Causes of the drastic loss of genetic variation in the Critically Endangered Formosa landlocked salmon of Taiwan

Abstract

The use of hatchery-reared fish to replenish existing threatened wild populations has been shown to reduce or change the natural genetic diversity of the wild populations. In this study, the genetic diversity of wild Formosa landlocked salmon Oncorhynchus formosanus in its main habitat of the Chichiawan Stream in Taiwan was examined after a large-scale escape of hatchery-cultivated fish. Approximately 3000 individuals (the descendants of only 5 pairs of wild salmon) escaped from an old hatchery when Typhoon Ariel breached the hatchery in the fall of 2004. The genetic diversity of the wild population was extremely low at that time, and declined further between 2004 and 2008 following the escape of hatchery fish. We hypothesize that the decline in genetic diversity of the wild population was mainly caused by a population bottleneck in 2005, and that genetic homogeneity since 2005 was caused by breeding of the escaped hatchery fish (which showed low genetic diversity) that survived the floods of 2004. This supports the possibility that the drastic decline in genetic diversity between 2004 and 2008 was caused by the genetic effects of the escaped hatchery fish, and demonstrates the risk of introducing hatchery fish into the wild.