Friday 30 March 2012
Brook lamprey are a small, primitive fish, which resemble eels. They are the smallest of the three lamprey species found in the UK. Brook lamprey are a non-migratory freshwater species, occurring in streams and occasionally in lakes in north-west Europe. Although quite a widespread species, they have declined in many areas of the UK and were thought to have disappeared from the River Chess. However, apparently triggered by the unseasonably warm weather, a number of these primitive, jawless fish were seen spawning in the river at Rickmansworth on Tuesday.
Chalk streams like the River Chess provide ideal conditions for this unusual fish as they provide high water quality, a clean gravel bed for spawning and soft silt for the larvae to live in. Brook lamprey spend the majority of their life as larvae known as ammocoetes, buried in the silty margins of the river. Blind and lacking the sucker-like mouthparts of the adults, they filter feed on algae and diatoms for anything up to seven years.
As temperatures rise in spring, the fully grown amoecetes undergo metamorphosis into the adults. Unlike their larger relatives the river lamprey and sea lamprey, which suck the blood of fish, brook lamprey do not feed when adult. Instead they migrate to suitable spawning gravels, congregating sometimes in quite large numbers to excavate a nest, using their suckers to pull up stones from the river bed. Once the nest is completed the males compete with each other to fertilise the eggs as the female releases them. After spawning the adults die leaving the eggs to develop within the gravel of the nest for about a month before hatching.