Pseudaspius sachalinensis is a freshwater fish species in the family Cyprinidae, commonly known in Japan as Ezo ugui (エゾウグイ). It is native to northeastern Asia, particularly Hokkaido in Japan, Sakhalin Island, and parts of the Russian Far East. The genus name Pseudaspius combines Greek pseudo- (“false”) with Aspius, a related genus of cyprinid fishes, indicating its taxonomic distinction from similar carp-like fishes. The species epithet sachalinensis refers to Sakhalin, one of the regions where the species is found. As a member of the carp family, it shares morphological and ecological traits with other freshwater cyprinids.

This species inhabits rivers, lakes, and brackish estuaries, showing adaptability to varying salinity conditions. It is a migratory fish in some parts of its range, moving between freshwater and coastal environments. Pseudaspius sachalinensis has an elongated, silvery body with relatively large scales and is adapted for active swimming in flowing water. Its diet consists of aquatic invertebrates, small fish, and plant material, reflecting an omnivorous feeding strategy typical of many cyprinids.

As a food source, Pseudaspius sachalinensis has been consumed regionally in northern Japan, particularly in Hokkaido, where it is caught in rivers and coastal waters. It may be prepared grilled, salted, simmered or served raw. The Ainu prepare the fish as ruipe, where the raw flesh is sliced while frozen to create a unique, crisp texture that melts in the mouth.

Compounds present in this species include common fish-derived components such as taurine, creatine, and bioactive peptides generated during digestion or processing, which have been studied for roles in muscle metabolism, antioxidant activity, and cardiovascular health. The flesh is relatively lean and contains typical freshwater fish nutrients, including high-quality protein and essential amino acids. It also provides micronutrients such as phosphorus and small amounts of omega-3 fatty acids, though generally in lower concentrations than marine fish.