SPATIO-TEMPORAL VARIATIONS IN FISH ASSEMBLAGES IN A TROPICAL REGULATED LOWER RIVER COURSE: AN ENVIRONMENTAL GUILD

Fish assemblage patterns were studied in the lower river reach of the Uthokawiphatprasit anti-salt dam, 6 km upstream from the Pak Panang River mouth, Southern Thailand, where the dam was opened occasionally depending on the upstream water level. Matrix data of the presence-absence of 71 fish species in 102 surveys was used in the analysis by applying a self-organizing map (SOM) model. The trained SOM (lattice 8 x 7) showed that after 6 years of operation, five assemblage patterns were distinguished. These patterns described the probability of the occurrence of fish in each fish environmental guild, which according to changes in flow and water geomorphology. Clusters Ia and Ib were mostly the surveys in upstream stations and occupied by fish in potamonic guilds, whereas the fish in the estuarine guild and marine guilds showed a high probability of occurring in clusters IIa, IIb and IIc, which belonged to the surveys in downstream stations. The surveys of the stations near the dam (i.e. stations 5 and 6) during the opening phase were contained in cluster IIb. The Kruskal-Wallis test showed that there was no statistical difference in the probability of occurrence among assemblages of the diadromous, catadromous and semi-anadromous fishes but not the amphidromous fishes, which had a low probability of occurrence in clusters Ia and Ib. The fish assemblages were arrayed along a longitudinal gradient, where salinity and pH were the most important controlling variables and explained 94.0% of the total inertia. Copyright (C) 2009 John Wiley & Sons, Ltd.