RIVER NAR CRAYFISH SURVEY 2010

Transcription

1 RIVER NAR CRAYFISH SURVEY 2010 Prepared by Steve Parnwell BA (Hons) AIEEM FLS with David and Gillian Shearing Registered in England & Wales Greenwillows Associates Ltd VAT Registration No: Registered Office: 88 High Street, Ramsey, Huntingdon, Cambridgeshire PE26 1BS

2 INTRODUCTION This survey was commissioned by the Norfolk Biodiversity Partnership with the main aim of obtaining a rapid assessment of the distribution and status of crayfish populations (both native and non-native) of the River Nar which is designated as a Site of Special Scientific Interest. Until recently there were no records of any crayfish (native or non-native) and the river was being considered as a good candidate to be made an Ark catchment. The Environment Agency issued trap licences in 2007 to a third party for the capture of Red Signal Crayfish (Pacifastacus leniusculus) for consumption. The understanding was that the Red Signal Crayfish were present in the lower reaches of the river (unit 40) but the exact location and spread is currently unknown to us at this point in time. The nearest known population of Red Signal Crayfish was recorded in the lower reaches of the River Babingley at TF approximately 8km or 5 miles from where the River Nar discharges in to the tidal River Great Ouse in Kings Lynn. This report outlines the methodology employed, the results obtained and recommendations for follow up actions.

3 METHODOLOGY The survey was carried out between the 18 th September 2010 and the 11 th October Using a 1:25,000 scale Ordnance Survey Explorer Map the extent of the River Nar was plotted and bridges and other easily accessible sites were marked. The river has its source in a field 2 kilometres North West of Mileham at TF and flows for a total of 42 km in roughly a Westerly direction before discharging in to the tidal Great Ouse in Kings Lynn at TF bridge sites, 3 fords and 2 easily accessible stretches of river were identified. These sites were visited sequentially in a downstream direction and were photographed and a habitat assessment was recorded. Each habitat assessment included estimates of the average water depth, channel width, water flow, water clarity, % of silt present. The presence of sand, gravel, cobbles, rocks, tree roots, weed and other potential crayfish habitat such as bricks, concrete etc. were also recorded. The water temperature at each site was recorded. Each site was also assessed for suitability for hand searching (to include kick sampling and netting if appropriate), trapping or night torching by boat or bank. In addition each site was graded on its overall suitability to support crayfish and any incidental sightings of any other significant species were noted. HAND SEARCHING If a site was assessed as suitable for hand searching (to include kick sampling and netting as appropriate) this was carried out during the first visit. A site was assessed as suitable for hand searching if it had permanent water, suitable crayfish habitat such as cobbles, rocks, tree roots, weed etc and was not excessively over shaded by trees or other vegetation. Each site assessed as suitable for hand searching was searched for a minimum of 20 minutes and an area of at least 20 metres upstream and downstream of the bridge or access site was searched. All potential refuges such as cobbles, rocks, bricks and lumps of concrete were carefully lifted to see if any creatures were beneath them. All of these refuges were carefully lowered back in to their original position after searching. Hands were used to investigate any overhangs, burrows, tree roots or crevices in the structure of the bridge. Kick sampling was carried out where the flow was fastest and netting was carried out in and under weed beds either where rooted to the bottom or occurring as hover. No stone was left unturned at each site unless it was in water that was too deep or silted. TRAPPING A site was trapped if it had a depth of water and/or silt which made wading to hand search potentially dangerous.

4 Another important consideration when assessing a site for its suitability for trapping was the possible presence of Water Voles (Arvicola amphibious). When the application for the trap licences from the Environment Agency was made the Agency took advice from Natural England who had concerns about trapping and the risk to Water Voles which are known to be present on the River Nar. The Agency made reference to some recent recommendations from David Rogers, Lead Partner, UK Biodiversity Steering Group (BAP) for the White-clawed crayfish. He has designed traps with an escapement hole built in to the top of a mesh trap which allows the fast and easy exit of any Water Vole that should enter the trap. Natural England stipulated that 50% of the traps should be modified in this way and paired with an unmodified trap when set. In this way the effectiveness of the escapement hole for an inadvertently trapped Water Vole to escape from could be tested whilst also comparing any effect on crayfish catch rates. However it transpired that no Water Voles or Crayfish were captured in either a standard trap or a modified one. The traps were modified using thin plastic 20 cms squares fixed to the inside of the traps which are of the minnow trap design. A 65 mm hole was then drilled through the mesh and then through the centre of the plastic square. The main difficulty when deploying these traps, even when they are fixed to another trap is to keep the hole at the top and if this is achieved keeping the bait container floating out of the escapement hole like a trapped Water Vole! To further minimize the chances of inadvertently trapping a Water Vole, care was taken to avoid setting the traps in areas where there were obvious signs of Water Vole activity such as burrows, latrines or feeding stations. Traps were then set at least 10 metres away from these areas of activity and always in the middle of the river with the traps lined up lengthwise. This would mean that the trap entrances are facing upstream or downstream and not across. This would hopefully minimize the chances of trapping a Water Vole as they tend to swim along the edge of the river or directly across either on the surface or underwater. The traps were baited with Salmon/Trout flavour cat food and in addition some Halibut oil fishing pellets. Both of these products release particles and scent downstream which experience has shown is effective in attracting crayfish in the vicinity to the bait in the traps. NIGHT TORCHING A site was assessed as suitable for night torching if there was clear, non turbulent water which was not choked with weed. Torching could be carried out by boat or from the bank if there was safe access. One site (High Bridge TF ) would have been ideal for torching by boat but launching a boat (and recovering it) would have been very difficult owing to the steep bank. Therefore it was decided that the survey would be conducted entirely from the bank. The torching was carried out using a 1 million candle powered torch starting at least 1 hour after sunset. Distances surveyed are detailed in the results appendix.

5 INCIDENTAL FINDINGS Other species of wildlife of note that were observed in the course of the survey were recorded. INFECTION CONTROL To prevent the potential spread of Crayfish plague (Aphanomyces astaci) all of the sites were visited sequentially in a downstream direction. Before returning to hand search, trap or night torch a site all equipment used was sterilised using household bleach diluted to a concentration of 100 ppm of free Chlorine.

6 RESULTS See results spread sheets for full details: Appendix 2 Habitat Assessments Appendix 3A Night Torching Appendix 3B Trapping Appendix 3C Hand Searching Appendix 3D Fish Species No Crayfish of any species native or non- native were found at any of the sites. A total of 23 sites were surveyed. 18 were bridge sites. 2 sites had easy access from a public road. 3 sites were fords. 13 sites were hand searched. 10 sites were trapped. 6 sites were night torched. 7 sites were assessed as being unlikely to support Crayfish. This was due to a combination of a lack of suitable water characteristics such as depth, flow, clarity and lack of suitable substrate or weed cover and presence of excessive silt or overhanging vegetation. 16 sites were assessed as being suitable or very suitable for supporting Crayfish because they had the right water conditions ie good depth, flow and clarity and an abundance of suitable habitat with little silting or overhanging vegetation. 10 species of fish were found during the course of the survey (see appendix) and most sites hand searched had an abundance of invertebrates present such as Fresh Water Shrimp (Gammerus pulex). A Stoat (Mustela erminea), frog (Rana temporaria) and a Little Grebe (Tachybaptus ruficollis) were seen. American Mink (Mustela vision) scats were found at site 1 near the source at Mileham. Otter (Lutra lutra) spraints were found as were Otter tracks but only in the lower reaches below Narborough. There were some very clear Otter tracks in the mud under bridge 23 in Kings Lynn where the river was tidal. Evidence of Water Vole activity was found at 4 sites above Narborough. South East of Litcham a very small tributary supplies the water for Bridge Farm fishing lakes. A walk around the Lakes revealed a pond choked with Australian Swamp Stonecrop (Crassula helmsii) and there were several stands of Gunnera manicata around the lakes. There is a definite possibility that fragments or seeds from these plants could find there way in to the main river especially in times of flood. Apart from these sightings, they were the only invasive non native plants seen during the survey.

7 DISCUSSION AND CONCLUSIONS The decision was taken to mainly target bridge sites for a number of reasons. 1) There was a very limited window for the survey and landowner permission, which often takes a long time to obtain, would not be needed. 2) Access from public roads was reasonably easy. 3) Bridge sites often have a concentration of the features that Crayfish prefer which are often lacking in the intervening sections of the river. These features include the presence of bricks, rubble and other debris which Crayfish use as hiding places. In addition the water level, flow and aeration are often increased at bridge sites which Crayfish also prefer. 4) The whole catchment could be covered reasonably evenly and be cost effective with a few exceptions in the lower reaches. 5) Similar methodologies have been used to good effect in other surveys such as the national Otter surveys conducted by the Vincent Trust. 6) It was possible to carry out a rapid survey of the River Nar for the presence of native or non-native Crayfish and carry out an assessment of the river s suitability to support Crayfish in the event of it being considered as an Ark catchment. The survey showed that in general the river was in good condition evidenced by the frequency that Bullheads (Cottus gobio) and Brown Trout (Salmo trutta) in particular and other fish were found. In addition there were good populations of invertebrates such as Fresh Water Shrimps (Gammarus pulex), various Mussels and snails. From Litcham downstream to Narborough the river exhibited the typical features found in a chalk stream/river ie fast flowing, clear, deep water with an abundance of fish, invertebrates and banks of water weed such as Water Cress (Nasturtium officinale) and Water Crowsfoot (Ranunculus aquatilis). At certain sites in the upper reaches, (9 and 16 for instance) the river seemed to have lost some water volume as compared with the site immediately upstream and this was possibly due to water abstraction by farmers or water companies. The lower reaches of the river below Narborough showed evidence of extensive channel management with wide, relatively straight sections contained in a deep trough of flood protection banks 7 metres high which gave it the appearance of a fen drain. Despite this management the river had surprisingly good depth, flow and clarity with many fish in residence which were seen mostly during the night torching. (Torching is a superb method for seeing what lives in a river. Hordes of Gudgeon (Gobio gobio) were seen at High Bridge as was a large Pike (Esox lucius), which had not been evident during the initial day time survey and around Narborough there were many large Rainbow Trout (Oncorhychus mykiss), Brown Trout (Salmo trutta) and Roach (Rutilus rutilus) to be seen.) Along the course of the River Nar there are at least 63 other waterbodies such as flooded pits, fish farms and ornamental estate lakes. Any of these could potentially contain native or non-native Crayfish. Surveying these waterbodies was outside of the scope of this survey. Enquiries were made at Bridge Farm Fishery TF and at Narborough Trout and Coarse fishery TF and neither reported that they had ever seen native or non-

8 native Crayfish in their waters or in the river. The Salmon and Trout Association who apparently control waters around Narborough were contacted, as were the fish farm between Narborough and West Acre at TF but no reply has been received yet. A total of 12 local walkers or anglers were asked about the presence of Crayfish in the River Nar and none had ever seen or heard of any being present. The fact remains that Red Signal Crayfish have been reported in the recent past below Narborough but none were found during the survey. Since completing the survey contact has been made with the person previously licensed to trap signal crayfish. He was able to confirm that Red Signal Crayfish had been present in 2007 and 2008 at Setchey Bridge on the River Nar at TF He had trapped a small number in total (15) from the area immediately down stream of the bridge in 2008 but had abandoned the trapping due to low catch returns and the fact that the traps were frequently interfered with. It remains unexplained why no crayfish were seen or trapped in this area as a total of 8 traps were set at the bridge and 14 about one mile downstream. It was noted that it was late in the year although the temperatures were perfectly acceptable. The temperature should be above 8 degrees C and survey temperatures were a minimum of 13 degrees. It would have been preferable preferred to do the trapping in September but it took 5 weeks to obtain the trap licences from the Environment Agency due to the concern about Water Voles expressed by natural England. The river had been through a recent flooded spell which could have re-located the Crayfish but then it would be expected to catch them downstream. Along the length of the river there are some significant barriers to impede the upstream progress of any Red signal Crayfish present below Narborough. There is a mill at Newton near to Castle Acre and at Narborough there is a 2.5 metre high weir on the main channel and a mill on the side stream. Due to the degree of hand searching/torching/trapping carried out in the upper reaches it is reasonable to conclude that there are no Crayfish present in the main river above Narborough.

9 RECOMMENDATIONS Systematically trap the river below Narborough next summer and yearly after this to chart any spread of Red Signal Crayfish upstream or downstream. Perform night torching on the stretch below Narborough by boat if possible. This is dependent on finding safe launch/ recovery areas and obtaining land owner permission. Failing this the torching should be carried out from the bank. Consider selecting a range of off river water bodies for follow up surveys to establish presence/absence of white clawed crayfish.

10 PHOTOGRAPHS Site 1 Site 2 Site 3 Site 4 Site 5 Site 6

11 Site 7 Site 8 Site 9 Site 10 Site 11 Site 12

12 Site 13 Site 14 Site 15 Site 16 Site 17 Site 18

13 Site 19 Site 20 Site 21 Site 22 Site 23 Bullhead at Site 15

14 APPENDIX ONE Survey Site Locations (Red Dots Denote Survey Sites)

15

16