A quick pond net dabble in the upper Glaven in March revealed one of the rivers least well known but most interesting inhabitants. Hidden in thick silt were a number of juvenile brook lamprey.
Britain has three species of lamprey, the brook lamprey, which grow to about 5 inches, and the larger river and sea lamprey. The lampreys are similar in shape and size to eels, but differ from other fish in that they have no jaw-bone, only a sucker disc.
Brook lamprey are found in the Glaven in large numbers. As is the case for each of the three British species the brook lamprey spawns in clean gravels in spring, digging redds in a similar way to trout or salmon. Once the juveniles hatch, they drift down-stream and settle into silt, where they spend several years filter-feeding before emerging as adults. At this stage both river and sea lamprey migrate to the sea to parasitize larger fish, while brook lamprey remain in the river and do not feed as adults.
It is likely that river and sea lamprey were common in the Glaven before the sluice gates were installed at Cley, which would have limited their entry to the river. In fact a large fresh looking sea lamprey was found washed up dead on Weybourne beach last summer so obviously they are still in the area…
This year we (Jonah and Carl) hunted for spawning brook lamprey on several occasions from late March as Jack Perks, a wildlife photographer that specialises in fish, wanted to capture this elusive event on film. Brook lamprey spawn when the water reaches 10°C and this year, as you might expect, things were late! But finally they went for it in late April. Jack could not get there in time…but next year we hope to get him on the spot!
Jonah Tosney and Carl Sayer (The lamprey hunters)