Effects of a dam and reservoir on the distributions and densities of macrofauna in tropical streams of Guam (Mariana islands)

Streams on the island of Guam were surveyed to compare species composition and densities of the most common species among seven rivers, of which three were above a dam and reservoir and three were unregulated. Rivers above th e reservoir were also compared to the outflowing river. Species found in al l of the rivers included the indigenous gobies Awous guamensis and Stiphodo n elegans and two introduced cichlid fishes, Oreochromis mossambicus and Ti lapia zillii. The gobies have modified pectoral fins that allow them to cli mb waterfalls and to traverse the spillway and enter the reservoir from dow nstream. The cichlids, however, are not morphologically adapted for climbin g and were introduced to the reservoir; they eventually moved into the rive rs. The freshwater shrimp Macrobrachium lar, smaller atyid shrimp of the ge nera Atya and Caridina, and the gastropod Thiara granifera were also found above the reservoir, in the outflowing river, and in the unregulated rivers . However, the flagtail Kuhlia rupestris and the neritid gastropods were no t found above the dam. In addition, the dam and reservoir appeared to affec t the densities of some species. The densities of the mountain goby Stiphod on elegans were significantly lower in streams above the reservoir than in unregulated streams. However, densities of the prawn Macrobrachium lar were somewhat higher in the streams above the reservoir, possibly a result of t he exclusion of the flagtails. The effects of a small dam and reservoir wit h a spillway on the largely amphidromous fauna of tropical island streams a re relatively benign, impinging primarily on species that are incapable of climbing the spillway or that are impeded in upstream migration by the rese rvoir.