ID in the bone

Ever inclined to make an inscription, human beings have figured out how to write their own messages in the heart of the [otolith]. By sequentially altering the temperature of the water in which salmon fry are hatched and raised, researchers can lay a distinctive “batch label” into the chemical layers of the otolith—a kind of barcode, inscribed in stone, and indelibly preserved within the maturing adult fish (a puckish early student of this technique used it to write “hi mom” in binary inside his experimental animal). Later, when these free-swimming creatures are captured at sea, each can be traced unfailingly to its hatchery of origin. Some five billion Pacific salmon have now been marked in this way, their inner qibla reconfigured to refer to their point of origin, and thus the point to which they seek return.

from The Orienting Stone