Community responses to dam removal in a subtropical mountainous stream

Dam removal has the potential to efficiently solve the problems caused by fragmented stream habitats but may simultaneously cause negative impacts on biotic communities. To conserve the critically endangered Formosan landlocked salmon (Oncorhynchus masou formosanus), a 15-m-tall check dam was partially removed from the Chichiawan Stream at the end of May 2011, before the flood season. Using this dam removal experience, we aimed to cast dam removal as an action comparable to a natural flood event. We applied a before-after-control-impact (BACI) design and quantified the environmental factors and major biotic communities at four sampling sites in the stream bimonthly before (2010) and after (2012 and 2013) the dam removal and monthly in the year of the dam removal (2011). After the dam removal, a faster current velocity and more turbid water were observed at the downstream sites, and the area's deposition consisted of small-grained sediments. Despite this, our results show that the dam removal was performed during a suitable period. There was no obvious influence on tadpoles as they metamorphosed into adult frogs and left the stream before the dam removal. Fish exhibited a greater resistance to the alteration in flow resulting from the dam removal. An increase in fish abundance at the upstream sites after the dam removal suggests that the corridors created by the dam removal allowed access to more habitats for the fish. In particular, the periphyton biomass and aquatic insect densities decreased markedly at the downstream sites after the dam removal, but they recovered within a year, demonstrating the resilience of these taxa. Coleoptera, Plecoptera and Trichoptera were more resistant than the periphyton, Diptera and Ephemeroptera after the dam removal and an extreme flood event. In conclusion, the responses of stream communities to dam removal were similar to the responses to an extreme flood event. To mitigate the impacts caused by dam removal, our results suggest that stream communities may respond to dam removal as a natural flow alteration if the timing of the dam removal occurs just before the flood season.