Relationship between the Formosan landlocked salmon Oncorhynchus masou formosanus population and the physical substrate of its habitat after partial dam removal from Kaoshan Stream, Taiwan

In order to protect and restore the population of the Formosan landlocked salmon (Oncorhynchus masou formosanus, Jordan and Oshima) (Salmoidae), an endemic and endangered species of the central Taiwanese highlands, Shei-Pa National Park partially removed 4 dams from Kaoshan Stream during 1999-2001. The purpose of this study was to evaluate the relationship between the abundance of the Formosan landlocked salmon and the physical substrate of its habitat as a consequence of habitat modifications caused by partial removal of the dams from Kaoshan Stream. After removal of the dams, fish were able to freely travel up and down the stream, but the relative slope of the riverbank became steeper, and the velocity of the river increased leading to more-serious erosion than was found prior to dam removal. We applied a principal component analysis (PCA) method to determine the most significant factors affecting the fish population. The 1st principal component of the fish population explained 79% of the total salmon population variance. The 1st 2 principal components of substrate rock size explained 72% of the total substrate variance. By examining a contour plot, we found that when the percentage of boulders was higher, the salmon population accordingly increased. Moreover, poor breeding years that highly impact the salmon population appeared to be a result of flooding caused by typhoons during the breeding season from late Oct. to Dec. Following flooding, it appears that the river substrate had a higher percentage of boulders, which possibly benefited the salmon population. Consequently, partial dam removal may have improved the survival rate of the salmon population in response to floods. http://zoolstud.sinica.edu.tw/Journals/47.1/25.pdf.