To understand the biodiversity and the periodic fluctuation of the fish in Shuangchi river estrarine of northeastern Taiwan, related with the season and the climate, we collected juvenile, young, immature, and adult fish individuals every night, with the tool of small trawl of fishing elvers on the west side of Fu-Long harbor from Dec 27, 1998 to May 31, 2000. Totally, we fished 243, 341 individuals of 20 orders, 76 families, 236 species, including post larvae, juvenile, young, immature, adult ones, and also obtained the periodic fluctuation and the body size of every kinds of fish for ecological study on estuarine fishes. The superior species whose amount is more than 1,000 are Mugilidae(Liza macrolepis, and Valamugil formosae, mainly), Engraulis japonicus, Anguilla japonica, Anguilla mamorata, Leiognathidae, Sardinella melanura, Sicyopterus micrurus, Terapon jarbua, Ambassis miops, Atherion elymus, Leiognathus lineolatus, Spratelloides gracilis, Bathygobius fuscus, successively. These superior species groups constituted approximately 97.2% of the total collection, and most of them are coastal epipelagic migratory or amphidromous fishes. Analyzing the lunar data, we found that the number of species and the total amount of fish collected had an inverse proportional relationship. The guild types of the superior fishes belong to coastal epipelagic migratory, amphidromous, and estuarine, mainly. Although the total amount of the fish whose habitat is around the rocky or sandy shore is less here, the number of species of these fish is more. Because the coast of the Northeastern Taiwan was mostly rocky, the large overslaugh in Shuangchi river estuary is unusual. The fish from Shuangchi river, Long-long brook, and Yenliao Bay coexisted here and there were large amount of fish larvae of various guild types living here. Therefore, it is important to protect the highly biodiversity and also economic fish larvae here.
