Thursday, 2 August 2012

American Signal Crayfish biosecurity on the River Kennet

The fact that the River Kennet is literally crawling with  Pacifastacus leniusculus, the American Signal Crayfish (ASC),  is nothing new, with The Field belatedly reporting "Danger Signs" in 2008, whilst the same year The Telegraph produced the headline "American Signal Crayfish wrecks UK waters", nearly twenty years after their introduction to the UK.

In 2011 the Crayfish issues on the Kennet again came to national attention after British Waterways responded to "American crayfish destroying Britain's footpaths" in Newbury with protective reinforcement of the banks with polypropylene mesh.

Protection of the banks of the Kennet Navigation on limited sections of the river may provide limited local defence from tunneling erosion, but also serves to concentrate the damage elsewhere.

A number of fisheries have employed the services of trappers in an attempt to reduce the impact of crayfish predation and environmental damage...

Crayfish traps on the River Kennet Navigation (and elsewhere in the region - see this week's report from the Oxford Mail) are said to produce substantive hauls for their licenced users...

...which is evidenced in such as this half-dustbin catch of ASC from the Sulhamstead loop.

 ...but whilst a conversation with a trapper on the lower Kennet produced the comments that the average size of trapped ASC was falling and that fisheries were pleased with the effects of taking out the larger individuals which was resulting in the recruitment of improved numbers of smaller fish with the potential to grow to specimen size, this might not be acheiving much in terms of reducing overall ASC numbers.  ASC are cannibalistic, which means that as the larger specimens get caught more smaller crayfish survive, and so the total biomass of crayfish in the river may actually increase.

ASC are only one of the six non-indigenous crayfish species in the UK; details of Narrow-clawed, Spiny-cheek, Noble, Virile and Red swamp crayfish can be found at Buglife's UK Crayfish Website. Of the six invasive alien species, the Noble is only currently known in Somerset and Gloucestershire and does not out-compete our single native species, the White-clawed crayfish, Austropotamobius pallipes.  The Narrow-clawed crayfish is recorded more widely in SE England and has the potential to outcompete the White-claws, but, like the Noble, it is susceptible to the fungal crayfish plague, Aphanomyces astaci.  Whilst the Virile and Red swamp crayfish are only currently recorded in London, they are, together with the Spiny-cheek (also resident in SE England) and the ASC (interactive distribution map here), immune to and carriers of crayfish plague.

For paddlers on the River Kennet biosecurity measures to avoid spreading crayfish plague are simple:

Further information on wider biosecurity measures for paddlers can be found here and watch this: