Masu Salmon

Masu salmon (Oncorhynchus masou) is a species of Pacific salmon native to the coastal rivers and seas of Japan, Korea, and the Russian Far East, making it the southernmost and westernmost distributed member of the Pacific salmon genus Oncorhynchus. It is closely related to, and in English commonly also called, cherry salmon. Masu salmon are unusual among Pacific salmon in that a portion of the population, particularly males, may mature and spawn without ever migrating to sea, remaining as smaller freshwater residents. The species has ecological significance throughout its native range and has experienced population declines in some rivers due to damming and habitat loss.

Masu salmon show the classic Pacific salmon body plan with some distinguishing features:

• Streamlined, slightly laterally compressed body, silvery overall with a bluish-green to olive back
• Small, scattered black spots present on the back and dorsal fin, generally fewer and smaller than in Chinook or coho salmon
• Small adipose fin and deeply forked tail typical of Pacific salmon
• At spawning, sides develop a pinkish-red flush, and males grow a pronounced hooked jaw (kype) and slightly humped back
• Sea-run adults reach up to about 50-71 cm, notably larger than the freshwater-resident form of the same species

Masu salmon are native to coastal river systems and adjacent seas of Japan, the Korean Peninsula, Sakhalin, and other parts of the Russian Far East, occupying the southernmost range of any Pacific salmon species. Sea-run individuals migrate relatively short distances compared to other Pacific salmon, generally remaining in the Sea of Japan and nearby coastal waters rather than ranging across the open North Pacific. Freshwater-resident individuals remain in cool, clear headwater streams year-round. The species depends on unobstructed river access for its spawning migration, and dam construction across parts of its range has reduced availability of historic spawning habitat.

Masu salmon display a flexible reproductive strategy: some individuals, especially males, mature entirely in freshwater without migrating to sea, a pattern less common among other Pacific salmon species. Sea-run masu salmon feed on small fish and invertebrates in coastal waters before returning to natal rivers to spawn, typically in autumn. During the spawning migration, adults stop feeding, develop breeding coloration, and males compete for access to females, with both resident and sea-run males sometimes participating in the same spawning event. Like other Pacific salmon, most adults die shortly after spawning, and their remains contribute nutrients to river ecosystems that support local wildlife.