2. Materials and methods
The Avondale Research Station is located in the Western Australian wheatbelt near Ž
X X
. Beverley 32
87 S, 116855 E . There were 25 clay ponds at the research station. Each pond was 1.5 m deep with a 10 = 10 m water surface area, 3:1 batter and a level floor.
Ponds were individually fenced to prevent movement of yabbies between treatments. All 25 ponds received water from the same supply dam. The ponds were filled prior to
stocking and thereafter topped up to replace water lost due to evaporation.
Eighteen of these 0.01-ha ponds were stocked with yabbies obtained from local harvestors at an initial density of 1rm
2
. Prior to stocking, the 1800 yabbies in this experiment were sexed by manual examination for the presence of penes at the base of
the fifth pair of periopods for males, or gonopores at the base of the third pair of periopods for females. Intersex animals were excluded from the study.
A completely randomized block design with two factors, sex and diet, was used. Ž
. Factor sex had three levels, male, female and mixed-sex 1:1 ratio . Factor diet also had
Ž .
y2 y1
Ž .
y2
three levels, Lupin Lupinus albus 2.5 g m week
, Lupin L. albus 5.0 g m
y1
Ž .
y2 y1
Ž week
and WA Crayfish Reference Diet CRD 2.5 g m week
Morrissy, 1990, .
1992; Lawrence et al., 1998 . There were two blocks: the first block consisted of northern ponds and the second block consisted of southern ponds.
A random sample of yabbies was weighed prior to stocking each treatment. The mean Ž
. Ž
. stocking weight S.E. of yabbies in this experiment was 19.2 0.3 g
n s 180 . Ž
. There was no significant difference P s 0.08 in the average initial weight for male
Ž . Ž
. Ž
. Ž .
19.3 0.2 g n s 90 and female 19.1 0.4 g
n s 90 yabbies by a two-tailed t-test.
The ponds were stocked in July 1995 and harvested after 175 days so as to include five months of the yabby breeding season. All yabbies were removed by first trapping
then draining the ponds to collect the remaining animals. Immediately after harvesting all yabbies were individually sexed and weighed.
Gross valuerha was calculated according to current industry values for standard size Ž
categories assuming that juveniles less than 30 g have no commercial value Lawrence, .
1998 .
3. Results
3.1. Water quality Ž
. The water quality of the research ponds Table 1 was typical of farm dams in the
Ž .
Western Australian wheatbelt Lawrence et al., 1998 . 3.2. Size-frequency distribution
Ž Construction of weight-frequency histograms showed cohorts of juveniles arising
. from reproduction in ponds during the experiment as animals with weights below the
initial stocking weights. The final density, including juveniles, was higher in mixed-sex
Table 1 Water quality of the research ponds at Avondale Research Station during the experiment
Parameter Units
Mean Alkalinity
mgrl 240
Hardness mgrl
185 Ca
mgrl 23
K mgrl
5 Na
mgrl 354
Zn total mgrl
0.02 Fe
mgrl -
0.05 Fe total
mgrl 0.31
Cu total mgrl
- 0.02
CO mgrl
27
3
Cl mgrl
440 P-SR
mgrl 0.02
HCO mgrl
240
3
SO –S mgrl
70
4
N–NO mgrl
- 0.02
2
N–NO mgrl
- 0.02
3
TDS mgrl
1050 Econd
mSrm 196
Temp 8C
22 pH
9.1
populations than the monosex populations. This was due to reproduction especially in Ž
. the mixed-sex ponds Table 2 . The decreased number of juveniles, and therefore lower
density, in monosex ponds favoured growth in these treatments. The mean weight of juveniles in mixed-sex ponds was 0.96 0.09 g, 1.00 0.77 g in male monosex ponds
Table 2 Ž
. Summary of results meanS.E. for monosex and mixed-sex populations of C. albidus
Ž .
Mixed-sex 1:1 Male monosex
Female monosex Ž
. Ž
. Ž
. ns6 ponds
ns6 ponds ns6 ponds
Ž . Weight
Initial g 19.20.3
19.30.2 19.10.4
Ž . Final adult g
46.10.6 60.60.6
43.60.3 Ž .
Final juvenile g 0.960.09
1.000.77 2.440.29
Ž .
Ž . Growth adults
Weight gain g 26.9
41.3 24.5
Ž . Growth raterweek g
1.08 1.65
0.98
2
Ž .
Density Initial no.rm
1 1
1
2
Ž .
Final adult no.rm 0.5
0.7 0.7
2
Ž .
Final juvenile no.rm 9.882.49
0.610.53 5.491.59
2
Ž .
Final adultqjuvenile no.rm 10.38
1.31 6.19
Ž . Biomass
Adult g 3095256
4161269 3300471
Ž . Juvenile g
930229 9795
1193256 Ž .
Total g 4025323
4258272 4493508
Ž .
Gross value USrha 2079164
3548292 2030363
Ž .
and 2.44 0.29 g in female monosex ponds Table 2 . Juveniles could therefore be distinguished from adults during the analyses of data.
All male yabby ponds gave a greater proportion of larger adult animals, and therefore more valuable size classes at harvest, than all-female and mixed-sex ponds. The size
frequency distribution of adult yabbies in female monosex, male monosex and mixed-sex treatments is shown in Fig. 1.
3.3. Growth and surÕiÕal of adults excluding juÕeniles Ž
. Analysis of variance revealed that neither the block effect
P s 0.17 nor the
Ž .
interaction between sex and diet P s 0.70 had a significant effect on yabby growth. Ž
. Ž
. Both diet P s 0.049 and sex P s 0.001 had a significant effect upon yabby growth.
For each diet treatment, the mean weight of yabby populations that contained only Ž
. males was greater than mixed or female populations Fig. 2 . Furthermore, growth of
yabbies was higher on the WA CRD in comparison to the standard industry practice of Ž
y2 y1
. Ž .
Ž
y2
feeding lupins 2.5 g m week
Fig. 2 . It required twice as much lupins 5 g m
Fig. 1. The weight-frequency distribution of female monosex, male monosex and mixed-sex yabby population Ž
. juveniles excluded .
Fig. 2. The mean increase in weight of mixed-sex and monosex yabby populations fed three different diets.
y1
. Ž
y2 y1
. week
to obtain similar growth to that achieved with WA CRD 2.5 g m week
Ž .
y2 y1
Fig. 2 . Consequently, monosex male populations fed WA CRD at 2.5 g m week
produced yabbies that grew, on average, 88 faster than those grown under the standard Ž
y2 y1
. industry practice of feeding mixed-sex populations with lupins 2.5 g m
week Ž
. Fig. 2 .
If ponds with the same sex treatment are grouped together, and juveniles excluded, Ž
. Ž
. Ž the final mean weight S.E. of yabbies in male-only ponds 60.6 0.6 g
n s 6 .
Ž . Ž
ponds was greater than that of female yabbies in monosex ponds 43.6 0.3 g n s 6
. Ž .
Ž . Ž
. ponds
P - 0.001 and mixed-sex yabby populations 46.1 0.6 g n s 6 ponds
Ž . Ž
. P - 0.0001
Table 2 . The male-only yabby ponds had a 52.8 higher growth rate Ž
. Ž
. 1.65 grweek than mixed-sex populations 1.08 grweek and 68.4 greater than
Ž . Ž
. female-only ponds 0.98 grweek
Table 2 . Female yabbies in monosex culture grew Ž
. slower than mixed-sex populations Table 2 , but the difference was not statistically
Ž .
significant P s 0.31 . Males in monosex culture grew 17 faster than the males in mixed-sex populations,
while females in monosex culture grew 31 faster than the females in mixed-sex Ž
. Ž .
populations P - 0.05 Table 3 .
Table 3 Ž
. Growth and survival meansS.E. of yabby males and females in mixed-sex and monosex populations
Ž .
juveniles excluded Males in
Males in Females in
Females in mixed-sex
monosex mixed-sex
monosex Ž .
Weight Initial g
19.20.3 19.30.2
19.20.3 19.10.4
Ž . Final g
54.40.6 60.60.6
37.90.6 43.60.3
Ž . Growth
Weight gain g 35.2
41.3 18.7
24.5 Ž
. Growth rate grweek
1.41 1.65
0.75 0.98
Ž . Survival
653.6 677.4
616.8 6010.8
A generalized linear model with logit link was used to examine the survival of Ž
. yabbies McCullagh and Nedler 1986 . The survival rate of yabbies in male monosex
Ž .
Ž .
ponds was 7 higher P s 0.005 than in female monosex ponds Table 3 . There was no significant difference in the survival of males in monosex culture and mixed-sex
Ž . Ž
. culture P s 0.86
Table 3 . Similarly, females in monosex and mixed-sex culture had Ž
. Ž .
the same survival P s 0.64 Table 3 .
3.4. Economic significance of monosex culture At current market prices, the ponds stocked with only male yabbies resulted in a 70
Ž .
greater gross value of animals produced than from the mixed-sex ponds P - 0.01
Ž .
Table 2 . However, there was no significant difference in the gross values of yabbies Ž
. produced from female-only and mixed-sex ponds P s 0.90 .
4. Discussion
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