Research Product

We used fish community data from trawl samples in >100 estuaries, bayous, and coastal lagoons of the Louisianan Biogeographic Province (Gulf of Mexico) to develop indicators of ecological condition. One data set, from which we derived reference values for fish community indicators, was based on bottom trawl samples collected from 367 randomly-located sites during the summers of 1992-1994. A second trawl data set with similar geographic scope from 2000-2004 was used to test the robustness of the indicators derived from the reference data set to new data. We constructed a fish community index (FCI) from three basic indicators: number of species per trawl, total abundance per trawl, and an index of trophic balance among four common feeding guilds. The FCI was not correlated with salinity over a range from freshwater to marine and hypersaline condi­tions (0-52 psu). Direct correlations between index and environmental variables generally were we, although some were significant (p < 0.05). The FCI was negatively associated with water clarity (secchi depth), water column depth, and sediment toxicity. There was a hyperbolic relationship between dissolved oxygen and maximum values of the index, and no significant correlations with (1) watershed land cover at the whole-estuary or estuary-complex scale, or (2) annual precipitation totals for states bordering the Gulf of Mexico. Values of all indicators increased between the two time periods. The FCI is a broad indicator of ecological condition for estuaries within the Louisianan Province, with data aggregated at scales ranging from large estuaries to the entire region. Sample density was insufficient to judge performance of the indicators or index at smaller scales