People longingly refer to Itou (sea-run taimen) as the “Phantom Fish” with a special sentiment. Although this reflects the species’ mysterious nature, a rare chance of encountering them is definitely disagreeable. According to previous records, sea-run taimen once lived in 42 river systems in the entire Hokkaido region, two in Aomori Prefecture, and one in Iwate Prefecture. As of now, however, there has been no report of their inhabitation in Aomori and Iwate, and even in Hokkaido, there are only eleven river systems in which they are still surviving: Six river systems are supporting stable populations whereas the remaining five are on the verge of losing them. Consequently, sea-run taimen were not only listed as Threatened IB (Species facing a high risk of extinction in the near future) in the Red List of Japan’s Ministry of Environment in 2007 but also designated as Critically Endangered in Hokkaido’s Red Data Book in 2001. In addition, the International Union for Conservation of Nature (IUCN) listed this species as Critically Endangered, the most threatened category. Instead of just allowing this crisis to worsen, we are committed to protecting the river systems where sea-run taimen currently exist in order to maintain their populations stably.

Dams and other transverse structures constructed in rivers are one of the culprits causing the populations of sea-run taimen to decrease. As mentioned in the “Biology of Itou” section, taimen repeatedly spawn during their lifetime and they migrate from the upper reaches of the river to the estuary and brackish water area every time they spawn. This species utilizes the whole river in its life history but obstructions built on their way back upstream make it impossible for taimen to run up the river. Although there have not been many large-scale dams constructed in the rivers inhabited by sea-run taimen as those rivers have gentle gradients and flow through wetlands, a numerous number of relatively small-sized river structures have been built instead such as drops with an elevation of around 1 meter, and culverts built upon forest road construction. These structures block sea-run taimen from reaching their upstream spawning sites, which leads their population size to decrease, and causes the species to eventually disappear from the rivers.

An interesting fact reveals in comparing the terrains of the rivers inhabited by sea-run taimen and those in which they have disappeared: Taimen are surviving in the rivers that flow gently and sometimes meander through wetlands and wildernesses that stretch over the cold region. This is probably because these river basins are not suitable for agricultural development, which allows their natural environment to remain intact. Rivers erode the outer banks when meandering and generate a sequence of pools and riffles. The area in which a pool, deeper and calmer area, turns to a shallower riffle is affluent and imperative as taimen’s spawning beds. However, channelization for the purpose of drainage, etc. destroys this riffle-pool sequence, depriving the adults of their spawning sites and the juveniles of feeding areas.

Pools with riparian trees overhanging the river are the cradles for juvenile sea-run taimen. Aquatic insects that grow on organic matter and nutrients such as fallen leaves serve as food for the fish. Fallen trees obstruct the water flow and make deep pools that provide them with the best refuge. Also, leaves of overhanging trees create shade and block the strong summer sunshine. However, river improvement work for flood control eliminates such precious riparian forests and dramatically deteriorates the rearing environment of juvenile taimen.