 %). The fish assemblage in the river and in the creeks were similar in terms of the number of species they had in common. However some species were predominantly found in one habitat.

The densities of the most important species in the river were influenced to a greater degree by site than by season, which decreased in a downstream direction. Freshwater outflow appears to be one of the major factors, governing the annual cycle and distribution pattern of the fish larvae in the Caeté River. Mean larval density was lowest in the upper reaches when freshwater discharge was at its maximum, which was attributed to increased seaward drift of larvae. The larval fish assemblage was most diverse in the early rainy season. Number of species and abundance were negatively correlated with salinity.

Environmental fluctuations, in part associated with lunar periodicity, such as tidal amplitude, may account for weekly variability of recruitment and reproduction of some fish species. Vertical migration was not a mechanism employed by most abundant species to enable retention in the river, since larval densities of these species were not significantly higher at the bottom on ebb tides. Diel effects on larval abundance were related to trophic activity and gear avoidance.