Prehistoric fish breeding in Cumbrian rivers!


Sea Lampreys (Petromyzon marinus) were a common sight in UK rivers but have suffered dramatic declines in the last few decades and are now an endangered species. These ancient snake-like creatures resemble an eel but unlike most fish, they lack scales, jaws and gill covers and have a cartilaginous skeleton. Their sucker-like mouths, pointy teeth and rasping tongue are by far their most unusual features.

Their creepy appearance is matched by their feeding strategy: they use their mouth to stick onto the side of fish, use their tongue to scrape a hole into the fishes flesh and then drink the fishes blood. They use their anticoagulant saliva to stop the blood from clotting until they have had their fill!

On the top of their heads they have a ‘pineal gland’ which detects changes in light and helps the lamprey detect its prey!

As their name suggests, Sea Lamprey spend most of their lives at sea but they return to freshwater environments to spawn. They rely on clean, undisturbed river systems to breed so the sightings of spawning Lampreys in Cumbria are an encouraging indication of the health of the regions rivers.

On the 18th June, with the help of local private fishery bailiff Chris Slater, staff from South Cumbria Rivers Trust filmed Sea Lamprey spawning in the River Leven. The fish were seen building their nests, which are called redds, in the gravel river bed. Several pairs of male and female fish were filmed excavating holes for their eggs.

Female Lampreys typically lay between 10,000 and 100,000 eggs in each redd. These eggs remain in the gravels for several weeks where the flowing water provides oxygen for the developing embryos. Once hatched, the young Lamprey larvae (called ammocoetes) move to slower flowing areas of the river where they spend the next few years in burrows in the silty riverbed. As the larvae grow, they undergo a metamorphosis from the blind toothless ammocoetes to the adult stage which has developed eyes and their characteristic sucker mouth. After metamorphosis, the adults migrate to sea where they feed on fish for around 2 years before returning to river to spawn.

The protection of this fascinating prehistoric creature is at the heart of South Cumbria Rivers Trusts work which involves habitat improvements which include river bank protection, reducing pollution and installing fish passes to allow eels and lamprey to navigate over manmade structures such as weirs.