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Posted: Monday 8th February 2016 by Living Rivers

Photo of a bullhead by David JohnsonBullhead (David Johnson)

In his latest blog, Living Rivers Officer David Johnson, takes a look at some of the larger specimens found at the bottom of the river sampling trays.

It has been a busy first month of the New Year getting planning work in place for Slimy Wrigglers and Riverfly Day and writing project reports for the Mimram. This has meant that I’ve been bound to my desk and had very little chance to visit any of our rivers. So you can imagine it was a moment of respite when the time came to monitor my monthly Riverfly site on the River Lea on a very cold frosty Saturday morning in Wheathampstead.

Despite only targeting four riverfly groups, there is often a large amount of interesting bycatch in Riverfly samples that people get excited about. “What is this fish?” Is a question I answer frequently, and not just locally. Nationally Riverfly monitors across the country show their enthusiasm and desire to learn uploading photos of the larger specimens they find swimming about their sample trays to social networking sites. I thought I would take the opportunity this month whilst work on the ground is quiet to share a bit more information about some of the non-Riverfly things you might find in the bottom of your tray.


BullheadBullhead (Cottus gobio) or the millers thumb (so called for its resemblance to the battered squashed thumbs of a miller), is a small fish with a wide flattened head, large mouth, prominent eyes large fins and a tapering body that would look less out of place in a coastal rock pool. In fact the Bullhead is the only native freshwater species of Sculpin found in Britain. The mottled brown bullhead - found in medium sized gravel and rocky bottomed streams - is reckoned to live its whole life under the same rock, venturing out to feed on the very same Riverfly larvae ARMI monitors look for. It is therefore not surprising that the bullhead is one of the most commonly caught species of fish I have observed in Riverfly monitoring.

Stone Loach

Stone loach (Barbatula barbatula) is one of only two freshwater loaches found in Britain, though you won’t find its cousin the spine loach in Hertfordshire. The stone loach is a small, slender fish with a long body and three pairs of barbels on its lower jaw. This yellowish-brown blotchy little fish is widespread in Britain’s chalk streams and upland rivers, though is a little known species even amongst staff here at the Trust. The stone loach is not targeted by anglers and is difficult to catch in electrofishing surveys so often goes unrecorded, yet shows up all too often in Riverfly samples thanks to its preference for burying itself into the gravel river bed from which it emerges at night to feed on Riverfly larvae and other small invertebrates.

American Signal Crayfish

CrayfishSignal crayfish (Pacifastacus leniusculus) you may unfortunately be all too familiar with. Voraciously eating fish eggs, destabilising river banks and out competing our native white claw crayfish, whilst simultaneously spreading to them the deadly crayfish plague. The signal crayfish is a rapidly reproducing non-native invasive species wreaking havoc to rivers and wildlife up and down Britain. Smaller specimens of the signal crayfish are most likely to show up in Riverfly samples, though growing up to 18 cm long, larger specimens can easily be spotted from the river bank. Easily identifiable as a crayfish from its lobster-like appearance, the signal crayfish can be distinguished from the native white claws (and other non-natives) by the bright red colouring on the underside of the claws. Unfortunately there are currently no effective methods to eradicate signal crayfish from river catchments, the best way being to prevent introduction at all, for that reason it is always important when you have undertaken any activity in your local river to follow the Check, Clean, Dry biosecurity measures produced by the Non-Native Invasive Species Secretariat, which not only helps prevent the spread of crayfish, but also many other invasive species like the demon shrimp and killer shrimp.


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