Effects of environmental variables upon the spatial and temporal structure of a fish community in a small, freshwater tributary of the Indian River Lagoon, Florida

A survey to monitor the distribution and abundance of fishes and selected invertebrates in the St. Sebastian River, Florida, was conducted from March 1999 through June 2000. We recorded a total of 181,854 individuals (representing 77 taxa) in 128 seine samples, and a single species, bay anchovy (Anchoa mitchilli), accounted for 84.4% of the animals collected. Seasonally, the species compositions of spring and fall samples were most similar ( percent similarity index (PSI)=95.3%), whereas those of summer and winter samples were most dissimilar (PSI=54.0%). The spring samples contained the greatest mean number of taxa (N0=59 taxa), and the fall samples had the fewest (N0=38). In addition, spring samples had the highest index of abundant taxa (Hill’s N1=10.6) and summer samples had the lowest (Hill’s N1=6.4), suggesting that approximately four more taxa were caught in greater abundance during spring than during summer. Community composition determined via canonical correspondence analysis revealed four assemblages: two seasonal groupings and two spatial groupings. Seasonal species assemblages were composed of a springsummer group characterized by recruits of Irish pompano (Diapterus auratus), snook (Centropomus undecimalis), and naked goby (Gobiosoma bosc), and a fallewinter group characterized by recruits of mullet (Mugil cephalus), spot (Leiostomus xanthurus), and croaker (Micropogonias undulatus). Spatial groupings were composed of a north prong group which included taxa with estuarine affinities, and a south prong group which included taxa with freshwater affinities. Community composition recorded during periods of controlled water releases and during periods of naturally high-water-flow differed, primarily because abundant estuarine taxa disappeared during the controlled release. The loss of these taxa suggests that when large amounts of fresh water enter the system during relatively short periods of time, the estuarine component of the community does not have time to adjust to the rapid decrease in salinity.