Lampreys are very rarely seen in our rivers as they have extremely reclusive and secretive habits. They remain hidden during all of their juvenile life, living in burrows within silt beds. After they emerge from their burrows as adults they are then largely nocturnal, undertaking their migrations at night. The best time to see them is at breeding time when they seem to throw off their otherwise reclusive habits.
When breeding the adults excavate a spawning pit in a gravel bed by moving stones with their sucker-like mouth. The size of stones that are moved varies, depending on the size and strength of each species. Sea lampreys are able to excavate the largest spawning pits, and it is not unknown for salmon fishermen wading in the river to stumble over them.
River and brook lampreys tend to breed during the spring (March to April), whereas sea lampreys breed in late spring and early summer (May to June). Several adult lampreys can breed together and breeding may extend over several days. The adults seem oblivious to human disturbance, and often spawn during the day in open areas of the river. This makes them vulnerable to predators such as herons, gulls and mink.
The fertility of lampreys varies with the smallest species, the brook lamprey, producing an average of around 1,500 eggs. In contrast, a far larger sea lamprey can produce more than 170,000 eggs. The eggs are small, white and deposited in the spawning pit where they stick to the stones. The eggs typically incubate for less then a month before hatching, and the emerging juveniles then swim or drift downstream to find suitable silt beds for burrowing.