Masu, or cherry salmon, is a species of salmon found only in the Western Pacific, from the Kamchatka Peninsula to Sakhalin Island, the Kuril Islands, Primorsky Krai and south through Korea, Japan and even Taiwan.
Japan is a major harvester of masu, but the fishery’s commercial importance is difficult to quantify because masu and pink harvest is recorded in aggregate. Historically, Japanese fisheries caught a portion of Russian-origin masu, a practice that likely continues. In Japan in-river recreational fishing for masu adults is illegal; fishing for juveniles is permitted.
Masu have the most limited distribution among Pacific salmon and are present only in the western Pacific. Masu may have diverged further from other salmonids during the last ice age, when lower sea levels likely created a brackish lake of the Sea of Japan. The island of Hokkaido is approximately the center of the range for masu salmon, where masu enjoy protection from harvest in 32 masu conservation rivers.
Masu salmon, found only in the Asian Pacific, have the most limited distribution and are the least abundant of the anadromous Pacific salmon.
Masu and pink are the smallest Pacific salmon. Adult masu measure up to 20 inches (50 cm) and weigh on average 4 to 5.5 pounds (2 – 2.5 kg).
Masu, like coho, reside 1 – 3 winters in freshwater; residence is longer in northerly populations. Masu spend one year in salt water with short ocean migrations, spending most of their time in either the Sea of Japan or the Sea of Okhotsk.
Spawning and rearing habitats for masu salmon are similar to those preferred by coho: fast water in the middle to upper reaches of river systems for spawning, and side channels and sloughs in the lower river for rearing. Although the anadromous fish die after spawning, some of the resident males may survive to spawn again.
Masu inspires debate about whether it is one species or two (amago (anadromous) and rhodurus (resident)). Some treat the southerly landlocked form separately, based on genetically linked coloration patterns. However, the two forms can hybridize and produce viable offspring.
The extensive freshwater life history stage of masu makes them particularly vulnerable to subsistence fishing and sportfishing pressure. They are harvested as juveniles by sportfishers and subsistence fishers in Russia and Japan. In Japan masu juveniles are the only salmon that can be legally caught in rivers. In Russia juvenile masu are sought after, particularly in rural communities. Most Russians, in fact, do not believe the juveniles are the same species as the robust mature fish that return to spawn.