Climate warming and the effectiveness of restoring longitudinal river connectivity for endangered non-anadromous Taiwan salmon (Oncorhynchus formosanus)
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River restoration is widely practiced to improve river ecosystem functions that have been degraded or impaired through anthropogenic alterations or other causes. Despite the significant amount of resources invested into river restoration efforts, the effectiveness of such actions is hard to evaluate due to the lack of research emphasizing long-term monitoring of the efficacy of restoration practices in areas being impacted by warming temperatures. In central Taiwan, a mid-sized dam was removed to restore longitudinal connectivity of the Cijiawan River with the goal to conserve the endangered Taiwan salmon (Oncorhynchus formosanus). However, the ecological benefits of dam removal, especially under the scenario of pre-existing habitat alterations due to land-uses and dams, need to be explored under a climate change scenario. The collective impacts on Taiwan salmon’s behavioral responses to certain hazardous conditions cannot be currently evaluated. The goals of this research were to assess the effectiveness of dam removal to restore river connectivity and to provide suitable thermal refugia for the Taiwan salmon under warmer climatic conditions. This research also needed to determine the threshold temperature that causes Taiwan salmon to express avoidance behavior and to migrate to cooler habitats. One ecological benefit of dam removal is the ability of Taiwan salmon to swim back to their home range after being washed over the dam (a washdown) and trapped at downstream river sections during extreme flood events. By analyzing historical precipitation data and the spatial population variations of the Taiwan salmon, Taiwan salmon population declines were associated with severe summer flood events, but the degree of decline was not correlated with the severity of the flood events. Also, the spatial distribution of the salmon populations after summer flood events did not fit the general patterns expected during a washdown return situation. Despite washdown return incidents occurring at local scales under specific biotic and abiotic conditions, the dam removal was not significantly correlated to the observed mortality rates of the Taiwan salmon. The dam removal did provide passage for salmon returning behavior to be expressed as they migrated back to their home ranges following the flood-induced displacement events. Another potential ecological benefit of dam removal for salmon is that downstream Taiwan salmon have access to upstream cold water habitat or thermal refugia when water temperatures exceed their thermal thresholds. A controlled temperature gradient flume experiment was used to identify the temperature threshold at which Taiwan salmon show a significant high temperature avoidance behavior. This experiment identified a water temperature threshold at 16.4oC where salmon express avoidance behavior. By monitoring the summer field temperature regimes on Cijiawan River, the temperatures of river sections below the Cijiawan Two dam regularly exceeded the 16.4oC temperature threshold for extensive periods during the day time in summer. In contrast, the upstream sections near Cijiawan Three dam were consistently thermally suitable for salmon during the same summer period. These results indicate that the restored longitudinal connectivity resulting from the removal of the dam has potentially significant benefits for downstream Taiwan salmon populations and their access to upstream cold water refugia under today’s thermal regimes. Climate change is predicted to have significant effects on freshwater ecosystems experiencing warmer temperatures and changing hydrologic regimes. By comparing current Taiwan salmon habitat conditions and usage behavior to the projected habitat changes under a climate change trend, the ecological effectiveness of dam removal to restore the longitudinal connectivity of the river appears to have a limited effect on the conservation of the Taiwan salmon populations. This study shows that the increase in the intensity of the extreme precipitation events as well as the warmer water temperatures are having a greater impact on salmon resilience that is not addressed by re-establishing the longitudinal connectivity of the river by the removal of the Cijiawan dam. The increasing flood intensity does not appear to increase the mortality rates of Taiwan salmon but does result in higher rates of salmon being washed down to the lower reaches of the river which are thermally unsuitable for extended hours during the daytime in the summer season. This research suggests that the greatest impact on salmon survival will be due to the rising river water temperatures resulting from climate change. In this study, approximately half of the river extent between Cijiawan One and Three dam was thermally unsuitable during the daytime period of the summer months. These temperatures will cause salmon to express their avoidance behavior and to migrate to the upper reaches of the river. Taiwan salmon may need to rely more on local small-scale cold water refuge to cool down during the nighttime periods. The restored longitudinal connectivity of the river provided by the removal of the Cijiawan dam does not mitigate future spatial thermal regime changes or the existence of other migration barriers. As a thermally sensitive freshwater species restricted to a confined river, Taiwan salmon and Cijiawan River demonstrate that non-anadromous landlocked salmon are extremely sensitive to the possible environmental impacts reported by future climate-change projections. This serves as a warning of the increased vulnerability of Taiwan salmon to climate change when they are restricted to inhabiting river systems isolated from marine environments. Taiwan salmon are highly vulnerable to the continued warming temperatures suggesting that maintaining viable population densities will be challenging. Most of the management tools such as dam removal or vegetation management in the riparian zones are not designed to address the impacts of future projected warming temperatures.
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