Seen young, Year-Fish are slips of silver no longer than a finger, with a dark spot behind the gill. Seen old, the same fish are as long as an arm, heavy-shouldered, their scales gone to tarnished pewter, fins ragged, jaws hooked, and a pale film over one eye. Between the two nothing is ever seen. The water does not ripple where they swim, and a jump, when it comes, is heard a few heartbeats after it is seen. The shallows smell of algae and old rain.

They live throughout the Mirror’s pool, the fry in the shallows near the stones and the old fish in the depths, which is where the water’s offset is longest. Pools of the Mirror elsewhere may hold them too; no one has counted.

They feed on midges at the surface and on the weed of the stones, as any pool fish does, and take no notice of watchers. What a watcher sees them doing has already been done, or is still to come. They cannot be caught from the shore: a hook let down reaches a fish that is not there yet.

They have no powers of their own; the Mirror’s Law of Lag is enough. A Year-Fish that is pulled out of the water by an Echo-Diver or a desperate traveller comes out as it is now, an ordinary fish of middle size, and dies as soon as it is taken from the zone.

Spawning is seen only as a cloud of fry that has already hatched; the old fish are seen dying, and their bodies sinking, but no body is ever found on the bottom.

They are harmless, and they are a useful gauge: guides judge the depth, and so the offset, of a stretch of water by whether its Year-Fish show young or old. A traveller who reaches in after one is the one in danger, since the Mirror takes those who touch the water.

The Echo-Divers are said to call the fish their teachers, because the fish live wholly in the present and are never seen there. Visitors tell of a man who watched the old fish die and then saw his own reflection turn old and grey beside it, and left that day.