Assessment of present habitat quality of main rivers in Taiwan for the Japanese eel (Anguilla japonica)

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The Japanese eel Anguilla japonica is a typical catadromous fish species that spawns in the sea and is distributed across the Western Pacific. Japanese eels are fish species of high economic value in East Asia. However, due to the high costs of complete aquaculture, currently the source of eel fry in farms still depends entirely on natural resources. Nevertheless, with the rapid development of aquaculture industry, the stock of Japanese eel has also sharply declined since 1970s, and to this day it is less than 10% of that observed in 1970. Habitat destruction, overfishing, and global climate change are all potential causes. In order to evaluate the habitat quality of the Japanese eel, our lab (2013) employed a new assessment indicator－Habitat Quality Index (HQI) that uses satellite remote sensing technology to assess the level of destruction of the Japanese eel habitats. Compared to past habitat survey methods, this method not only saves time and costs, but it also allows frequent monitoring and acts in concert with water quality indicators to implement appropriate response mechanism at any time. In this study, we use the satellite images provided by the Center for Space and Remote Sensing Research of the National Central University, as well as the information on river pollution from the Environmental Protection Administration, Executive Yuan of Taiwan, to evaluate the newest data of the HQI of a total of 38 rivers in Taiwan from 2014-2017. Every river has at least one assessment data for each season. The results showed that the HQI of rivers in the northern part of Taiwan across the four seasons were 13.23, 14.2, 13.46, 13.05; in the eastern part were 6, 6.24, 5.86, 5.63; and in southwestern rivers were 22.87, 28.08, 27.03, 21.63. The habitat quality of northern and eastern rivers is relatively stable across the four seasons, while there is noticeable seasonal variability in southwestern rivers in which the habitat quality declines especially during the winter and spring. Finally, this study calculated the HQIs of a total of 38 studied rivers in Taiwan from 2014-2017, and the sum was 44.97. After deducting the HQI of 12 small and medium sized rivers newly added to the assessment, the value obtained was 41.46. There is a trend of slight increase compared to our lab research (2016) of 38.14 obtained by evaluating a total of 26 rivers in Taiwan in the 2010s. Possible reasons for this trend may be a decrease of the levels of river pollution, and the use of higher-resolution satellite imagery. This study uses satellite imagery and river pollution data to assess the habitat quality of Japanese eel in a total of 38 rivers in Taiwan. The results said that the difference of HQI of rivers depended on the seasons in Taiwan. Therefore, in order to facilitate the conservation of Japanese eel, the governments should choose the rivers that have higher and more stable HQI as the priority selection of rivers to formulate the protective standard.The final assessment results can serve as a reference by county and city governments for developing Japanese eel protection measures.