Surimi is a process, not a species. Minced white fish flesh is rinsed with cold water in several cycles, which removes fat, blood, enzymes and the water-soluble proteins that carry most of the flavour and odour, leaving concentrated myofibrillar protein. Cryoprotectants such as sugars are added so the paste survives freezing.
Alaska pollock dominates the supply because of scale, texture and cost. It is caught in enormous volume in the Bering Sea and Sea of Okhotsk, its flesh is mild and white, and its protein forms a strong elastic gel when salted and heated. That gel is what gives a crab stick its characteristic snap and shreddable grain.
The crab flavour is added afterwards, typically as extract, and colour is painted on the outside, which is why the red is only a surface layer. The Japanese product kamaboko uses the same technology without pretending to be shellfish. Because surimi is defined by process rather than by species, the fish in any given package can vary, and label practice differs between jurisdictions.