Photo: Piscary

Iwame trout

Oncorhynchus iwame · Salmonidae

Freshwater
Salmonids
Hard

Size
25 cm

Where to fish

River

Habitat icons: Flaticon

The iwame is an extremely rare depigmented form of the Japanese yamame, recognizable by its nearly silver coloration without the typical salmonid markings.

Calendar

Its range

This salmonid exists only in Japan, and its range is not a territory but a list. The known populations are landlocked, each in its own basin — the Ôno, the Niyodo, the Inabe, the Ibi, the Sakawa, the Hananuki. Six rivers, six nuclei with no contact between them.

A map of that shape records a partition rather than a conquest. A salmonid that no longer goes down to the sea is a prisoner of its own network; each basin then evolves on its own account and receives nothing from outside. A local extinction is repaired by no natural recolonisation, and what is observed in one of these rivers does not transfer to the other five.

Breeding

The ovary of this fish ripens as one block. Its organisation is synchronous and its fecundity determinate: the stock of oocytes is built before the season, is not added to along the way, and the female releases what she formed, with no second attempt before the following year.

That arrangement ties the spawning to the condition of the female several months before it happens. A poor growing season is paid for at the next reproduction, and nothing corrects the shortfall in between. In a fish shut inside a small mountain basin, where the food available depends on a narrow flow regime, the rigidity counts double.

Areas

Isolated Japanese mountain torrents
Remote headwater river sections
Deep pools below waterfalls
Rocky riffles with strong current

Techniques

Dry fly fishing (very stealthy approach)
Traditional tenkara
Very light lure fishing
Free-drifting nymph

Baits

Imitative dry flies
Artificial nymphs
Natural insects
Micro-spoons

Conservation

Nobody knows what state this fish is in. No monitoring says whether numbers are rising, falling or holding: the last international examination, in August 1996, stopped at the finding that the data were not there, and it has been carried forward since without review.

Thirty years of silence weigh more here than they would elsewhere. The fish is split into six nuclei, none of which can relieve another, so a population can go out without the apparent total moving much and without anything registering that it has gone. The rest of the file is empty in the same way: the animal falls under neither CITES nor the convention on migratory species, and it presents no danger to people.