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Aspects of the introduction of “signal crayfish”, Pacifastacus leniusculus (Dana), into the southern United Kingdom. 1. Growth and survival
A~uaeulture, 58 (1986) 27-44
Elsevier Science Publishers B.V., Ams~r~m
27 - Printed in The Netherlan~
Aspects of the Introduction of “Signal Crayfish”, Pacifastacus leniusculus (Dana), into the Southern United Kingdom. 1. Growth and Survival J.B. HOGGER T~ames Water, alert
House, Vastern Road, Reedit
RGl8DB {Great Brittany
(Accepted 3 June 1986)
ABSTRACT Hogger, J.B., 1986. Aspects of the introduction of “signal crayfish”, Pacifastacus leniusculus (Dana), into the southern United Kingdom. 1. Growth and survival. Aquaculture, 58: 27-44. A large number of sites throughout the United Kingdom have been stocked with signal crayfish, Pucifastacus 1eniu.wulu.s.This paper describes the growth and survival of an enclosed population
of this species in the southern United Kingdom. Analysis of size data from trapping records indicates that growth in the study population is at least as good as that reported from elsewhere in Europe. maximum sizes of 75.0 mm and 68.5 mm carapace length, for males and females, respectively, were recorded. Growth of the initial implants was very good but declined slightly as density increased and habitat resources were utilised, Approximately 20% survival of introduced juveniles to cat&able adults was calculated from estimates of population size. These data also permitted calculation of adult density as 1.8 mm’. Subsequent harvesting had a detectable effect on catch per unit effort. The benefits and drawbacks of introducing a fast growing, alien species, resistant to the crayfish plague are discussed in relation to the status of Austropotamobius pallipes, the single, disease-susceptible native species.
INTRODUCTION
Freshwater crayfish are of value in both ecological and economic terms and there is currently considerable interest in the United Kingdom regarding their production, particularly exotic species, for the table. This interest is illustrated by the fact that over 200 sites in the United Kingdom had received implantations of imported crayfish by the end of 1983 ( Hogger, 1984). The distribution of the single native species, Austropotumobiuspallipes (Lereboullet) , is largely controlled by natural influences (Jay and Holdich, 1981) whereas the range of the principal introduced species, the “signal crayfish”, Pucifustcxus kniusculus (Dana), is determined artificially. Until 1982 there was no control over the importation of alien crayfish species into the United Kingdom so P. Zeniusculus has been introduced into a wide range of habitat types; from farm ponds and
0 1986 Elsevier Science Publishers B.V.
28
Fig. 1. Distribution of known int~du~ions of the “signal crayfish”, P. ~e~~~cu~~, in the United Kingdom. 0 10 km O.S. square into which at least one introduction of P. Zeniusculus is known to have been made during 1976-1982. (after Hogger, 1984).
gravel pits to established fish farms, with varying degrees of security. Throughout the United Kingdom introductions have been more or less restricted to hardwater areas and by proximity to a source of supply. Most introductions have consequently been made in the south and east of England with few into Scotland and none into Ireland. About 20% of these sites are located in the Thames catchment in central southern England (Fig. 1) . The number of crayfish implanted into individual sites ranges from tens to thousands per year. The majority of those imported are supplied by a hatchery in Sweden (Richards and Fuke, 1977) but latterly movement of P. Zeniusculus from established populations in the United Kingdom has increased in importance. The information on growth and survival presented here, together with data concerning production potential and morphometry ( Hogger, in prep. ) , pro-
vide a basis on which and ecological and economic assessment of introductions of P. Zeniusculus can be made. Such an assessment is of considerable importance for any species being considered as a candidate for aquaculture ( Webber and Riordan, 1976), particularly when the species involved is alien to both the country and continent. Basic information on the biology of P. Zeniusculus has been obtained by studying a recently introduced population for a period of 4 years from the date of the final introductions. Other sites within the Thames catchment have also been sampled, less frequently, in order to assess the success of introductions in a wide range of sites and to compare the results from the principal study site and the rest of the southern United Kingdom. Study site A lake on the Stratfield Saye Estate, Hampshire (NGR: SU711619), approximately 1.0ha in area and with a mean depth between 0.75 m and 1.0m (maximum 4.0 m ) , was stocked with imported juvenile ( 0 + ) P. leniusculus at the rate of 500 per year in 1977,1978 and 1979. The main water inflow to the lake originates from a similar lake 0.5 km upstream and both lakes are fed by local runoff. Land use proximal to the upper lake is predominantly mixed woodland whereas that adjacent to the study site is grazed by cattle. The study lake is retained by an earth dam and discharges to the River Loddon, a tributary of the River Thames, some 1.0 km distant. Samples of lake water were collected periodically for chemical analysis, the results of which are listed in Table 1. Submerged aquatic macroph~es were sparse and limited to small beds of Elodea cunudensis ( Canadian pondweed). Marginal plants, mainly Carex sp. (sedges) and Juncus sp. (rushes) were restricted to an area on the west bank which was fenced to exclude cattle. On the east bank was located a clump of mature Fugus syluatica (beech) and Pinus sy~uestr~ ( Scats pine) trees. Large flocks of water fowl were frequently observed on the lake, particularly in winter. These consisted mainly of Brunta cunudensis (Canada geese) whilst Anus platyrhyncos (mallard), Fulica atra (coot) and Ardea cinereu (heron) were often also present. Site prepurution Prior to stocking with crayfish the lake was drained and crushed chalk spread on the bed. In addition a large quantity of builders rubble (approximate mean diameter = 100 mm) was spread along the west shore, in a band nominally 3.0 m wide, to provide additional crayfish habitat. All fish were removed during draining and only a small number of tenth, Tinca tincu, were subsequently introduced.
N.B.Values as mg I
25.02
’ unless otherwise stated.
14.25
115
120
97
18.77
148
149
14.3 0.11
14.2 0.20
208
8.1 0.01
32
33
29.97
117
152
14.0 0.01
32
0.4
0.4
5.3
0.3
32
0.028
0.046
74.0 5.1
0.043
59.8 5.8
8.35
4.6 67.1 10.0
7.47 16.0
10.00
6.11.1979
0.025
57.5 5.7
6.30
6.15
8.0
oxygen % Saturation Ammoniacal nitrogen Unionised ammonia as N TOW oxidiwd &sN Chloride asc1 silica as sio Orthophosphate aSP Total hardness as C&o:, Alkalinity 89 CaCo, Total c~or~pkyll
86.3 2.0
2.8 73.5 13.0
2.2 N.D. 12.5
5.2 N.D. 19.0
PH Suspended sotids (104OC) BOD (ATU) COD Temperature (“C) Dissolved
7.52 13.3
7.53 18.2
7.53 14.0
09‘50
10.00
19.30
Time
31.10.1979
25.10.1979
5.9.1979
Date
Chemical water quality (Stratfield &ye, 1979-1982)
TABLE 1
N.D.
60
123
11.8 0.02
106
53
134
5.4 0.01
26
0.7
1.30 30
0.002
78.9 0.36
8.8
3.5 35.0 10.5
7.52 N.D.
09.00
24.4.1980
0.005
91.8 0.92
12.20
4.1 N.D. 3.5
19.0
7.69
08.30
12.1960
88.6
54
126
5.5 0.01
26.1
55
142
5.6 0.006
26
26.2
61
141
ND.
ND.
81
105 92 5.21
130
144
13.8 0.05
32.8
54
71 57.6
142
5.0 0.09
29
0.4
0.002
65.6 0.40
128
6.2 0.01
22
1.0
1.0 27
0.004
76 0.63
6.75
4.1 58.9 14
5.6 N.D. 15 7.65
7.37 40.7
09.30
13.5.1982
7.39 N.D.
oB.oB
11.6.1981
0.012
78.7 1.9
8.5
5.1 52 9.0
66
1.57
09.06
21.11.1980
130
12.1 0.17
11.1 0.14 6.8 0.01
0.2
0.4
30
0.022
0.006
28
11.8 3.3
1.20
19.8 2.1
1.95
1.9 82.1 14.5
0.7 79.4 16.0
29
0.4
0.6
0.7 27
0.015
91.8 1.02
0.001
80.7 0.42
9.45
2.7 N.D. 14
3.0 N.D. 10.5 9.0
N.D.
7.40 2
09.00
1.10.1960
6.96 6.0
08.30
09.00 7.77
24.9.1980
17.7.1960
18.0
1.12
08.30
7.5.1960
0.002
71.4 0.49
8.05
3.6 48.1 10.0
7.42 13.0
08.30
30.4.1980
31
0
IO
20
30
40 C.L.
SO
60
70
80
50
60
70
80
(mm)
30 Ovk$crous
=
20 z Etl 5 z
IO
0
0
10
20
30
10 C.C.
irnrn!
Fig. 2. Size dist~bution of crayfish in sampies collected from Stratfield Saye, 1979-1982. METHODS
Overnight trapping was used at this site to sample the crayfish population, supplemented by the hand collection of juveniles. Unmodified crayfish traps with a mesh size of 7.5 mm (Westman et al,, 1979) were baited with fresh coarse fish, usually carp or tenth as these proved most effective, laid near dusk and lifted soon after dawn. The time that traps were in position varied according to the time of year. Traps only consistently caught crayfish of carapace length > 30 mm (Fig. 2 ) . All crayfish were measured to the nearest mm using vernier calhpers. Carapace length (C.L. ) , from the tip of the rostrum to the posteriomedial rim of the cephalothorax, was the standard feature used (Brown, 1979). This measurement was selected in preference to post-orbital C.L., measured to the eye notch, in order to facilitate comparison with the majority of published data. Additional measurements were also collected on other occasions to enable a fuller morphometric analysis to be carried out ( Hogger, in prep. ) . Mark-recapture experiments were carried out in 1979,1980,1981 and 1983 in order that estimates of abundance could be calculated. Marking was achieved by cauterisation, enabling date and individual-speci~c marks to be applied according to a s~ndard scheme ( Abrah~sson, 1965).
32
StutisticaZanalysis Estimates of the size of the trappable population were calculated using four different methods, i.e., “Petersen”, “Schnabel”, “Bailey” and “Jolly” (Ricker, 1975). Some of the assumptions made during the use of these methods are clearly not met by the conditions of the current study so the estimates obtained can only be considered approximations, Trapping success has been expressed as catch per trap per unit time to compare the “catchability” of adult crayfish on different occasions and at different sites. The size composition of the trapped crayfish samples has been determined by separating the data into their Gaussian components (Bhattacharya, 1967). The distinct size groups within the population, corresponding to moult classes, as identified by this analysis, have been plotted against time. Utilising knowledge of the maximum possible age of the study population and the observed size of hand-caught juveniles, together with published data on the age-size relationship of this species in both Europe and North America, average growth curves have been constructed. RESULTS
Analysis of the length-frequency data has permitted generalised growth curves, with males and females shown separately, to be drawn for the period 1977-1981 (Fig. 3). Also shown is a curve constructed using only data collected in 1979 and relating to the crayfish implanted in 1977 and 1978. During the mark-recapture investigation of population size, 63 individually marked crayfish were recaptured. Of these, 27 showed an increase in C.L. indicative of at least one moult (Table 2). Moult frequency is unknown, however, and can only be intuitively deduced using published data (Mason, 1970,1975; Flint, 1975a). It is therefore not legitimate to express overall growth in terms of moult increment, using these data, and the relationship of pre-moult size to post-moult size cannot be established. Estimates of population size, calculated using a number of different methods, are listed in Table 3. The extent of remixing of marked individuals was checked by comparing the ratio of marked to unmarked crayfish in subsequent samples usingx2. Satisfactory mixing was shown to have occurred on all occasions (PKO.005). DISCUSSION
Growth Growth in this population has generally been good, with maximum sizes of 75.0 mm CL. and 68.5 mm CL. being achieved by males and females, respec-
33
70
60
2 50 s _; u
40
30
20
10
0
1
2 AGE
3
4
5
(years)
Fig. 3. Proposed growth curvec~for P. leniusculus at Stratfield Saye.
tively, after 4-5 years. Comparable published data from newly established populations elsewhere in Europe indicate maxima of 71.0 mm CL. for males and 60.0 mm C.L. for females (Laurent, 1.980). Elsewhere in the southern United Kingdom maximum sizes of 83.0 mm C.L. for males and 76.0 mm for females have been recorded (M. Moore, personal communications 1983 1. A growth model, such as that formulated by Von Bertafanffy CRicker, 1975 ) f can be applied to the data obtained in this study. The growth coefficient (Ic) , a measure of the rate at which length approaches a maximum, calculated using
34 TABLE 2 Growth increments of individually marked crayfish (Stratfield Saye, 1979-1981) (a) Males (CL. mm)
NO. 10 9. 8.1979
15. 8.1979 21. 6.1979 24.10.1979 30.10.197s 5.11.1979 31. 1.1960 23. 4.1966 9. 7.1980
24
31
32
34
40
51
65.6
60.1 60.1 60.0
45.6 47.1
53.2 53.1
45.1
44.0
73
110
131
59.5
53.0 53.5
65.0
(RI
62.0 (R)
162
179
193
53.0
41.0
42.0
55.0 55.0
42.5
43.5 43.0
313
49.9
57.0
60.0 65.0 68.0 65.5
59.5
(R) 16. 7.1980 23. 7.1980 23. 9.1980 20.11.1960 10. 6.1981
68.5
61.5
31.5 35.0
64.5 47.0
Recaptures
1
1
4
1
2
1
I
1
2
1
2
2
2
1
6.75
3.5
3.0
5.25
1.25
3.5
5.4
3.8
12.
3.0
170
197
226
250
306
46.5
38.5 51.5 43.3
43.0
Growth increment (mm)
15.6
2.2
4.95
2.1
11.35 14.4
17.5
2.5
% of pre M.&L.
31
3.3
8.2
4.6
21
32
40
4.4
65
70
120
43.5 63.0 tB>
40.5 fB1
16
11
fb f Females (C.L. mm ) NO. 4 9. 8.1979 15. 8.1979 21. 8.1979 24.10.1979 5.11.1979
7.1980 7.1960 9.1960 6.1981
37
39
53.2
47.3 47.4
53.3
122
43.4 43.4
9. 71980 16. 23. 23. 10.
35
54.0 (R) 52.5 52.0
51.0
58.8 53.0 (B) 57.0 (R)
62.0
52.0 46.0
44.0 66.0 53.0
46.5 56.0
56.0
Recaptures
3
1
3
2
1
1
1
1
1
1
1
1
1
Growth 8 increment (mm)
6.85
0.8
4.15
3.65
3.2
4.5
3.0
3.6
6.5
4.8
4.5
3.5
4.5
1.5
8.7
7.2
5.4
4.6
8.6
8.7
8.1
8.7
I of pre M.C.L.
20
IO
14
51.5
12
N.B. R = Individ~l removed: B = Ovigerous female. OnIy individually recogniaable crayfish that had increased in size are included. 19 males+ 1’7 females recaptumd>once (max. three recaptures) showed no size increase.
35 TABLE 3 Estimates of population size calculated from mark-recapture
(Stratfield Saye, 1979-1983)
Date
Estimate ( + 95% confidence limits)
Method
August 1979 August 1979 November 1979 November 1979 Autumn 1979 (combined data) July 1980 1979 and 1980 (combined data) June 1981 June 1981 May 1983
193+ 81 296 145+ 54 22 462rt 123 240f 81 715% 351 390 404 2062 ? 1397 2203 I! 1260 1938 f 1075
Adjusted Petersen Jolly Adjusted Petersen Jotly Adjusted Petersen Schnabel Adjusted Petersen Schnabel Schumacher Adjusted Petersen Bailey Bailey
N.B. Crayfish juveniles were introduced in June 1977,1978 and 1980.
this model is 0.47. This high value confirms the rapid growth rate apparent in Fig. 3. The generalised growth curves (Fig. 3) suggest that rapid initial growth of the original implants occurred compared with average growth figures for the period 1979-1981. This difference in growth rates could be explained by the presence of large beds of Elodea canadensis prior to stocking with crayfish (A. Ellis, personal communication, 1979). This plant, known to be a popular food for this and other crayfish species (Dean, 1969; Flint, 1975a), subsequently declined in abundance until, in 1982, only a few sparse patches remained. As P. leniusculus have been shown to consume aquatic macrophytes as 20% of the juvenile diet and 65% of the adult diet in Lake Tahoe (Flint and Goldman, 1975) it is assumed that the crayfish were responsible for stripping the weed from the lake. These observations imply that P. ~ni~cuZ~ could be useful as an agent of weed control as has been suggested elsewhere for this species (Blake and Laurent, 1982 ) , A. astucus ( Abrahamsson, 1966) and Orconectes causeyii (Dean, 1969). In addition, the population size increased after 1979, due to natural recruitment, and as the density of adults increased a reduction in growth rate due to competition would presumably have resulted. Comparison of the data presented here with published information for this and other crayfish species (Fig. 4) reveals that this study population has a faster growth rate than populations in Sweden ( Abrahamsson, 1973a), Finland (Westman, 1973)) Canada (Mason, 1974, 1975) and the U.S.A. (Flint 1975a, b) . This difference is attributed to the longer growing season, with water temperatures > 10°C for about 6 months, and the comparatively low stocking density of 0.05 m--2 year-’ achieved through introductions. No data are avail-
36
90
80
o----<)
i.
c.-.-.
Canada (Mason, 1976)
Tahoe,
Callfornla
(Cukerzle
!Fll"t.
e--a
tlthuan~a
.-*
Sweden
iAbrahsmsson,
&----A
France
- I" cuiture
W
England (Hagger, 198&l
1975b.‘
+ Terentjew,
1979)
1971a)
(Laurent,i’)BOl
70
6C
-z e
5(
-i Li
4(
31
21
11
0
1
2
3
4 AGE
5
6
7
8
(years)
Fig. 4. Growth curves for P. leniuscuhs - published data.
able to substantiate the former proposal other than that published for A. pul@es in the United Kingdom (Brown, 1979; Pratten, 1980) and A, mtacus in Sweden ( Abr~amsson, 1973a) and the fact that hatching has been observed earlier in this locality than has been reported from anywhere. else in the wild. When compared with the growth rates of other crayfish species it can be seen that P. leniwculus is both faster growing and of a larger final size than all other coldwater species in the northern hemisphere (Fig. 5). The’limited amount of individual moult-increment (M.I.) data available from this study suggests that moulting results an increase in CL. ranging from 3.8% to 16% in males and 4.8% to 12% in females. There is considerable individual variation, however, and not all of the measured M.I. values agree with published M.I. equations (Emadi, 1974; Flint, 1975a, b). In addition, M.I. values are reported to decline as crayfish increase in size, e.g., from ll%-12% at 30 mm C.L. to 7% at 50 mm C.L., although the weight increment remains at a
37
w m*
38
constant 22%-24% (Mason, 1970,19X5). Variation in M.I. in this population is comparable with the 1.5%-l&2% reported for a population of P. leniusculus in Canada (Mason, 1974). Population size
Mark-recapture methods have been widely used for estimating the size of fish and small mammal populations. There are, however, inherent drawbacks in using such methods and their use for estimating the size of crayfish populations has been criticised (Brown and Brewis, 1979). Their study demonstrated that population estimates calculated from trapping results consistently underestimate population size by a factor of three compared to hand collection. In view of this, several methods of calculating estimates from trap data have been used in this study. The data available from the majority of sampling occasions in this study are most suited to the calculation of adjusted Petersen estimates (Ricker, 1975). This method appears to underestimate population size compared with, for example, Jolly’s stochastic model (see Table 3). Higher estimates but with decreased variance result from calculations using Bailey’s triple-catch method. This method of estimating population size would therefore be the most suited to the needs of crayfish farmers as it increases accuracy whilst not requiring excessive additional trapping. The figures obtained (Table 3) indicate that initial population size was between 200 and 300 but declined to 150-200 by November 1979. This was presumably due to the large proportion of females that would be ovigerous at that time and would consequently less readily enter the traps. Thereafter the estimated size of the trappable population increased to 2000-2200 by 1981 and was still at this level in 1983. The 95% confidence limts of these estimates, however, average 44% (26%-67% ) and this greatly exceeds the ? 10% considered acceptable (Momot, 1967). In addition, the figures make no allowance for the number of animals removed by the owner and it was not possible to obtain reliable information concerning this point. Despite this the figures do provide a guide to the order of magnitude of the population of catchable adults. From a knowledge of the number of crayfish juveniles introduced an approximate survival value of 20% of the 1978/1979 implants to late 1979 can be calculated. The juveniles introduced in 1979 would not be included in this estimate as they were not trappable at that time. Such a figure compares well with published survival data (Table 4) and may reflect the relatively low stocking density and the absence of predators at this site. Elsewhere in the southern United Kingdom sampling has established the survival to maturity of introduced P. ~ni~cul~ at six of the 14 sites investigated (Hogger, 1984).
39 TABLE 4 Published data on the survivat of introduced crayfish Species
Location
Habitat type
Stage
Elapsed time
Estimated survival
Reference
P. leniusculua
Sweden
Lake
12 months
21%
Abrahamsson. 1973b.
France France France Canada Canada Canada Canada
Pond Pond Pond Creek Creak Tanks Raceways
Juveniles CO+) Juveniles Juveniles Adults Adults Adults Juveniles Adults
17 Months 48 manths 12 months 4 months Overwinter
9.3%-13% 7% 72% 66%-70% 25% 13% 65%
Vigneux, 1979 Laurent, 1986 Laurent, 1980 Mason, 1974 Mason, 1974 Mason, 1974 Mason, 1974
Sweden Sweden
Lake (11 Lake (2)
Adults Juveniles?
24 months 36 months
4% 27%
Abrahamsson, 1972 Abrahamsson, 1972
Australia
Ponds
65%
Jones. 1981
A.
astocus
C. tenuimanus
-
Density The trapping records show that the crayfish in this lake have a distinctly clumped distribution, presumably due to preferential selection of suitable habitat. Little credibility can therefore be attached to an estimate of density. A value of 0.22 adult crayfish per m2 can be calculated for 1981 and this compares poorly with most published information. However, such data generally relate to the area actually occupied by the crayfish which may be a bed area as little as 12% of the total surface area ( Abrahamsson and Goldman, 1970). If allowance is made for this then the adult crayfish density in the occupied areas of this lake may be of the order of 1.8 per m2. Alternatively the ratio of numbers of crayfish to the length of suitable bank may be a more useful relationship. At this site only 250 m of the total bank length of 650 m is considered suitable as habitat and from the current results there may be up to 12 adult crayfish per m of such a bank. Consequently if the margins of this lake were improved in terms of crayfish habitat then the holding capacity of the lake could theoretically by doubled. ~igni~cant increases in production potential could be predicted for other crayfish “farms” were such habitat improvements unde~aken. Catch per unit effort In order to relate sampling success to the estimated population size the number of crayfish caught on each sampling occasion has been expressed as catch per trap per night (Fig. 6). This index of catchability has increased from almost 1.0 in 1979 to 10.4 in 3.981.The harvesting of an unrecorded number of adults in 1981 and 1982 caused this index to drop to 3 but it had increased again to
DATE
Fig. 6. Catchability of P. le~i~cul~ (Stratfield &ye, 1979-1983).
6.3 prior to harvesting in 1983. Catchability figures of O.l-18.5 from other populations of P. leniwculw have been reported ( Abrahamsson and Goldman, 1970; Westman and Pursiainen, 1979) but all relate to well established populations or those originating from introductions of adults. An index of comparative cat~hability would seem to have potential as a practical tool in monitoring commercially managed crayfish populations. In order to quantify the relationship of catchability and population size a calibration exercise on a crayfish population of known size and in a similar moult and reproductive condition to the study population would be required. In addition to the figures for growth and survival the current study has also provided some further information relevant to the management of such crayfish populations in the United Kingdom. Although newly moulted crayfish, stage Al, A2 and I3 (Stevenson, 19X), do not readily enter traps, some were
caught and indicated that the main periods of moulting in this population are May and September. Newly moulted adults were, however, trapped as late as November. An average of 68% of trapped females at relevant times of the year (October-May) were ovigerous and their minimum size was 38.5 mm C.L. The earliest of these was caught on 24 October and the latest on 10 June. Eggs were generally found to hatch in May, the earliest date that a female with young was observed was 13 May. CONCLUSIONS
From the information presented here it can be seen that P. 1e~i~cuZ~ are able to become successfully established in suitable situations in the United Kingdom. They have been shown to grow relatively rapidly to maturity and marketable size and therefore would appear to be good subjects for crayfish farming. Current information from commercial sources clearly indicates considerable marketing potential for this species both in the United Kingdom and elsewhere in Europe (K. Richards, personal communication, 1983). There are, however, numerous drawbacks in introducing and establishing populations of an alien species in the United Kingdom. Most important among these are possible effects on populations of the indigenous crayfish A. pu~lipes. This latter species is highly susceptible to the “crayfish plague” caused by the fungus Ap~uno~~ces astaci (Schikora) and to which P. Zeni~cu~~ is largely resistant (Unestam, 1969). The possible role of P. ~en~uscuZ~ as a source of infection and the consequent effects of its introduction on the native species have been discussed (Goddard and Holdich, 1979). In addition, as P. Zeniuscub is both larger and faster growing than A. pallipes it may compete successfully for food, habitat and breeding partners. Comparative studies currently being undertaken in the United Kingdom (D. Holdich, personal communication, 1985; G. Warner, personal communication, 1985 ) should provide valuable information regarding this competitive relationship. The benefits of exploiting populations of native crayfish rather than introducing alien species with their attendant problems have been discussed (Goddard and Holdich, 1979). It seems unlikely that this will occur, however, now that P. leniusculus is widely established in the United Kingdom. Current legislation requires that introductions of non-native species to the wild are licenced (Wildlife and Countryside Act, 1981) and commercial crayfish farms must now be registered (Diseases of Fish Act, 1985). A suggested voluntary code of practice for crayfish farmers and importers concerning introductions and movements and the possible establishment of zones containing populations of native crayfish to which no introductions will be made may assist in protecting some populations of A. pa&es from the effects of introducing other species. Recent reports of mass mortalities of A. pullipes populations due to the crayfish plague (Alderman et al., 1984) may strengthen such efforts to
42
minimise the impact of crayfish introductions on native populations in the United Kingdom. ACKNOWLEDGEMENTS
Thanks are due to Dr. M.C. Dart, formerly Director of Scientific Services, Thames Water, for permission to carry out this study, and to N.J. Nicolson, Environmental Services Manager, Thames Water, for permission to publish this work. The views expressed are those of the author and are not necessarily those of Thames Water.
REFERENCES Abrahamsson, S.A.A., 1965. A method of marking crayfish Astacus astacus (L.) in population studies. Oikos, 16: 228-231. Abrahamsaon, S.A.A., 1966. Dynamics of an isolated population of the crayfish Astacus astacus (L.). Oikos, 17: 98-107. Ab~hamsson, S.A.A., 1972. Fecundity and growth of some populations of Astaeus astacus (L.) in Sweden; with special regard to introductions to northern Sweden. Rep. Inst. Freshwater Res. Drottningholm, 52: 23-37. Abrahamsaon, S.A.A., 1973a. The crayfish Astacus ustacus (L.) in Sweden and the introduction of the crayfish P~c~f~t~~~ le~i~c~~~ (Dana). Freshwater Crayfish, 1: 27-40. Abr~~on, S.A.A., 1973b. Meth~ for the gyration of crayfish waters in Europe. The development of an industry for the production of the young of P~if~t~~ leniusculus (Dana) ~ Freshwater Crayfish, 1: 203-210. Abrahamsson, S.A.A. and Goldman, C.R., 1970. Distribution, density and production of the crayfish Pacifastacus leniwculus (Dana) in Lake Tahoe, California-Nevada. Oikos, 21: 83-91. Alderman, D.J., Polglase, J.L., Frayling, M. and Hogger, J.B., 1984. Crayfish plague in Britain. J. Fish. Dis., 7: 401-405. Arrignon, J., 1975, Crayfish farming in France. Freshwater Crayfish, 2: 105116. Bhattacha~a, C.G., 1967. A simple method of resolution of a distribution into Gaussian components. Biometrics, March, pp. 115-129. Blake, G. and Laurent, P.J., 1982. Le faucardage par des ticrevisses; result&s preliminaires. Bull. Mens. Sot. Linn. Lyon, 51: 203-208. Brown, D.J., 1979. A study of the population biology of the British freshwater crayfish Austro~ta~bi~ pulses (~~~ullet). PH.D. thesis, Durham Univ., Great Britain, 262 pp. (unpublish~) . Brown, D.J. and Bowler, K., 1979. The relationship between size and age throughout the life-cycle of Austropotamobiuspallipes (Lereb.) . Freshwater Crayfish, 4: 35-42. Brown, D.J. and Brewis, M., 1979. A critical look at trapping as a methodof sampling a population of Austrapotamobius pallipes (Lereboullet) in a mark and recapture study. Freshwater Crayfish, 4: 159-164. Cukerzis, J. and Terentjew, A., 1979. Acclimatisation de Pacif~tac~ ~eni~~~~ (Dana) dans un lac isole. Pi&c. Fr., 56: 13-16 (translated by PJ. Laurent). Dean, J.L., 1969. Biology of the crayfish Onconectes causeyii and its use for the control of aquatic weeds in trout lakes. Tech. Paper U.S. Bur. Sport Fish and Wild., 24: 15 pp. Emadi, H., 1974. Culturing conditions and their effects on survival and growth of the crayfish
43 Pacifastacus leniusculus trowbridgii (Dana). Ph.D. thesis, Oregon State Univ., U.S.A. (unpublished), Flint, R.W., 1975a. Natural history, ecology and production of the crayfish Pacifastucus leniusculus (Dana) in a subalpine lacustrine environment. Ph.D. thesis, Univ. California, Davis, U.S.A. (unpublished), 150 pp. Flint, R.W., 1975b. Growth in a population of the crayfish Pacifastacus leniusculus (Dana) from a subalpine lacustrine environment. J. Fish. Res. Board Can,, 32: 2433-2440. Flint, R.W. and Goldman, CR., 1975. The effects of a benthic grazer on the primary productivity of the littoral zone of Lake Tahoe. Limnol. Oceanogr., 20: 935-944. Goddard, J.S. and Holdich, D.M., 1979. Explore the native potential first. Fish Farmer, 2: 47. Hogger, J.B., 1984. Aspects of the biology and distribution of freshwater crayfish in the Thames ~~hment. Ph.D. thesis ( Unpubl. ) , C.N.A. A., United Kingdom, 340 pp. Hogger, J,B., in prep. Aspects of the intr~uction of the “signal crayfish”, Pac~f~tac~ leniusculus (Dana) into the southern United Kingdom, 2. Morphometry and production potential. Jay, D. and Holdich, D.M., 1981. The distribution of Austropotamobiuspallipes (Lereboullet) in British waters. Freshwater Biol., 11: 121-129. Jestin, J.M., 1979. Croissance et developpement de l’ecrevisse americaine Orconectes limosus dans le Lac de Cretail (Val de Marne, France). Freshwater Crayfish, 4: 65-72. Jones, J.B., 1981. Growth of two species of freshwater crayfish (Paranephrops st.) in New Zealand. N.Z.J. Mar. Freshwater Res., 15(l): 15-20. Kossakowski, J. and Orzechowski, D., 1975. Crayfish Orconectes limosus in Poland. Freshwater Crayfish, 2: 31-4’7. Laurent, P.J., 1980. Utili~tion des Btangs pour la production d’ecrevisses. In: R. Billard (Editor), La pisciculture en Etang. Natl. Rech. Agron., Paris, pp. 333-342. Mason, J.C., 1970. Egg laying in the western North American crayfish P~if~tac~ tro~bridgii (Dana). Crustaceana (Leiden ) ,19: 37-44. Mason, J.C., 1974. Aquaculture potential of the freshwater crayfish (Pacifastacus) . 1. Studies during 1970. Fish. Res. Board Can. Tech. Suppl., 440: 43 pp. Mason, J.C., 1975. Crayfish production in a small woodlandstream. Freshwater Crayfish, 2: 449-480. Momot, W.T., 1967. Population dynamics and productivity of the crayfish Orconectes uirilis (Hagen) in a marl lake. Am. MidI. Nat., 78: 55- 81. Momot, W.T. and Gowing, H., 1977. Response of the crayfish Orconectes uirilis to exploitation. J. Fish. Res. Board Can., 34(8): 1212-1219. Pratten, D.J., 1980. Growth in the crayfish Austropotamobiuspallipes (Lereb.). Freshwater Biol., lo: 401-402. Reynolds, J.D., 1979. Crayfish ecology in Ireland. Freshwater Crayfish, 4: 215-220. Richards, K.J. and Fuke, P., 1977. Freshwater crayfish - the first centre in Britain. Fish Farm. Int., 4: 12-15. Ricker, W.E., 1975. Computation and interpretation of biological statistics of fish populations. Bull. Fish. Res. Board Can., 191: 882 pp. Stevenson, J.R., 1975. The moulting cycle of the crayfish:recognising the moulting stages, effects of ecdysone and changes during the cycle. Freshwater Crayfish 2: 255-269. Unestam, T., 1969. Resistance to the crayfish plague in some American, Japanese and European crayfishes. Rep. Inst. Freshwater Res. Drottningholm, 49: 202-209. Vigneux, E., 1979. P. leniuscuEus (Dana) et A. 1eptodactylu.s (Esch.) premiere bilan d’exploitation en Btangs. Freshwater Crayfish, 4: 227-234. Webber, H.H. and Riordan, P.F., 1976. Criteria for candidate species for aquaculture. Aquaculture, 7: 107-123. Westman, K,, 1973. The population of the crayfish Astacus astacus (L.) in Finland and the introduction of the American crayfish P~if~tac~ leni~c~~~ (Dana). Freshwater Crayfish, 1: 41-50.
44
Westman, K. and Pursiainen, M., 1979. Development of the European crayfish Astacus astacus and the American crayfish P~if~t~~ ~~i~cu~~ population in a small Finnish lake. Freshwater Crayfish, 4: 243-250. Westman, K., Pursiainen, M. and Viikman, R., 1979. A new folding trap model which prevents crayfish from escaping. Freshwater Crayfish, 4: 235-242.
Elsevier Science Publishers B.V., Ams~r~m
27 - Printed in The Netherlan~
Aspects of the Introduction of “Signal Crayfish”, Pacifastacus leniusculus (Dana), into the Southern United Kingdom. 1. Growth and Survival J.B. HOGGER T~ames Water, alert
House, Vastern Road, Reedit
RGl8DB {Great Brittany
(Accepted 3 June 1986)
ABSTRACT Hogger, J.B., 1986. Aspects of the introduction of “signal crayfish”, Pacifastacus leniusculus (Dana), into the southern United Kingdom. 1. Growth and survival. Aquaculture, 58: 27-44. A large number of sites throughout the United Kingdom have been stocked with signal crayfish, Pucifastacus 1eniu.wulu.s.This paper describes the growth and survival of an enclosed population
of this species in the southern United Kingdom. Analysis of size data from trapping records indicates that growth in the study population is at least as good as that reported from elsewhere in Europe. maximum sizes of 75.0 mm and 68.5 mm carapace length, for males and females, respectively, were recorded. Growth of the initial implants was very good but declined slightly as density increased and habitat resources were utilised, Approximately 20% survival of introduced juveniles to cat&able adults was calculated from estimates of population size. These data also permitted calculation of adult density as 1.8 mm’. Subsequent harvesting had a detectable effect on catch per unit effort. The benefits and drawbacks of introducing a fast growing, alien species, resistant to the crayfish plague are discussed in relation to the status of Austropotamobius pallipes, the single, disease-susceptible native species.
INTRODUCTION
Freshwater crayfish are of value in both ecological and economic terms and there is currently considerable interest in the United Kingdom regarding their production, particularly exotic species, for the table. This interest is illustrated by the fact that over 200 sites in the United Kingdom had received implantations of imported crayfish by the end of 1983 ( Hogger, 1984). The distribution of the single native species, Austropotumobiuspallipes (Lereboullet) , is largely controlled by natural influences (Jay and Holdich, 1981) whereas the range of the principal introduced species, the “signal crayfish”, Pucifustcxus kniusculus (Dana), is determined artificially. Until 1982 there was no control over the importation of alien crayfish species into the United Kingdom so P. Zeniusculus has been introduced into a wide range of habitat types; from farm ponds and
0 1986 Elsevier Science Publishers B.V.
28
Fig. 1. Distribution of known int~du~ions of the “signal crayfish”, P. ~e~~~cu~~, in the United Kingdom. 0 10 km O.S. square into which at least one introduction of P. Zeniusculus is known to have been made during 1976-1982. (after Hogger, 1984).
gravel pits to established fish farms, with varying degrees of security. Throughout the United Kingdom introductions have been more or less restricted to hardwater areas and by proximity to a source of supply. Most introductions have consequently been made in the south and east of England with few into Scotland and none into Ireland. About 20% of these sites are located in the Thames catchment in central southern England (Fig. 1) . The number of crayfish implanted into individual sites ranges from tens to thousands per year. The majority of those imported are supplied by a hatchery in Sweden (Richards and Fuke, 1977) but latterly movement of P. Zeniusculus from established populations in the United Kingdom has increased in importance. The information on growth and survival presented here, together with data concerning production potential and morphometry ( Hogger, in prep. ) , pro-
vide a basis on which and ecological and economic assessment of introductions of P. Zeniusculus can be made. Such an assessment is of considerable importance for any species being considered as a candidate for aquaculture ( Webber and Riordan, 1976), particularly when the species involved is alien to both the country and continent. Basic information on the biology of P. Zeniusculus has been obtained by studying a recently introduced population for a period of 4 years from the date of the final introductions. Other sites within the Thames catchment have also been sampled, less frequently, in order to assess the success of introductions in a wide range of sites and to compare the results from the principal study site and the rest of the southern United Kingdom. Study site A lake on the Stratfield Saye Estate, Hampshire (NGR: SU711619), approximately 1.0ha in area and with a mean depth between 0.75 m and 1.0m (maximum 4.0 m ) , was stocked with imported juvenile ( 0 + ) P. leniusculus at the rate of 500 per year in 1977,1978 and 1979. The main water inflow to the lake originates from a similar lake 0.5 km upstream and both lakes are fed by local runoff. Land use proximal to the upper lake is predominantly mixed woodland whereas that adjacent to the study site is grazed by cattle. The study lake is retained by an earth dam and discharges to the River Loddon, a tributary of the River Thames, some 1.0 km distant. Samples of lake water were collected periodically for chemical analysis, the results of which are listed in Table 1. Submerged aquatic macroph~es were sparse and limited to small beds of Elodea cunudensis ( Canadian pondweed). Marginal plants, mainly Carex sp. (sedges) and Juncus sp. (rushes) were restricted to an area on the west bank which was fenced to exclude cattle. On the east bank was located a clump of mature Fugus syluatica (beech) and Pinus sy~uestr~ ( Scats pine) trees. Large flocks of water fowl were frequently observed on the lake, particularly in winter. These consisted mainly of Brunta cunudensis (Canada geese) whilst Anus platyrhyncos (mallard), Fulica atra (coot) and Ardea cinereu (heron) were often also present. Site prepurution Prior to stocking with crayfish the lake was drained and crushed chalk spread on the bed. In addition a large quantity of builders rubble (approximate mean diameter = 100 mm) was spread along the west shore, in a band nominally 3.0 m wide, to provide additional crayfish habitat. All fish were removed during draining and only a small number of tenth, Tinca tincu, were subsequently introduced.
N.B.Values as mg I
25.02
’ unless otherwise stated.
14.25
115
120
97
18.77
148
149
14.3 0.11
14.2 0.20
208
8.1 0.01
32
33
29.97
117
152
14.0 0.01
32
0.4
0.4
5.3
0.3
32
0.028
0.046
74.0 5.1
0.043
59.8 5.8
8.35
4.6 67.1 10.0
7.47 16.0
10.00
6.11.1979
0.025
57.5 5.7
6.30
6.15
8.0
oxygen % Saturation Ammoniacal nitrogen Unionised ammonia as N TOW oxidiwd &sN Chloride asc1 silica as sio Orthophosphate aSP Total hardness as C&o:, Alkalinity 89 CaCo, Total c~or~pkyll
86.3 2.0
2.8 73.5 13.0
2.2 N.D. 12.5
5.2 N.D. 19.0
PH Suspended sotids (104OC) BOD (ATU) COD Temperature (“C) Dissolved
7.52 13.3
7.53 18.2
7.53 14.0
09‘50
10.00
19.30
Time
31.10.1979
25.10.1979
5.9.1979
Date
Chemical water quality (Stratfield &ye, 1979-1982)
TABLE 1
N.D.
60
123
11.8 0.02
106
53
134
5.4 0.01
26
0.7
1.30 30
0.002
78.9 0.36
8.8
3.5 35.0 10.5
7.52 N.D.
09.00
24.4.1980
0.005
91.8 0.92
12.20
4.1 N.D. 3.5
19.0
7.69
08.30
12.1960
88.6
54
126
5.5 0.01
26.1
55
142
5.6 0.006
26
26.2
61
141
ND.
ND.
81
105 92 5.21
130
144
13.8 0.05
32.8
54
71 57.6
142
5.0 0.09
29
0.4
0.002
65.6 0.40
128
6.2 0.01
22
1.0
1.0 27
0.004
76 0.63
6.75
4.1 58.9 14
5.6 N.D. 15 7.65
7.37 40.7
09.30
13.5.1982
7.39 N.D.
oB.oB
11.6.1981
0.012
78.7 1.9
8.5
5.1 52 9.0
66
1.57
09.06
21.11.1980
130
12.1 0.17
11.1 0.14 6.8 0.01
0.2
0.4
30
0.022
0.006
28
11.8 3.3
1.20
19.8 2.1
1.95
1.9 82.1 14.5
0.7 79.4 16.0
29
0.4
0.6
0.7 27
0.015
91.8 1.02
0.001
80.7 0.42
9.45
2.7 N.D. 14
3.0 N.D. 10.5 9.0
N.D.
7.40 2
09.00
1.10.1960
6.96 6.0
08.30
09.00 7.77
24.9.1980
17.7.1960
18.0
1.12
08.30
7.5.1960
0.002
71.4 0.49
8.05
3.6 48.1 10.0
7.42 13.0
08.30
30.4.1980
31
0
IO
20
30
40 C.L.
SO
60
70
80
50
60
70
80
(mm)
30 Ovk$crous
=
20 z Etl 5 z
IO
0
0
10
20
30
10 C.C.
irnrn!
Fig. 2. Size dist~bution of crayfish in sampies collected from Stratfield Saye, 1979-1982. METHODS
Overnight trapping was used at this site to sample the crayfish population, supplemented by the hand collection of juveniles. Unmodified crayfish traps with a mesh size of 7.5 mm (Westman et al,, 1979) were baited with fresh coarse fish, usually carp or tenth as these proved most effective, laid near dusk and lifted soon after dawn. The time that traps were in position varied according to the time of year. Traps only consistently caught crayfish of carapace length > 30 mm (Fig. 2 ) . All crayfish were measured to the nearest mm using vernier calhpers. Carapace length (C.L. ) , from the tip of the rostrum to the posteriomedial rim of the cephalothorax, was the standard feature used (Brown, 1979). This measurement was selected in preference to post-orbital C.L., measured to the eye notch, in order to facilitate comparison with the majority of published data. Additional measurements were also collected on other occasions to enable a fuller morphometric analysis to be carried out ( Hogger, in prep. ) . Mark-recapture experiments were carried out in 1979,1980,1981 and 1983 in order that estimates of abundance could be calculated. Marking was achieved by cauterisation, enabling date and individual-speci~c marks to be applied according to a s~ndard scheme ( Abrah~sson, 1965).
32
StutisticaZanalysis Estimates of the size of the trappable population were calculated using four different methods, i.e., “Petersen”, “Schnabel”, “Bailey” and “Jolly” (Ricker, 1975). Some of the assumptions made during the use of these methods are clearly not met by the conditions of the current study so the estimates obtained can only be considered approximations, Trapping success has been expressed as catch per trap per unit time to compare the “catchability” of adult crayfish on different occasions and at different sites. The size composition of the trapped crayfish samples has been determined by separating the data into their Gaussian components (Bhattacharya, 1967). The distinct size groups within the population, corresponding to moult classes, as identified by this analysis, have been plotted against time. Utilising knowledge of the maximum possible age of the study population and the observed size of hand-caught juveniles, together with published data on the age-size relationship of this species in both Europe and North America, average growth curves have been constructed. RESULTS
Analysis of the length-frequency data has permitted generalised growth curves, with males and females shown separately, to be drawn for the period 1977-1981 (Fig. 3). Also shown is a curve constructed using only data collected in 1979 and relating to the crayfish implanted in 1977 and 1978. During the mark-recapture investigation of population size, 63 individually marked crayfish were recaptured. Of these, 27 showed an increase in C.L. indicative of at least one moult (Table 2). Moult frequency is unknown, however, and can only be intuitively deduced using published data (Mason, 1970,1975; Flint, 1975a). It is therefore not legitimate to express overall growth in terms of moult increment, using these data, and the relationship of pre-moult size to post-moult size cannot be established. Estimates of population size, calculated using a number of different methods, are listed in Table 3. The extent of remixing of marked individuals was checked by comparing the ratio of marked to unmarked crayfish in subsequent samples usingx2. Satisfactory mixing was shown to have occurred on all occasions (PKO.005). DISCUSSION
Growth Growth in this population has generally been good, with maximum sizes of 75.0 mm CL. and 68.5 mm CL. being achieved by males and females, respec-
33
70
60
2 50 s _; u
40
30
20
10
0
1
2 AGE
3
4
5
(years)
Fig. 3. Proposed growth curvec~for P. leniusculus at Stratfield Saye.
tively, after 4-5 years. Comparable published data from newly established populations elsewhere in Europe indicate maxima of 71.0 mm CL. for males and 60.0 mm C.L. for females (Laurent, 1.980). Elsewhere in the southern United Kingdom maximum sizes of 83.0 mm C.L. for males and 76.0 mm for females have been recorded (M. Moore, personal communications 1983 1. A growth model, such as that formulated by Von Bertafanffy CRicker, 1975 ) f can be applied to the data obtained in this study. The growth coefficient (Ic) , a measure of the rate at which length approaches a maximum, calculated using
34 TABLE 2 Growth increments of individually marked crayfish (Stratfield Saye, 1979-1981) (a) Males (CL. mm)
NO. 10 9. 8.1979
15. 8.1979 21. 6.1979 24.10.1979 30.10.197s 5.11.1979 31. 1.1960 23. 4.1966 9. 7.1980
24
31
32
34
40
51
65.6
60.1 60.1 60.0
45.6 47.1
53.2 53.1
45.1
44.0
73
110
131
59.5
53.0 53.5
65.0
(RI
62.0 (R)
162
179
193
53.0
41.0
42.0
55.0 55.0
42.5
43.5 43.0
313
49.9
57.0
60.0 65.0 68.0 65.5
59.5
(R) 16. 7.1980 23. 7.1980 23. 9.1980 20.11.1960 10. 6.1981
68.5
61.5
31.5 35.0
64.5 47.0
Recaptures
1
1
4
1
2
1
I
1
2
1
2
2
2
1
6.75
3.5
3.0
5.25
1.25
3.5
5.4
3.8
12.
3.0
170
197
226
250
306
46.5
38.5 51.5 43.3
43.0
Growth increment (mm)
15.6
2.2
4.95
2.1
11.35 14.4
17.5
2.5
% of pre M.&L.
31
3.3
8.2
4.6
21
32
40
4.4
65
70
120
43.5 63.0 tB>
40.5 fB1
16
11
fb f Females (C.L. mm ) NO. 4 9. 8.1979 15. 8.1979 21. 8.1979 24.10.1979 5.11.1979
7.1980 7.1960 9.1960 6.1981
37
39
53.2
47.3 47.4
53.3
122
43.4 43.4
9. 71980 16. 23. 23. 10.
35
54.0 (R) 52.5 52.0
51.0
58.8 53.0 (B) 57.0 (R)
62.0
52.0 46.0
44.0 66.0 53.0
46.5 56.0
56.0
Recaptures
3
1
3
2
1
1
1
1
1
1
1
1
1
Growth 8 increment (mm)
6.85
0.8
4.15
3.65
3.2
4.5
3.0
3.6
6.5
4.8
4.5
3.5
4.5
1.5
8.7
7.2
5.4
4.6
8.6
8.7
8.1
8.7
I of pre M.C.L.
20
IO
14
51.5
12
N.B. R = Individ~l removed: B = Ovigerous female. OnIy individually recogniaable crayfish that had increased in size are included. 19 males+ 1’7 females recaptumd>once (max. three recaptures) showed no size increase.
35 TABLE 3 Estimates of population size calculated from mark-recapture
(Stratfield Saye, 1979-1983)
Date
Estimate ( + 95% confidence limits)
Method
August 1979 August 1979 November 1979 November 1979 Autumn 1979 (combined data) July 1980 1979 and 1980 (combined data) June 1981 June 1981 May 1983
193+ 81 296 145+ 54 22 462rt 123 240f 81 715% 351 390 404 2062 ? 1397 2203 I! 1260 1938 f 1075
Adjusted Petersen Jolly Adjusted Petersen Jotly Adjusted Petersen Schnabel Adjusted Petersen Schnabel Schumacher Adjusted Petersen Bailey Bailey
N.B. Crayfish juveniles were introduced in June 1977,1978 and 1980.
this model is 0.47. This high value confirms the rapid growth rate apparent in Fig. 3. The generalised growth curves (Fig. 3) suggest that rapid initial growth of the original implants occurred compared with average growth figures for the period 1979-1981. This difference in growth rates could be explained by the presence of large beds of Elodea canadensis prior to stocking with crayfish (A. Ellis, personal communication, 1979). This plant, known to be a popular food for this and other crayfish species (Dean, 1969; Flint, 1975a), subsequently declined in abundance until, in 1982, only a few sparse patches remained. As P. leniusculus have been shown to consume aquatic macrophytes as 20% of the juvenile diet and 65% of the adult diet in Lake Tahoe (Flint and Goldman, 1975) it is assumed that the crayfish were responsible for stripping the weed from the lake. These observations imply that P. ~ni~cuZ~ could be useful as an agent of weed control as has been suggested elsewhere for this species (Blake and Laurent, 1982 ) , A. astucus ( Abrahamsson, 1966) and Orconectes causeyii (Dean, 1969). In addition, the population size increased after 1979, due to natural recruitment, and as the density of adults increased a reduction in growth rate due to competition would presumably have resulted. Comparison of the data presented here with published information for this and other crayfish species (Fig. 4) reveals that this study population has a faster growth rate than populations in Sweden ( Abrahamsson, 1973a), Finland (Westman, 1973)) Canada (Mason, 1974, 1975) and the U.S.A. (Flint 1975a, b) . This difference is attributed to the longer growing season, with water temperatures > 10°C for about 6 months, and the comparatively low stocking density of 0.05 m--2 year-’ achieved through introductions. No data are avail-
36
90
80
o----<)
i.
c.-.-.
Canada (Mason, 1976)
Tahoe,
Callfornla
(Cukerzle
!Fll"t.
e--a
tlthuan~a
.-*
Sweden
iAbrahsmsson,
&----A
France
- I" cuiture
W
England (Hagger, 198&l
1975b.‘
+ Terentjew,
1979)
1971a)
(Laurent,i’)BOl
70
6C
-z e
5(
-i Li
4(
31
21
11
0
1
2
3
4 AGE
5
6
7
8
(years)
Fig. 4. Growth curves for P. leniuscuhs - published data.
able to substantiate the former proposal other than that published for A. pul@es in the United Kingdom (Brown, 1979; Pratten, 1980) and A, mtacus in Sweden ( Abr~amsson, 1973a) and the fact that hatching has been observed earlier in this locality than has been reported from anywhere. else in the wild. When compared with the growth rates of other crayfish species it can be seen that P. leniwculus is both faster growing and of a larger final size than all other coldwater species in the northern hemisphere (Fig. 5). The’limited amount of individual moult-increment (M.I.) data available from this study suggests that moulting results an increase in CL. ranging from 3.8% to 16% in males and 4.8% to 12% in females. There is considerable individual variation, however, and not all of the measured M.I. values agree with published M.I. equations (Emadi, 1974; Flint, 1975a, b). In addition, M.I. values are reported to decline as crayfish increase in size, e.g., from ll%-12% at 30 mm C.L. to 7% at 50 mm C.L., although the weight increment remains at a
37
w m*
38
constant 22%-24% (Mason, 1970,19X5). Variation in M.I. in this population is comparable with the 1.5%-l&2% reported for a population of P. leniusculus in Canada (Mason, 1974). Population size
Mark-recapture methods have been widely used for estimating the size of fish and small mammal populations. There are, however, inherent drawbacks in using such methods and their use for estimating the size of crayfish populations has been criticised (Brown and Brewis, 1979). Their study demonstrated that population estimates calculated from trapping results consistently underestimate population size by a factor of three compared to hand collection. In view of this, several methods of calculating estimates from trap data have been used in this study. The data available from the majority of sampling occasions in this study are most suited to the calculation of adjusted Petersen estimates (Ricker, 1975). This method appears to underestimate population size compared with, for example, Jolly’s stochastic model (see Table 3). Higher estimates but with decreased variance result from calculations using Bailey’s triple-catch method. This method of estimating population size would therefore be the most suited to the needs of crayfish farmers as it increases accuracy whilst not requiring excessive additional trapping. The figures obtained (Table 3) indicate that initial population size was between 200 and 300 but declined to 150-200 by November 1979. This was presumably due to the large proportion of females that would be ovigerous at that time and would consequently less readily enter the traps. Thereafter the estimated size of the trappable population increased to 2000-2200 by 1981 and was still at this level in 1983. The 95% confidence limts of these estimates, however, average 44% (26%-67% ) and this greatly exceeds the ? 10% considered acceptable (Momot, 1967). In addition, the figures make no allowance for the number of animals removed by the owner and it was not possible to obtain reliable information concerning this point. Despite this the figures do provide a guide to the order of magnitude of the population of catchable adults. From a knowledge of the number of crayfish juveniles introduced an approximate survival value of 20% of the 1978/1979 implants to late 1979 can be calculated. The juveniles introduced in 1979 would not be included in this estimate as they were not trappable at that time. Such a figure compares well with published survival data (Table 4) and may reflect the relatively low stocking density and the absence of predators at this site. Elsewhere in the southern United Kingdom sampling has established the survival to maturity of introduced P. ~ni~cul~ at six of the 14 sites investigated (Hogger, 1984).
39 TABLE 4 Published data on the survivat of introduced crayfish Species
Location
Habitat type
Stage
Elapsed time
Estimated survival
Reference
P. leniusculua
Sweden
Lake
12 months
21%
Abrahamsson. 1973b.
France France France Canada Canada Canada Canada
Pond Pond Pond Creek Creak Tanks Raceways
Juveniles CO+) Juveniles Juveniles Adults Adults Adults Juveniles Adults
17 Months 48 manths 12 months 4 months Overwinter
9.3%-13% 7% 72% 66%-70% 25% 13% 65%
Vigneux, 1979 Laurent, 1986 Laurent, 1980 Mason, 1974 Mason, 1974 Mason, 1974 Mason, 1974
Sweden Sweden
Lake (11 Lake (2)
Adults Juveniles?
24 months 36 months
4% 27%
Abrahamsson, 1972 Abrahamsson, 1972
Australia
Ponds
65%
Jones. 1981
A.
astocus
C. tenuimanus
-
Density The trapping records show that the crayfish in this lake have a distinctly clumped distribution, presumably due to preferential selection of suitable habitat. Little credibility can therefore be attached to an estimate of density. A value of 0.22 adult crayfish per m2 can be calculated for 1981 and this compares poorly with most published information. However, such data generally relate to the area actually occupied by the crayfish which may be a bed area as little as 12% of the total surface area ( Abrahamsson and Goldman, 1970). If allowance is made for this then the adult crayfish density in the occupied areas of this lake may be of the order of 1.8 per m2. Alternatively the ratio of numbers of crayfish to the length of suitable bank may be a more useful relationship. At this site only 250 m of the total bank length of 650 m is considered suitable as habitat and from the current results there may be up to 12 adult crayfish per m of such a bank. Consequently if the margins of this lake were improved in terms of crayfish habitat then the holding capacity of the lake could theoretically by doubled. ~igni~cant increases in production potential could be predicted for other crayfish “farms” were such habitat improvements unde~aken. Catch per unit effort In order to relate sampling success to the estimated population size the number of crayfish caught on each sampling occasion has been expressed as catch per trap per night (Fig. 6). This index of catchability has increased from almost 1.0 in 1979 to 10.4 in 3.981.The harvesting of an unrecorded number of adults in 1981 and 1982 caused this index to drop to 3 but it had increased again to
DATE
Fig. 6. Catchability of P. le~i~cul~ (Stratfield &ye, 1979-1983).
6.3 prior to harvesting in 1983. Catchability figures of O.l-18.5 from other populations of P. leniwculw have been reported ( Abrahamsson and Goldman, 1970; Westman and Pursiainen, 1979) but all relate to well established populations or those originating from introductions of adults. An index of comparative cat~hability would seem to have potential as a practical tool in monitoring commercially managed crayfish populations. In order to quantify the relationship of catchability and population size a calibration exercise on a crayfish population of known size and in a similar moult and reproductive condition to the study population would be required. In addition to the figures for growth and survival the current study has also provided some further information relevant to the management of such crayfish populations in the United Kingdom. Although newly moulted crayfish, stage Al, A2 and I3 (Stevenson, 19X), do not readily enter traps, some were
caught and indicated that the main periods of moulting in this population are May and September. Newly moulted adults were, however, trapped as late as November. An average of 68% of trapped females at relevant times of the year (October-May) were ovigerous and their minimum size was 38.5 mm C.L. The earliest of these was caught on 24 October and the latest on 10 June. Eggs were generally found to hatch in May, the earliest date that a female with young was observed was 13 May. CONCLUSIONS
From the information presented here it can be seen that P. 1e~i~cuZ~ are able to become successfully established in suitable situations in the United Kingdom. They have been shown to grow relatively rapidly to maturity and marketable size and therefore would appear to be good subjects for crayfish farming. Current information from commercial sources clearly indicates considerable marketing potential for this species both in the United Kingdom and elsewhere in Europe (K. Richards, personal communication, 1983). There are, however, numerous drawbacks in introducing and establishing populations of an alien species in the United Kingdom. Most important among these are possible effects on populations of the indigenous crayfish A. pu~lipes. This latter species is highly susceptible to the “crayfish plague” caused by the fungus Ap~uno~~ces astaci (Schikora) and to which P. Zeni~cu~~ is largely resistant (Unestam, 1969). The possible role of P. ~en~uscuZ~ as a source of infection and the consequent effects of its introduction on the native species have been discussed (Goddard and Holdich, 1979). In addition, as P. Zeniuscub is both larger and faster growing than A. pallipes it may compete successfully for food, habitat and breeding partners. Comparative studies currently being undertaken in the United Kingdom (D. Holdich, personal communication, 1985; G. Warner, personal communication, 1985 ) should provide valuable information regarding this competitive relationship. The benefits of exploiting populations of native crayfish rather than introducing alien species with their attendant problems have been discussed (Goddard and Holdich, 1979). It seems unlikely that this will occur, however, now that P. leniusculus is widely established in the United Kingdom. Current legislation requires that introductions of non-native species to the wild are licenced (Wildlife and Countryside Act, 1981) and commercial crayfish farms must now be registered (Diseases of Fish Act, 1985). A suggested voluntary code of practice for crayfish farmers and importers concerning introductions and movements and the possible establishment of zones containing populations of native crayfish to which no introductions will be made may assist in protecting some populations of A. pa&es from the effects of introducing other species. Recent reports of mass mortalities of A. pullipes populations due to the crayfish plague (Alderman et al., 1984) may strengthen such efforts to
42
minimise the impact of crayfish introductions on native populations in the United Kingdom. ACKNOWLEDGEMENTS
Thanks are due to Dr. M.C. Dart, formerly Director of Scientific Services, Thames Water, for permission to carry out this study, and to N.J. Nicolson, Environmental Services Manager, Thames Water, for permission to publish this work. The views expressed are those of the author and are not necessarily those of Thames Water.
REFERENCES Abrahamsson, S.A.A., 1965. A method of marking crayfish Astacus astacus (L.) in population studies. Oikos, 16: 228-231. Abrahamsaon, S.A.A., 1966. Dynamics of an isolated population of the crayfish Astacus astacus (L.). Oikos, 17: 98-107. Ab~hamsson, S.A.A., 1972. Fecundity and growth of some populations of Astaeus astacus (L.) in Sweden; with special regard to introductions to northern Sweden. Rep. Inst. Freshwater Res. Drottningholm, 52: 23-37. Abrahamsaon, S.A.A., 1973a. The crayfish Astacus ustacus (L.) in Sweden and the introduction of the crayfish P~c~f~t~~~ le~i~c~~~ (Dana). Freshwater Crayfish, 1: 27-40. Abr~~on, S.A.A., 1973b. Meth~ for the gyration of crayfish waters in Europe. The development of an industry for the production of the young of P~if~t~~ leniusculus (Dana) ~ Freshwater Crayfish, 1: 203-210. Abrahamsson, S.A.A. and Goldman, C.R., 1970. Distribution, density and production of the crayfish Pacifastacus leniwculus (Dana) in Lake Tahoe, California-Nevada. Oikos, 21: 83-91. Alderman, D.J., Polglase, J.L., Frayling, M. and Hogger, J.B., 1984. Crayfish plague in Britain. J. Fish. Dis., 7: 401-405. Arrignon, J., 1975, Crayfish farming in France. Freshwater Crayfish, 2: 105116. Bhattacha~a, C.G., 1967. A simple method of resolution of a distribution into Gaussian components. Biometrics, March, pp. 115-129. Blake, G. and Laurent, P.J., 1982. Le faucardage par des ticrevisses; result&s preliminaires. Bull. Mens. Sot. Linn. Lyon, 51: 203-208. Brown, D.J., 1979. A study of the population biology of the British freshwater crayfish Austro~ta~bi~ pulses (~~~ullet). PH.D. thesis, Durham Univ., Great Britain, 262 pp. (unpublish~) . Brown, D.J. and Bowler, K., 1979. The relationship between size and age throughout the life-cycle of Austropotamobiuspallipes (Lereb.) . Freshwater Crayfish, 4: 35-42. Brown, D.J. and Brewis, M., 1979. A critical look at trapping as a methodof sampling a population of Austrapotamobius pallipes (Lereboullet) in a mark and recapture study. Freshwater Crayfish, 4: 159-164. Cukerzis, J. and Terentjew, A., 1979. Acclimatisation de Pacif~tac~ ~eni~~~~ (Dana) dans un lac isole. Pi&c. Fr., 56: 13-16 (translated by PJ. Laurent). Dean, J.L., 1969. Biology of the crayfish Onconectes causeyii and its use for the control of aquatic weeds in trout lakes. Tech. Paper U.S. Bur. Sport Fish and Wild., 24: 15 pp. Emadi, H., 1974. Culturing conditions and their effects on survival and growth of the crayfish
43 Pacifastacus leniusculus trowbridgii (Dana). Ph.D. thesis, Oregon State Univ., U.S.A. (unpublished), Flint, R.W., 1975a. Natural history, ecology and production of the crayfish Pacifastucus leniusculus (Dana) in a subalpine lacustrine environment. Ph.D. thesis, Univ. California, Davis, U.S.A. (unpublished), 150 pp. Flint, R.W., 1975b. Growth in a population of the crayfish Pacifastacus leniusculus (Dana) from a subalpine lacustrine environment. J. Fish. Res. Board Can,, 32: 2433-2440. Flint, R.W. and Goldman, CR., 1975. The effects of a benthic grazer on the primary productivity of the littoral zone of Lake Tahoe. Limnol. Oceanogr., 20: 935-944. Goddard, J.S. and Holdich, D.M., 1979. Explore the native potential first. Fish Farmer, 2: 47. Hogger, J.B., 1984. Aspects of the biology and distribution of freshwater crayfish in the Thames ~~hment. Ph.D. thesis ( Unpubl. ) , C.N.A. A., United Kingdom, 340 pp. Hogger, J,B., in prep. Aspects of the intr~uction of the “signal crayfish”, Pac~f~tac~ leniusculus (Dana) into the southern United Kingdom, 2. Morphometry and production potential. Jay, D. and Holdich, D.M., 1981. The distribution of Austropotamobiuspallipes (Lereboullet) in British waters. Freshwater Biol., 11: 121-129. Jestin, J.M., 1979. Croissance et developpement de l’ecrevisse americaine Orconectes limosus dans le Lac de Cretail (Val de Marne, France). Freshwater Crayfish, 4: 65-72. Jones, J.B., 1981. Growth of two species of freshwater crayfish (Paranephrops st.) in New Zealand. N.Z.J. Mar. Freshwater Res., 15(l): 15-20. Kossakowski, J. and Orzechowski, D., 1975. Crayfish Orconectes limosus in Poland. Freshwater Crayfish, 2: 31-4’7. Laurent, P.J., 1980. Utili~tion des Btangs pour la production d’ecrevisses. In: R. Billard (Editor), La pisciculture en Etang. Natl. Rech. Agron., Paris, pp. 333-342. Mason, J.C., 1970. Egg laying in the western North American crayfish P~if~tac~ tro~bridgii (Dana). Crustaceana (Leiden ) ,19: 37-44. Mason, J.C., 1974. Aquaculture potential of the freshwater crayfish (Pacifastacus) . 1. Studies during 1970. Fish. Res. Board Can. Tech. Suppl., 440: 43 pp. Mason, J.C., 1975. Crayfish production in a small woodlandstream. Freshwater Crayfish, 2: 449-480. Momot, W.T., 1967. Population dynamics and productivity of the crayfish Orconectes uirilis (Hagen) in a marl lake. Am. MidI. Nat., 78: 55- 81. Momot, W.T. and Gowing, H., 1977. Response of the crayfish Orconectes uirilis to exploitation. J. Fish. Res. Board Can., 34(8): 1212-1219. Pratten, D.J., 1980. Growth in the crayfish Austropotamobiuspallipes (Lereb.). Freshwater Biol., lo: 401-402. Reynolds, J.D., 1979. Crayfish ecology in Ireland. Freshwater Crayfish, 4: 215-220. Richards, K.J. and Fuke, P., 1977. Freshwater crayfish - the first centre in Britain. Fish Farm. Int., 4: 12-15. Ricker, W.E., 1975. Computation and interpretation of biological statistics of fish populations. Bull. Fish. Res. Board Can., 191: 882 pp. Stevenson, J.R., 1975. The moulting cycle of the crayfish:recognising the moulting stages, effects of ecdysone and changes during the cycle. Freshwater Crayfish 2: 255-269. Unestam, T., 1969. Resistance to the crayfish plague in some American, Japanese and European crayfishes. Rep. Inst. Freshwater Res. Drottningholm, 49: 202-209. Vigneux, E., 1979. P. leniuscuEus (Dana) et A. 1eptodactylu.s (Esch.) premiere bilan d’exploitation en Btangs. Freshwater Crayfish, 4: 227-234. Webber, H.H. and Riordan, P.F., 1976. Criteria for candidate species for aquaculture. Aquaculture, 7: 107-123. Westman, K,, 1973. The population of the crayfish Astacus astacus (L.) in Finland and the introduction of the American crayfish P~if~tac~ leni~c~~~ (Dana). Freshwater Crayfish, 1: 41-50.
44
Westman, K. and Pursiainen, M., 1979. Development of the European crayfish Astacus astacus and the American crayfish P~if~t~~ ~~i~cu~~ population in a small Finnish lake. Freshwater Crayfish, 4: 243-250. Westman, K., Pursiainen, M. and Viikman, R., 1979. A new folding trap model which prevents crayfish from escaping. Freshwater Crayfish, 4: 235-242.