Japanese Eels Face Threat From Warming Rivers

The distribution of Japanese eels (Anguilla japonica) at the northern edge of the species' range appears to be shaped by river water temperature, which is influenced by watershed geology and land use. Osamu Kishida and colleagues conducted electrofishing surveys in 105 rivers across southern Hokkaido, Japan, capturing 222 Japanese eels from 52 rivers. The authors used structural equation modeling that incorporated catch per unit effort, environmental variables, and estimates of glass eel recruitment—the number of juvenile eels that enter rivers from the sea, where they are born— based on an ocean circulation model. The analysis showed that eels were more abundant in rivers with higher summer water temperatures. Rivers with watersheds dominated by farmland and urban areas tended to have warmer summer temperatures, while rivers in watersheds with volcanic geology, particularly younger volcanic formations, had cooler summer temperatures. The volcanic geology's high permeability allows rapid infiltration of precipitation, resulting in spring-fed rivers that remain cooler in summer. According to the authors, global warming combined with urbanization may facilitate northward expansion of Japanese eel populations, though expansion in this region may be constrained by past volcanic activity that shapes the thermal environment of rivers.