What Causes the Drastic Loss of Genetic Variation and Skewed Sex Ratio of Endangered Formosa Landlocked Salmon in Taiwan?

The use of hatchery-reared fish to replenish existing threatened wild populations has been shown to reduce or change natural genetic diversity among wild populations. In this study, the genetic diversity of wild Formosa landlocked salmon (Oncorhynchus formosanus) in its main habitat of Chichiawan Stream was examined after the large-scale escape of hatchery-cultivated fish. About 3,000 individuals, the descendants of 5 pairs of wild salmon, escaped from the old hatchery near Dam 2, when Typhoon Ariel breached the hatchery in the fall of 2004. Extremely low genetic diversity of wild Formosa landlocked salmon was reconfirmed and further declined between 2004 and 2008. Loss of genetic diversity does not seem to have happened in the winter of 2004. We hypothesize that the decline in genetic diversity of wild population was mainly caused by population bottlenecks in 2005, and genetic homogeneity since 2005 was due to the breeding of many escaped-hatchery fish with low genetic diversity, surviving the floods of 2004. The present study supports the possibility that the drastic decline of genetic diversity between 2004 and 2008 was caused by the genetic effects of escaped-hatchery fish. We also examined the effect of estrogenic pollutants on the sex ratio of this population.