One of the likely causes of the decrease in the abundance of the Japanese eel Anguilla japonica is the reduction of refuge habitats, such as rocky areas or rock piles, due to artificial river renovations. To determine whether the presence of rock piles affects predation on this species, we compared the survival rate between groups kept with the rock-pile structure (experimental group) and groups kept without the rock-pile structure (control group) in simulated natural conditions. The survival rate of the experimental group was found to be significantly higher than that of the control group, strongly indicating that the rock piles function as vital shelters to protect Japanese eels from bird predators.
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