The iwame trout (Japanese: 岩女魚, romanized: イワメ, lit. 'rock lady fish') or markless trout is a variety of salmonid fish inhabiting some freshwater systems of Japan. It is an intra-specific mutant morph of the masu salmon (Oncorhynchus masou) that is characterized by a lack of the lateral camouflage spots (parr marks) typical of juvenile salmon in most populations. The iwame is a stream-resident morph that does not migrate to the sea as adults. This morph is recessively inherited and occurs together with the standard phenotype in some populations.

This morph occurs in the landlocked amago subspecies (Oncorhynchus masou ishikawae) in western Japan, and apparently also in the nominate subspecies yamame (Oncorhynchus masou masou), if these are not identical.

The morph was scientifically described as a distinct species Oncorhynchus iwame in 1961 by Kimura & Nakamura. In the 2000s, the iwame morph was shown to occur as a polymorphism in random mating populations of O. masou, and is no more thought to have taxonomical value.

• Kano, Y., Shimizu, Y., Kondou, K. 2006. Sympatric, simultaneous, and random mating between markless trout (iwame; Oncorhynchus iwame) and red-spotted masu salmon (amago; Oncorhynchus masou ishikawae). Zoological Science 23:71-77. doi:10.2108/zsj.23.71
• Kano, Y., Kondou, K., Shimizu, Y. 2009. Present status and conservation of the markless forms of stream-resident masu salmon Oncorhynchus masou (the so-called 'iwame') in Japanese mountain streams. Ichthyological Research 57:78-84 doi:10.1007/s10228-009-0130-1
• Kottelat, M. (1996). "Oncorhynchus iwame". IUCN Red List of Threatened Species. 1996: e.T15320A4513336. doi:10.2305/IUCN.UK.1996.RLTS.T15320A4513336.en. Retrieved 17 November 2021.

v t e Trout and char of the world
Trout - Salmonidae
Trout of Europe and Atlantic basin Salmo	Adriatic trout Brown trout Fibreno trout Ferox trout Flathead trout Garda lake trout Gillaroo Isli trout Lake (lacustrine) trout Marble trout Mediterranean trout Ohrid trout Prespa trout River trout Sea trout Sevan trout Sonaghen
Trout of the Pacific basin Oncorhynchus	Apache trout Coastal cutthroat trout Cutthroat trout Gila trout Golden trout Mexican golden trout Rainbow trout Steelhead
True char Salvelinus	Arctic char Blunt-nosed Irish charr Brook trout Bull trout Cole's char Coomsaharn char Dolly Varden trout Haddy charr Kirikuchi char Lake char Lake trout Lough Melvin charr Orkney charr Silver trout Small-mouthed char Whitespotted char
Media related to Trout at Wikimedia Commons