The salmon, which returns from the sea to the very river of its birth to spawn, holds a special place in the mythology of the Celtic world. In Irish tradition the Salmon of Knowledge fed on the nine hazelnuts of wisdom that fell into a sacred well, taking into itself all the knowledge of the world; whoever ate the salmon would gain that wisdom. The young Fionn mac Cumhaill acquired it by accident, burning his thumb on the cooking fish and putting it to his mouth, so that ever after he had only to bite his thumb to know what he needed, the salmon standing, throughout Celtic lore, for the deepest wisdom and the oldest memory.
Science has since explained the natural fact behind the salmon's mythic homing: the biologist Arthur Hasler demonstrated in the 1950s and after that salmon imprint on the distinctive chemical signature of their home stream as juveniles and use that memory, detected through smell, to navigate back to the same river, and often the same stretch of gravel, to spawn years later, a genuine olfactory feat that gives literal grounding to the fish's old reputation for carrying the oldest and deepest memory. A species distinction sharpens the myth: the salmon of Irish and British tradition is the Atlantic salmon, Salmo salar, which can survive spawning and return to the sea to breed again, unlike the Pacific salmon of the genus Oncorhynchus, which die after their single spawning run, so the fish that carried the deepest wisdom in Celtic lore was, biologically, also one capable of repeating the journey.