Elimination of signal crayfish from the Glaven tributaries? Well “attempted elimination” would be a better description as we have not reached the end of the exercise by a long way. Indeed some experts says we never will!
Three years ago a population of signal crayfish were discovered in some small ponds (Lawn Farm ponds) at the top of Watering Lane, an important tributary of the Glaven in the Kelling-Letheringsett area. Signals were also found in the stream below the ponds, but for some as yet unexplained reason these pernicious and unwanted invaders had not got past Letheringsett Lake (lake towards the end of the tributary). We jumped at the opportunity of tackling this issue while there was still a chance of containing the problem.
The favoured route was biocide treatment, but after considerable investigation the idea had to be abandoned. This left the option of trapping and in view of the very small water courses and ponds involved we thought it well worth a go…
So in April 2011 no fewer than 15 traps and monitoring devices were installed along Watering Lane. The traps were of the barrier type (right across the stream), drop traps (boxes inserted into the stream bed into which the crayfish fall) and monitoring traps (opened pipes which the crayfish retreat into as refuges).
As of today we have removed 400 signals from Watering Lane and we still have three drop traps in place. All of this has been possible thanks to the continued support of Letherinsett estate who have tolerated the invasion of their space with some considerable understanding.
Signal catches in Watering Lane are now much more infrequent and generally consist of tiny animals likely hatched last year. We are confident, therefore, that the immediate threat to the Glaven’s native white-clawed crayfish population has been resolved. But…we will continue until we are completely confident that Watering Lane is free of signals and the Glaven’s white clawed crayfish population is safe.
In the summer of 2011 we also decided on tackling the signals in Lawn Farm ponds (from whence the Watering Lane infestation had originated). Thanks to the enthusiastic support of Lawn Farm’s owners we started an intensive programme of trapping using no fewer than 20 fyke net type traps kindly provided by the Environment Agency (who at all times have guided, helped and authorised our activities). The results have been breathtaking!
We were catching 200 to 350 mature signals per day at the start (for a brief period my popularity was high as I gave away very delicious eatable signal crayfish to friends). This honeymoon period did not last long…maybe a few months.
As we made inroads into the mature population catches of mature crayfish started to drop. However, it soon became obvious that vast numbers of immature fish were escaping through the relatively course mesh of the traps. Various methods were attempted to solve this problem, including pulling womens’ tights over the traps! This worked well but was cumbersome, time consuming and shops in Holt were beginning to look askance at me as I bought yet more pairs of tights!!
Willie Weston of Blakeney Lobster fame came to my rescue with the loan of 6 beautiful lobster pots especially designed to trap crayfish down to a very small size. These 6 traps have saved my determination to push on as they are robust, very easy to empty and easy to bait (using fish shop leftovers).
The net effect of all this is that after 300+ days of daily trapping we have removed 10,350 signal crayfish from the Lawn Farm ponds!!!
I have been hugely encouraged by Martin Pugh of Essex Wildlife Trust (very knowledgeable about all crayfish aspects) Lorraine Marks of the Environment Agency (whose idea it was to use the tights) and Jeff Compton who is analysing all the catch information and reflecting on what we have learnt. He forecasts that the signal crayfish will eventually succumb…I do hope he is right!!!
Watch this space for occasional updates on the situation…
Robin Combe (Chairman of the RGCG)