Effects of dam removal on resident fish movement in Cijiawan River, Taiwan
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Freshwater fish change their movement patterns in response to natural or anthropogenic changes in habitat. Dam removal, a major restoration practice to restore river longitudinal connectivity, can affect river habitat with changes in flow regimes and morphological changes due to the sudden release of dam-trapped sediments. In Taiwan, a mid-size dam, Cijiawan #1 dam, was removed in 2011 to provide access to upstream habitat for the endangered target species, Taiwan salmon (Oncorhynchus masou formosanus). A three-phase radio telemetry tracking experiment was conducted to identify the movement patterns before, during and after dam removal. Some tagged fish were also displaced downstream to observe homing behavior. The movements of non-displaced fish showed reduced movement over time and persistence of location which indicates that the influence of disturbance due to capture and tagging was temporary. Displaced fish showed significant long distance movement toward their original home range immediately after release, but became as sedentary as non-displaced fish over time. The displaced fish homing behavior was limited by the barrier effect before dam removal, but daily movements and total absolute stream distance covered both increased after dam removal as fish began accessing upstream habitat. During the dam removal phase, fish below the dam moved very little when the heavy machinery was in the stream and then made long distance movements when upstream habitat was accessible after deconstruction. The rapid response of Taiwan salmon in this study confirmed that the new habitat opened by the dam removal project could be utilized almost immediately, but the sensitivity to habitat degradation indicates that extra attention to short-term habitat impacts should be considered for future stream restoration practices.
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