This article reports on the success of reintroducing native crayfish (Austropotamobius pallipes) in the Sherston and Tetbury Avon, following extinction of the population from crayfish plague. The authors describe and review the survey methods that were used and identify a survey technique that was found to be the most rapid and robust for monitoring crayfish populations. Such a survey technique could be adopted as a standard method

Fraylin, Martin

Spink, Joanna

English

Aquatic Commons

POST-PLAGUE REINTRODUCTTON OF CRAYFISH 59 
AN ASSESSMENT OF POST-PLAGUE REINTRODUCED NATIVE 
WHITE-CLAWED CRAYFISH, A USTROPOTAMOBIUS PALLIPES, IN 
THE SHERSTON AVON AND TETBURY AVON, WILTSHIRE 
JOANNA SPINK AND MARTIN FRAYLING 
(J. M. Spink & M. W. Frayling, Environment Agency, Rivers House, 
East Quay, Bridgwater, Somerset TA6 4YS, England). 
[Tel: 01278 457333] 
Introduction 
In this article we report on the success of reintroducing native crayfish 
(Austropotamobius pallipes) in the Sherston and Tetbury Avon, following 
extinction of the population from crayfish plague. We describe and review the 
survey methods that were used and identify a survey technique that we found 
to be the most rapid and robust for monitoring crayfish populations. Such a 
survey technique could be adopted as a standard method. 
Surveys of native crayfish were carried out during the summers of 1998 and 
1999. The primary objective was to assess native crayfish numbers at the 
reintroduction sites and also to survey sites further upstream and downstream 
to ascertain if there had been any migration of crayfish from the sites used for 
restocking (Figs 1-3). 
A pollution incident in April 1998, involving the synthetic insecticide 
cypermethrin, resulted in a significant reduction in the numbers of native 
crayfish - along the upper reaches of the Sherston Avon. This incident 
strengthened the need for a standard survey technique that could rapidly assess 
the health of crayfish populations in a watercourse. Our method has been 
incorporated into long-term monitoring strategies, to ensure the effective 
conservation of the last remaining native crayfish colonies on the Bristol Avon 
(Skinner 1998). 
Crayfish plague on the Sherston and Tetbury Avon 
Crayfish plague is caused by the fungus Aphanomyces astaci, resulting in 
100% mortality of infected animals. The plague fungus has had a devastating 
impact on populations of native crayfish. The initial mortality on the Bristol 
Avon was recorded in 1981 at Hyam Farm on the Sherston Avon (Fig. 3). The 
upper limit of deaths was located at a weir in Easton Grey. Crayfish above this 
weir remained free from plague throughout 1982 and 1983 (Lowery et al. 
1986). By 1983 crayfish plague had also reached the Tetbury Avon. 
Previously thriving crayfish populations on the Avon tributaries had long 
been established, with no reported mortality in over 40 years. The effects of 
crayfish plague were dramatic; the riverbed was strewn with dead and dying 
crayfish (Alderman 1993). Signal crayfish Pacifastacus leniusculus are known 
60 JOANNA SPINK AND MARTIN FRAYLING 
FIG. 1. Two sampling sites for crayfish. Above: Easton Grey on the Sherston Avon. 
Below: Merchants Farm on the Tetbury Avon. 
POST-PLAGUE REINTRODUCTION OF CRAYFISH 61 
FIG. 2. Sampling for crayfish on the Sherston Avon, using a quadrat (above) and a 
glass-bottomed bucket (below). 
62 JOANNA SPINK AND MARTIN FRAYLING 
FIG. 3. Outline map of the Sherston Avon and Tetbury Avon, Wiltshire, showing the 
main river network and sites mentioned in the text. Malmesbury is downstream. 
carriers of the fungus, but none have been found in either the Sherston or 
Tetbury Avon and the source of the plague remains uncertain. 
POST-PLAGUE REINTRODUCTION OF CRAYFISH 63 
The reintroduction programme on the Sherston and Tetbury Avon 
The reintroduction programme, the first of its kind in the UK, was carried out 
by one of us (MWF), a Fisheries officer of the Environment Agency. Frayling 
anticipated the need to encourage and try to restore native crayfish back to 
these tributaries. After consulting MAFF and English Nature, licensed 
reintroductions began in the 1980s and early 1990s, along both the Sherston 
Avon and the Tetbury Avon. Holding ("canary") cages were set up in both 
branches of the Avon, to check that crayfish plague had not returned. Ten 
healthy crayfish were placed into each holding cage, approximately 46x46x31 
cm in size. The cages were inspected every few days over a period of a year on 
the Sherston Avon and over six months on the Tetbury Avon, and no crayfish 
became infected. It was concluded that plague had probably died out after the 
native crayfish population became extinct in the early 1980s. 
The first reintroduction was in 1982, when 150 crayfish were directly 
released into the Sherston Avon at Fosse Mill. This paved the way for further 
reintroductions in 1986, 1987 and, finally, in 1994. Eight sites were used for 
reintroducing crayfish, five on the Sherston Avon and three on the Tetbury 
Avon (Table 1; Fig. 1). Crayfish released in the Sherston Avon were taken 
from this river between Pinkney and Sherston village, where the population 
had not been affected by plague. Those introduced on the Tetbury Avon were 
taken from the Mells River in Somerset, a site known to be clear of plague. 
Table 1. Reintroduction sites on the Sherston and Tetbury Avon, showing locations, numbers of 
crayfish released (sexes where recorded; b = berried females) and the date of reintroduction. 
Survey of crayfish sites in 1998 and 1999 
The five reintroduction sites on the Sherston Avon were sampled between July 
and August 1998. On the Tetbury Avon, two of the reintroduction sites were 
sampled in September 1998, and the third site was sampled in July 1999. 
64 JOANNA SPINK AND MARTIN FRAYLING 
Attempts were also made to define the distribution limits of the native crayfish 
population, by surveying stretches of river between the reintroduction sites. All 
of the latter were known to support good populations before the 1980s, i.e. 
before the advent of crayfish plague (Fig. 3). 
On the Sherston Avon, two survey methods were used to assess crayfish 
numbers at each of the reintroduction sites: turning stones by hand within 1 m2 
quadrats, and setting crayfish traps. Both methods are semi-quantitative and 
therefore give an indication of the relative numbers present at each site, rather 
than actual population sizes. Only quadrats were used on the Tetbury Avon, as 
time limitations prevented us from setting traps. 
Quadrat sampling/turning stones 
A 50 m stretch of river was measured alongside the site of reintroduction and 
five quadrats were placed on the riverbed at 10 m intervals. For each quadrat, 
the left, middle or right side of the channel was selected at random drawn from 
a bag. The same procedure was used on 50 m stretches immediately upstream 
and downstream. The riverbed within each 1 m2 quadrat was searched while 
looking through a glass-bottomed bucket, which gave a clear view through the 
water. The operator moved in an upstream direction to minimise disturbance of 
silt, lifting stones carefully within each quadrat and placing exposed crayfish 
into a bucket (Fig. 2). The bucket contained aquatic plant material to provide 
cover for the crayfish. A standard pond-net was placed downstream of the 
quadrat, to catch crayfish that escaped or were picked up by the current. Within 
each of the quadrats the aquatic vegetation was removed, placed in the pond-
net and searched for young crayfish. 
For each crayfish caught, we recorded the following information: sex, 
carapace length (tip of rostrum to back of the carapace), claw absence/ 
regeneration, stage of moult and disease incidence. Crayfish were then returned 
to the river. 50 m stretches were then surveyed above and below the middle 
stretch to ascertain the degree of crayfish migration from the point of 
reintroduction. 
A manual survey by turning stones within 1 m2 quadrats was also undertaken 
at sites interspersed between the reintroduction sites, except that only one 50 m 
stretch was sampled and no traps were set. Due to time constraints the same 
procedure was also undertaken at Boakley Farm on the Tetbury Avon. 
Figure 4 shows the total numbers of crayfish caught in 5 m2 of riverbed over 
each 50-m section on the Sherston and Tetbury Avon. On both rivers, crayfish 
appear to be found only where they were reintroduced, except at Hyam Farm 
downstream of the sluice, on the Sherston Avon, where no crayfish were 
recorded. The riverbed is silty at this point, perhaps encouraging migration of 
crayfish away from the site. A silty substratum might also explain the zero 
numbers recorded at some of the other 50 m sections at Hyam Farm. At 
POST-PLAGUE REINTRODUCTION OF CRAYFISH 65 
FIG. 4. Total numbers of crayfish caught in five 1-m2 quadrats in 50 m sections of river 
at the site of reintroduction (solid bars) and above and below the site (open bars). 
Above: Tetbury Avon. Below: Sherston Avon. 
66 JOANNA SPINK AND MARTIN FRAYLING 
Boakley Farm only one 50 m section was surveyed (at the reintroduction point) 
because the upstream and downstream sections were too deep for sampling. 
The relatively high numbers of crayfish at Fosse Mill on the Sherston Avon 
and at Shipton Mill on the Tetbury Avon (Fig. 4), may be attributed to the 
good habitat conditions found here, and also might be due to the initial high 
numbers of crayfish reintroduced at these two sites. 150 crayfish were released 
at Fosse Mill and 127 at Shipton Mill, compared with fewer than 100 released 
at the other reintroduction sites. 
Sampling stretches of river between the reintroduction sites revealed no 
crayfish, with the exception of Foxley Grove on the Sherston Avon. At the 
reintroduction sites where crayfish numbers are relatively high, the population 
has spread for about 150 m on both sides but additional survey work, 
traversing further upstream and downstream, would be required to determine 
the precise distances travelled. The migration distance was not sufficiently 
extensive to allow convergence of crayfish populations between the 
reintroduction sites. Poor habitat conditions could be a possible explanation. 
Soon after the pollution incident on the Sherston Avon in 1998, numerous 
crayfish were seen dead or dying in the upper reaches of the river. However, 
although mortalities were noted at Easton Grey and Fosse Mill further 
downstream, the numbers there appeared to be small. 
Trapping survey at the reintroduction sites 
The trapping survey was carried out only on the Sherston Avon. Traps were 
baited with mackerel and left in suitable crayfish habitats, i.e. alongside 
vertical banks or in deep pools. Traps were tied to branches or stones to ensure 
that they were not carried away in the current and were left fully submersed in 
the water. 
A triple catch survey was employed; i.e. traps were set at the same place for 
three consecutive trappings over a period of about 2 weeks. Traps were left 
overnight and collected the following day. Captured crayfish were removed 
from the traps and marked with a number, using a permanent marker pen; the 
same details for each crayfish were recorded as in the manual survey. Crayfish 
were immediately released back into the location where they had been caught. 
Traps were then removed for a period of 5-6 days, as this was thought to 
reduce the chance of crayfish becoming habituated or "trap happy" and 
therefore they would be sufficiently hungry to feed again; traps were then set 
again in the same place. From these triple-catch recordings, a crayfish 
population estimate at each site was obtained using Bailey's modification of 
the Peterson estimate (or Lincoln Index) (Holdich 1990). 
Due to low numbers of crayfish at the three reintroduction sites at Hyam 
Farm, a mark-recapture study could not be conducted. Therefore results are 
presented only for Fosse Mill and Easton Grey (Table 2). 
Evaluation of sampling methods 
Numerous variables are associated with trapping and monitoring crayfish 
populations. Our results indicated that, in general, traps caught larger 
individuals than when turning stones by hand, suggesting that the two methods 
have sampled different age-classes within the populations. This size selectivity 
may occur due to the different habitat preferences of adults and juveniles. The 
latter are more likely to be found in the shallow, gravel marginal areas (Blake 
& Hart 1993) whereas the older, trappable crayfish tend to shelter either in 
deep pools and/or in burrows and tree-root systems. Up to certain depths it is 
possible to sample these deeper pooled sections by hand, if a dry suit is worn. 
Brown & Brewis (1978) found a significant three-fold underestimation of 
population size when using mark-recapture estimates, and therefore strongly 
recommended that trapping should be used only as an auxiliary sampling 
method in population studies. Woodland (1967) reviewed crayfish behaviour in 
response to trapping. Not only the responses to the trap itself and crayfish 
already within it will be important in trap-behaviour, but also whether the 
animals are exposed to a trap at a suitable sampling time; e.g. reproductive 
females may remain in their hides and are not sampled. 
Turning stones by hand within a 1 m2 quadrat appeared to be effective. Very 
few crayfish were seen evading capture. However, it is limited to relatively 
clear, shallow water and it is not possible to sample depths greater than arm's-
length; the maximum depth sampled using this method was 70 cm. Sampling 
methods such as stone-turning are not subject to the same limitations as 
trapping. In theory, all animals present in the sampled area have an equal 
chance of being caught. Therefore this sampling method is much less biased, 
and all year-classes are sampled (Brown & Bowler 1977). The most accurate 
method of estimating population size and density is to drain the site inhabited 
by crayfish, and collect and record all of the animals revealed. However, few 
sites containing crayfish would be suitable for complete draining and 
consequently this method is of limited practical use (Hogger 1988). 
Concluding remarks 
This study has revealed that the first attempt at a restocking programme by 
Frayling in the 1980s and early 1990s, following the destruction of the native 
POST-PLAGUE REINTRODUCTION OF CRAYFISH 67 
66 JOANNA SPINK AND MARTIN FRAYLING 
Boakley Farm only one 50 m section was surveyed (at the reintroduction point) 
because the upstream and downstream sections were too deep for sampling. 
The relatively high numbers of crayfish at Fosse Mill on the Sherston Avon 
and at Shipton Mill on the Tetbury Avon (Fig. 4), may be attributed to the 
good habitat conditions found here, and also might be due to the initial high 
numbers of crayfish reintroduced at these two sites. 150 crayfish were released 
at Fosse Mill and 127 at Shipton Mill, compared with fewer than 100 released 
at the other reintroduction sites. 
Sampling stretches of river between the reintroduction sites revealed no 
crayfish, with the exception of Foxley Grove on the Sherston Avon. At the 
reintroduction sites where crayfish numbers are relatively high, the population 
has spread for about 150 m on both sides but additional survey work, 
traversing further upstream and downstream, would be required to determine 
the precise distances travelled. The migration distance was not sufficiently 
extensive to allow convergence of crayfish populations between the 
reintroduction sites. Poor habitat conditions could be a possible explanation. 
Soon after the pollution incident on the Sherston Avon in 1998, numerous 
crayfish were seen dead or dying in the upper reaches of the river. However, 
although mortalities were noted at Easton Grey and Fosse Mill further 
downstream, the numbers there appeared to be small. 
Trapping survey at the reintroduction sites 
The trapping survey was carried out only on the Sherston Avon. Traps were 
baited with mackerel and left in suitable crayfish habitats, i.e. alongside 
vertical banks or in deep pools. Traps were tied to branches or stones to ensure 
that they were not carried away in the current and were left fully submersed in 
the water. 
A triple catch survey was employed; i.e. traps were set at the same place for 
three consecutive trappings over a period of about 2 weeks. Traps were left 
overnight and collected the following day. Captured crayfish were removed 
from the traps and marked with a number, using a permanent marker pen; the 
same details for each crayfish were recorded as in the manual survey. Crayfish 
were immediately released back into the location where they had been caught. 
Traps were then removed for a period of 5-6 days, as this was thought to 
reduce the chance of crayfish becoming habituated or "trap happy" and 
therefore they would be sufficiently hungry to feed again; traps were then set 
again in the same place. From these triple-catch recordings, a crayfish 
population estimate at each site was obtained using Bailey's modification of 
the Peterson estimate (or Lincoln Index) (Holdich 1990). 
Due to low numbers of crayfish at the three reintroduction sites at Hyam 
Farm, a mark-recapture study could not be conducted. Therefore results are 
presented only for Fosse Mill and Easton Grey (Table 2). 
Holdich & R. S. Lowery), pp. 115-144. Chapman and Hall Ltd, London. 
Holdich, D. M. (1990). A preliminary impact assessment of an invading signal 
crayfish population upon both the native population and general stream 
ecology of Broadmead Brook, Wiltshire. Unpublished report, Nottingham 
University to the Environment Agency. 
Lowery, R. S., Hogger, J., Polglase, J. L. & Alderman, D. J. (1986). Crayfish 
mortalities in UK rivers. Freshwater Crayfish 6, 234-238. 
Skinner, A. (1998). Freshwater crayfish on the Bristol Avon. Unpublished 
M.Sc. report. 
Woodland, D. J. (1967). Population study of a freshwater crayfish Cherax 
albidus Clark. Unpublished Ph.D. Thesis. The University of New England, 
Armidale, New South Wales. 


An assessment of post-plague reintroduced native white-clawed crayfish, a Ustropotamobius pallipes, in the Sherston Avon and Tetbury Avon, Wiltshire

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An assessment of post-plague reintroduced native white-clawed crayfish, a Ustropotamobius pallipes, in the Sherston Avon and Tetbury Avon, Wiltshire

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