Change of crab and goby fauna in the mitigation tidal flats in the Naka River estuary

In the mouth of the Naka River in Tokushima Prefecture, approximately 24% of the estuarine environment has been altered because of heightening of the embankment as a countermeasure for the earthquake and tsunami in 2012. Artificial tidal flats, aimed at conserving the habitat of endangered benthic species, were created for mitigation of negative impact due to the construction. To assess the effectiveness of the mitigation, we set nine survey sites in the area, and conducted monitoring the sites 10 times between 2013 and 2017. Thirty three species of crabs and 27 species of gobies were found in the surveys. All threatened and near threatened crabs and gobies found during the first survey were collected during the subsequent surveys as well, suggesting that the habitat of these species has not disappeared due to the construction. We confirmed a decrease in the mean mud content in most of the survey sites in winter 2015, due to a large flush in summer 2014. Contrarily, the mean mud content increased at the mitigation sites due to the introduction of silt dredged from a neighboring river after the large flush. Nine survey sites were analyzed separately in the high (four sites) and low compartments (five sites) according to the range of elevation. The results of the high compartment sites, including two mitigation sites, suggested that the sites had fauna similar to that of the reference sites, after about two and a half years from the construction. On the contrary, the results of the low compartment sites suggested that the sediment condition and fauna changed in most of the sites due to the large flush. However, the dredged silt was supplied around the mitigation sites. Therefore, the fauna similar to that of before the embankment construction was formed around the mitigation sites after its construction.