Neglected forests in Gifu Prefecture have caused a significant decline in ayu fish catches in the Nagara River over the last 30 years [1].
This environmental shift highlights the critical link between upland forest management and river health, showing how land neglect can destroy aquatic habitats downstream.
According to Seiro Shinoda, a professor emeritus at Gifu University, forests that are left untended for long periods cause the soil to dry out [2]. Shinoda said that invasive tree species in these neglected areas absorb excessive moisture from the ground. Furthermore, the dense canopy of these trees blocks rain from reaching the forest floor, which inhibits the growth of underbrush and further dries the soil [2].
When heavy rains occur, the dried-out soil cannot absorb water effectively, leading to massive amounts of sediment flowing into the river [1, 2]. This influx of soil degrades the river environment and destroys the specific habitats required for ayu to thrive.
Local fishers have noted that the river's physical structure is also changing. Kentaro Hiraku, a Nagara River fisherman, said the river is being straightened, which removes the series of rapids and deep pools that fish depend on. He also said that the river mouth weir, constructed approximately 30 years ago, remains a primary obstacle for the fish [3].
While some views suggest that the overall decline of the Japanese fishing industry may be a misconception, the specific trend for freshwater fish like ayu in this region remains a stark decrease [1, 3]. The combination of forest neglect and river engineering has created a challenging environment for the species to survive.
“"Long-term neglect causes the soil to dry out," said Professor Emeritus Seiro Shinoda.”
The situation in the Nagara River demonstrates that river conservation cannot be isolated from forestry. When the 'green dam' function of a forest fails due to lack of maintenance, the resulting sediment runoff creates a cascading failure that affects biodiversity and the local economy. This suggests that restoring fish populations requires a holistic approach that includes both thinning invasive forest species and removing man-made river obstructions.
