Background. Because estuarine fish communities undergo temporal changes in abundance and species composition, the influence of environmental factors on diel and seasonal variation in the structure of a fish community was evaluated. This information is necessary to extend our knowledge of fish communities in tropical estuaries. The objective of the study was to test the hypothesis that the total number of fish, species richness and fish composition would undergo temporal variations related to the variability of environmental conditions. Materials and methods. In Pueblo Viejo Lagoon (western Gulf of Mexico), six 24-h sampling cycles every 2 months were carried out. Fish were collected every 2 h during each 24-h cycle using a beach seine net. Data were analysed using repeated measures PERMANOVA and canonical correspondence analysis. Results. Diel variation in total fish number and species richness tended to peak at dawn and dusk, with significant differences between the periods of day, night and twilight. Total number of fish had significant pulses during the dry season and rainy season, whereas species richness showed a pulse in March–July. Fish community structure was mainly influenced by salinity, temperature, rainfall, and seagrass cover. Seasonally, three fish groups were observed. The fish assemblage during the rainy season was mainly characterized by low-salinity tolerant species. The fish assemblage during the cold-dry period was dominated by species with low-temperature affinities whereas filter-feeding species tended to be dominant during the warm-dry period. Differences in fish assemblages among diel periods were mainly due to some species were typically diurnal, nocturnal or crepuscular. Conclusion. This study allowed determining that total fish abundance and richness are related at diel level with the transition period between the activities of diurnal and nocturnal species, and with fish strategies of predator avoidance and foraging. At seasonal level, these seem to be coupled to periods of high productivity in the system. Moreover, the fish composition showed a seasonal succession of species, which may be determined by a combination of temporal changes in salinity, rainfall, temperature, and by the different feeding strategies of species.

PERMANOVA, constrained ordination, Ruppia maritima, seasonal succession, twilight pulses