A recent phylogeographic assessment that included samples from the Far-East Eurasian continent, Sakhalin Island, Japan (including Hokkaido, Honshu, Shikoku, Kyushu), and Taiwan) indicated no marked regional or subspecies-specific mtDNA haplotype associations, with the exception of an O. masou subspecies, a lacustrine form endemic to Lake Biwa, an ancient lake in central Honshu (Yamamoto et al. 2019). This analysis include the landlocked subspecies O. masou formasanus, which showed no diagnostic population features differing from the other species.

Oncorhynchus masou is found in the north-western Pacific. The overall population trend of this species is unknown. Threats to this species include habitat degradation from blockages to spawning routes, illegal fishing, and potential over-exploitation. It occurs within multiple protected areas and is protected under fishing regulations. Being widespread, protected under fishing regulations and occurring in protected areas, this species is assessed as Least Concern.

This species is found in the north-western Pacific, ranging from the Korean Peninsula to the west of Kamchatka (Kamchatka River), including the South Kurils (Kunashir and Iturup), the islands of the Japanese archipelago, and Taiwan.

In Kamchatka, this species is the least abundant species of all the Pacific salmon species, generally not accounted for by fishery statistics and at a stable abundance (Tokranov and Seiko 2006). In the Amur, abundance is not determined, but at present juveniles of this species are one of the most numerous components of fish communities in tributaries of the lower Amur (Antonov et al. 2019). The overall population trend of this species is unknown.

Oncorhynchus masou is the most thermophilic representative of Pacific Salmon, whose distribution in the sea is limited by isotherms 8-12 °C (Tokranov and Sheiko 2006, Antonov et al. 2019).

A polymorphic species, it is known to easily form resident forms. Some males (dwarf resident) do not go out to sea, instead maturing in the river, and do not necessarily die after spawning (Tokranov and Sheiko 2006, Antonov et al. 2019). No dwarf females have been observed in the Amur, in contrast to other regions (Tokranov and Sheiko 2006, Antonov et al. 2019).

Spawning in the Amur begins in May with a peak in July and spawning in August (Tokranov and Sheiko 2006, Antonov et al. 2019). Spawning usually takes place in the upper reaches of rivers, where there are usually no spawning grounds for other salmon species (Tokranov and Sheiko 2006, Antonov et al. 2019). This species builds redds and bury their eggs in small fast flowing tributaries with stony-gravel substrate. Young fish in the river ("parr") live in the river for one to two years (commonly two) feeding on amphibious insects and their larvae (Tokranov and Sheiko 2006, Antonov et al. 2019). This species migrates to the sea in summer in marine silver colouring (smolts), entering the Pacific Ocean, bypassing the northern tip of Sakhalin Island and further into the Sea of Okhotsk to the Kuril Islands (Tokranov and Sheiko 2006, Antonov et al. 2019). The migration of Amur Masu Salmon is shorter than that of the Chum and Pink Salmons (Tokranov and Sheiko 2006, Antonov et al. 2019). Most fish live in the sea for two years, less often one or three. Oncorhynchus masou within the Amur, spends one year in the sea, near Hokkaido Island. It returns to rivers at the age of 3-5 years (Tokranov and Sheiko 2006, Antonov et al. 2019).

This species is rare and under threat from commercial fishing, although it is not considered very commerically important. Also, this species faces threats of mass catch of juveniles (parrs), which local amateur fishermen consider a separate species of fish, as well as illegal fishing and blockage of migration routes to spawning grounds in rivers (Antonov et al. 2009).

The factor limiting the number of Cherry Salmon in Kamchatka is temperature limits, as climatic conditions in the rivers of the peninsula, as well as in the waters of the surrounding seas, are much more severe than in the areas of the main range of the species, where its ecological optimum is located. In Kamchatka, it is found only in rivers located to the south of 59° 00' N (Tokranov and Seiko 2006). Other threats include changing climatic conditions in places of spawning and sea foraging (Antonov et al. 2009).

This is a commercial species, but it is not very important. In recent years, official fishing for Cherry Salmon has been prohibited almost everywhere in Russia, and only six tons are allowed for aquaculture purposes (Reshetnikov et al. 2002). This species is artificially propagated in Primorye and Sakhalin (Reshetnikov et al. 2002).

This species is in the Red Book of the Khabarovsk Krai, where it was listed in the 1990s without biological justification, despite the fact that in all other areas remained a commercial species (Antonov et al. 2019). In recent years, official fishing for cherry salmon has been prohibited almost everywhere in Russia, and only six tons are allowed for aquaculture purposes (Reshetnikov et al. 2002). This species is protected as an object of illegal fishing in the lower reaches of the Amur River (Antonov et al. 2019). This species occurs in multiple protected areas within its range, including Tumninsky State Nature Reserve, Russia and Shiretoko Nachional Park, Japan. Further research on population dynamics is recommended.

While focus on species-level status assessments are an important first step, the IUCN Species Survival Commission (SSC) Salmon Specialist Group (SSG) emphasizes the need to characterize status of Pacific Salmon at a more granular, population-level scale (identified as “subpopulations” in the IUCN Red List Guidelines) to provide meaningful guidance to stem the loss of biodiversity across the natural range of the species. There are many examples of declines in wild Pacific Salmon in both North America and Asia, particularly in the southern portion of their range given the degree of degradation and fragmentation of habitat there and the more immediate risk of climate change impacts. At the same time, there are large-scale ocean drivers that appear to be affecting species broadly across the North Pacific, regardless of their freshwater origin. Two excellent examples exist of assessment approaches and policies in the US (Waples 1991) and Canada (DFO 2005, COSEWIC 2018) that establish an effective framework for Pacific Salmon conservation. These efforts involve identifying population units based on a variety of criteria including examination of traits that are important in the evolutionary process and future adaptation. In these examples, assessments are conducted at a more granular, population-level, resulting in listings for individual population units, with identification of needed conservation actions specific to each unit. An example of assessing range-wide status of the species and at the individual subpopulation level in the IUCN Red List now exists for Oncorhynchus nerka (Rand 2011). While the amount of effort required to rigorously assess the species is substantial, we encourage efforts like this applied to the other species in the genus.