In the autumn, winter and spring there are lampreys in Maretarium. There are no lampreys in the tank in the summer because they die after spawning in the early summer. Lampreys that have reached spawning age, that is the age of six to seven years, migrate from the sea to a river in September or October. The lampreys in Maretarium are caught in the Kymi river in the autumn. After migrating to the river the lampreys do not eat because their intestines have atrophied. The lampreys live through the winter without eating. The live on the reserves they have gathered during the summer. Lampreys do not move much during the winter, but they use a lot of energy to mature spawn and milt. Lampreys lose weight and even some of their length during the winter. When the water temperature in the tank rises to six degrees centigrade after the cold winter, it is time for clearing the spawning hole and wild mating: the male attaches itself on the back or neck of a female and wraps its tale near the female’s vent. Then the spawn and milt are laid on the bottom of the spawning hole. The female whisks some sand and gravel on top of the eggs. One female can spawn with several males. Sometimes there are up to six males attached to one female simultaneously.
After spawning the lampreys get tired. Lamprey’s organs start to degenerate already in the winter before the beginning of spawning. Especially controlling the fluid balance becomes difficult. As the water gets warmer the water content of the lampreys increase rapidly, they swell and finally die. The dead lampreys are not replaced in Maretarium until the next autumn when lampreys migrate to the river. Those lampreys that have not yet reached spawning age and live in the sea, are not taken into Maretarium because they attach themselves onto other fish and suck the meat of even live fish with their mouth.
The lamprey larvae that hatch from the spawn are called ammocoetes larva. They dig themselves into bottom mud up to their head. After three to four years the larvae undergo a metamorphosis, and the young lampreys float to the sea with the flood the next autumn. There is also a lake form of lamprey, called Brook Lamprey, which inhabits the lakes Saimaa and Päijänne.