Abstract
The Formosan landlocked salmon (Oncorhynchus masou formosanus) is an endangered endemic species found only in the basin of the Chichiawan Stream in the mountains of Central Taiwan. Census observations recorded from 1987 to 2004 showed that the regulation of the Formosan salmon population is density-independent, possibly due to the fluctuating environment. In order to better understand and predict the population dynamics, stage-structured population matrix models were constructed for juveniles, subadult and adult stages using the census data. We applied the least-squares solution to determine stage-specific parameters including the growth rates, survival rates and fecundity for the matrix models. The derived population growth rates determined by the matrix models indicated that the Formosan salmon population is declining in the Chichiawan Stream but has been increasing in a tributary, the Kaoshan Stream since 2001, when dams on that stream were demolished. In general, the numbers of each stage of the Formosan salmon could be well predicted by the matrix models. In the Chichiawan Stream, the model performance was better for subadults than for adults or juveniles. The lower predictability of adult numbers across summers might be a result of the low capacity of large-sized adult salmon to seek refuge when typhoons cause floods in the shallow streams. The lower predictability of juvenile numbers from winter to spring suggests that other intrusive factors or interactions with other communities might be involved in determining the fecundity and require further study. Stagestructured population matrix models appear to be a useful tool for the management and conservation of the Formosan salmon.