SPATIAL AND TEMPORAL CHANGES OF FORMOSAN LANDLOCKED SALMON (ONCORHYNCHUS-MASOU-FORMOSANUS) IN CHICHIAWAN STREAM, TAIWAN

Spatial and temporal changes of formosan landlocked salmon (Oncorhynch us masou formosanus) in Chichiawan stream, Taiwan. Bull. Inst. Zool., Academia Sinica 32(2): 87-99. The population abundance of Formosan lan dlocked salmon (Oncorhynchus masou formosanus) in Chichiawan Stream wa s estimated visually by snorkeling over a time period from September, 1987 to January, 1991. The salmon population decreased during this tim e. Yearly fluctuation in abundance was larger in young than in adult s almon; young salmon were more abundant in summer than in winter. Salmo n abundance decreased downstream, while the temporal variability of ab undance increased. In the area just above a sand-retention dam, salmon abundance was lowest and temporal variability was highest. During the study period, salmon abundance in sections with high salmon density s howed a greater tendency to decrease; at the same time, the stream bec ame wider, shallower, and slower. The decrease of stream depth was the major factor affecting adult salmon density, although it was not sign ificant for young salmon. Typhoon Lynn, which occurred during the salm on breeding season, destroyed the cohort born in 1987. Salmon populati on size was also decreased by habitat degradation caused by natural di sturbances (i.e. typhoons, floods, etc.) and man-made factors (i.e. th e construction of sand-retention dams, agricultural activities, etc.).