Signal Crayfish Everywhere!

 

Signal Crayfish (note the white spot 'signals' on the claws)

I was recently doing some survey work in Settrington, North Yorkshire within the Beck and could not help but find huge numbers of Signal Crayfish within the stream bed. Searching beneath logs, amongst roots, beneath overhanging vegetation stands and around marginal and submerged vegetation was particularly productive for finding them. Sampling gravels revealed large numbers of tiny juveniles which presumable seek out this habitat due to the smaller cavities it provides as well as the lack of larger cannibalistic adult animals, within it. Herein lies one of the issues with Signal Crayfish, ultimately when they have eaten all the fish and all the invertebrates they happily maintain their population by eating themselves, keeping the water course as a lifeless crayfish soup. 

This particular population are rumoured to have originally been released in the large lake at Settrington House, and subsequently have found their way along the attached Settrington Beck. I am not sure how true this is or the timescales involved but it makes perfect sense. Based on my finding the stream had good numbers of both Minnow and Stone Loach, and invertebrate numbers were interesting too with lots of gammarids and burrowing stonefly nymphs (Ephemeridae), however both caddis fly, other species of mayfly and stonefly nymphs seemed lower than what I would expect which is possibly evidence that the population is beginning to become suppressed by crayfish predation. Both Gammarids and Ephemerid nymphs burrow and can quickly avoid the risk of predation being less likely to encounter Crayfish below the riverbeds surface.   

Of the Crayfish themselves only Signal were recorded which isn't too surprising, White Clawed Crayfish rarely (if ever) persist within areas with such high competition from Signal Crayfish, not to mention the dreaded crayfish plague which blights our native White Clawed Crayfish. This is a large subject and one I will probably delve into more detail another time. There had been old records of White Clawed Crayfish here although I am not sure of the validity of these records, or whether White-Clawed Crayfish ever existed here and were in fact misidentification of Signal Crayfish, which seems plausible. 

The section of Beck surveyed was relatively short <200m but the distribution of Crayfish was interesting. Signal Crayfish are an adaptable species and readily flourish in silty habitats avoided by White-clawed. Although it was clear to see an increase in abundance here with far more Signal Crayfish found from areas of clean riverbed with gravels and rock lacking silt, with the density dropping considerably from areas with a silty bed. 

Interestingly too was the age structure within this section with around 60% being yearlings, 35% being 1-2 years old and the remainder being adults which were scarce. This is most likely due to the large animals being concealed in burrows during the day. This watercourse lacks many large rocks or refugia along its bed to support larger animals, with the banks instead riddled with large crayfish burrows. These large animals will emerge during the cover of darkness to feast on other river inhabitants, including the smaller subadult crayfish in other parts of the watercourse. Interestingly less juveniles occurred in the same sections as burrows, whether this is just a coincidence or some kind of avoidance is hard to say from this limited sampling. Either way the inhabitants of this small watercourse are in serious trouble, with the masses of Signal Crayfish which are present.