|Although there are few records of lampreys in the site, there are known populations within the Cleddau Rivers and the fish must therefore pass through both the open coast waters and the Milford Haven waterway to access the rivers systems.
Lampreys are a primitive type of fish that have a distinctive suckered mouth rather than jaws, quite unlike any other fish in Britain. Eel-like lampreys parasitise other fish; by fastening on to the living fish, lampreys rasp into the flesh and feed on the body fluids. Sea (Petromyzon marinus) and river (Lampetra fluviatilis) lampreys spend their adult life in the sea or estuaries but spawn and spend the juvenile part of their life cycle in rivers.
River lampreys (Lampetra fluviatilis) are anadromous. After five years in fresh water the river lamprey spends one or two years in the sea (probably staying near the shore) before migrating back up river, perhaps hitching a ride (and getting a meal at the same time) from another fish. It does not feed once it has reached fresh water and spawns in the upper reaches of the river the next spring.
Sea lampreys (Petromyzon marinus) are larger than river lampreys but have a similar life cycle. They do, however, venture further out to sea and spawn in lower reaches of the rivers than the river lampreys. Pollution and obstacles to migration (such as dams or weirs) can cause decline in numbers of both species.