Estuarine fish community structure in a submerged aquatic vegetation habitat: seasonal and diel variations

In order to analyze the temporal variations of an estuarine fish community in a submerged aquatic vegetation habitat, we completed six bi-monthly 24 h sampling cycles (using a seine net) over a 1-year period (1989-1990). A total of 17,105 individuals, corresponding to 66 fish species were captured. Species richness and diversity showed significant differences among months and its seasonal variation was mainly related to temperature and salinity. At diel scale, richness was higher at night and diversity during the day, although these parameters showed no significant differences between these two periods. A non-metric multidimensional scaling showed that the ordination of the samples was mainly related to seasonal variation, which showed significant differences in species composition between months (MANOVA). Despite a wide dominance of Anchoa mitchilli throughout the year, seasonal changes in species composition were due to the cyclical succession of populations of other species, primarily determined by their physiological tolerances and secondly, by feeding patterns. Although there were no significant changes between day and night in overall species abundance, cluster analyses showed that coincidentally in months with lower nocturnal light (new moon and first quarter), diurnal and nocturnal samples were clearly segregated, and associated alternatively with samples of twilight hours. During daytime, pelagic-planktophagous species were abundant and benthic-predatory ones in the night, whereas the most abundant species showed a twilight pattern of activity. So, seasonal changes in fish community are mainly related to physicochemical conditions, whereas diel variation was slightly related to the light/darkness cycle.