Is sluice gate management beneficial for spawning migrations? The case of the shark catfish (Helicophagus waandersii) in the Mun River below the Pak Mun Dam,Thailand

The reproductive ecology of shark catfish Helicophagus waandersii in the Mun River, the largest Mekong tributary in Thailand, was studied from July 2003 to December 2004. The aim of the study was to determine whether shark catfish, which contribute greatly to fish catches, benefits in terms of spawning, from the management regime of the Pak Mun Dam to open all sluice gates annually for 4 months during the rainy season. In total, 228 females were studied. The highest average gonadosomatic index (4.42% ± 1.90%) was observed in the rainy season in June 2004. Fecundity ranged from 21 550 to 191 540 eggs. Three hormonal profiles viz. testosterone (T), 17 β‐estradiol (E₂), and 17, 20 β‐dihydroxy‐4‐pregnen‐3‐one (17, 20 βP) were determined by radioimmunoassay. The highest levels of T (0.96 ng ml⁻¹), E₂ (14.37 pg ml⁻¹), and 17, 20 βP (0.81 ng ml⁻¹) were respectively measured in April to May, July, and May. Changes in the maturity stage and the annual sex hormonal profiles revealed that the spawning period of H. waandersii is from May to June. There was no spatial statistical difference (p > 0.05) for each sex steroid profiles. The results indicated that H. waandersii spawn during the rainy season in the rapid areas, both downstream and upstream, and the sluice gates opening management regime for 4 months in the rainy season is of advantage for H. waandersii in terms of feeding and nursery grounds in the upstream area. Copyright © 2006 John Wiley & Sons, Ltd.     

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 × 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 © 2009 John Wiley & Sons, Ltd.