I've stood above spawning American brook lampreys and taken pictures, measured nests, and even touched the lampreys without disturbing them. Female sea lampreys are known to lay more than 100,000 eggs, but native lampreys lay far fewer. Just as salmon do, lampreys die when they finish spawning. Juveniles resemble adults, but they lack eyes and broad fins and have a folded, hoodlike mouth instead of a sucking disk. Unless disturbed, the ammocoetes remain buried in sediment, growing to night crawler size while filtering bacteria, algae, and other microscopic food particles.
Researchers rely on electrofishing units to coax ammocoetes from their muddy homes.
Before the advent of electrofishing, researchers scooped buckets of mud from likely habitat and spread the mud on the stream bank, hoping to spot ammocoetes wriggling through the goo. My students at Winona State University and I usually have the best luck finding ammocoetes around submerged woody debris, where sluggish eddies apparently concentrate lamprey foods. We have collected as many as 20 ammocoetes near a single submerged log. In late summer the largest, oldest ammocoetes begin the gradual changes that transform them into adults. Most obviously, they develop eyes, broader fins, and suction-cup mouths.