A . Linnane et al. J. Exp. Mar. Biol. Ecol. 249 2000 51 –64
55
2.5. Feeding Prior to the introduction of postlarvae into the experimental system, the unit was
seeded with brine shrimp Artemia sp.. Initially, 100 g of hatched cysts were released, with 50 g added every 2 days subsequently. As the temperatures within the system
dropped during the long-term study, the feeding ration was reduced to 50 g every 4 days. The Artemia were hatched by placing the cysts into a 50 l conical vessel containing
aerated, 1 mm U.V. filtered seawater at a temperature of 188C for a period of 12 h.
2.6. Data analyses The results were analysed using Chi-square, t and ANOVA tests. Where multiple
comparisons were made over the 9 month study, the probability of Type 1 error was adjusted via the Bonferroni procedure and significance was accepted at P , 0.01. Where
the ANOVA analysis indicated significant differences, a Tukey test Zar, 1996 was used to examine differences between pairs of treatments. The calculations were performed
with MINITAB MINITAB Inc., U.S.A and SYSTAT Wilkinson et al., 1992
statistical software.
3. Results
3.1. Survival and distribution within substrata Survival of lobsters from postlarvae to 1 month old ranged from 5 to 14 Table 1.
2
As postlarvae did not settle in equal proportions as hypothesised x 5 49.31, df 5 3,
P , 0.05 the null hypothesis concerning lobster distribution was rejected. In cases where mussel shell and coralline algae were chosen, animals were found sheltering in
Table 1 The number of lobsters found in test substrata and the end of each 1 month trial and overall mean densities.
Significance tests between substrata are also included. Means were calculated from 30 quadrats of each substratum during 3 replicate trials. Each trial had an initial release of 4000 postlarvae
Trial No. of lobsters per substratum
number Mussel
Cobble Coralline
Sand Total
Survival shell
algae 1
213 124
47 384
9.6 2
78 113
26 217
5 3
350 187
9 546
14 F
df P
2
Mean n m 21.3
14.1 2.7
28.8 2
, 0.05 S.E.
2.53 1.44
0.5 Single-factor ANOVA.
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. Linnane et al. J. Exp. Mar. Biol. Ecol. 249 2000 51 –64
the interstitial spaces provided by the substratum. Lobsters in cobble, as well as utilising interstitial spaces, constructed burrows in the sand beneath the stones. Frequently, two
juveniles were observed sheltering under the same cobble stone. As there were no differences in the mean size of EBP animals among substrata after 1
month, data from all 3 substrata were pooled. Surviving 1 month benthic recruits formed a clear mode of size classes between 6 and 8 mm carapace length CL in the size
distributions of lobsters sampled from all three trials Fig. 2. There was a significant
2
variance P , 0.05 in the density of juveniles per m within mussel shell, cobble, and coralline algae substrata Table 1. Post hoc analysis Tukey test, Zar, 1996 identified
a significant variation P , 0.05 between all pairwise mean comparisons of each substratum.
3.2. Long term study – number of lobsters per substratum
2
The number of lobsters m of cobble remained relatively constant throughout the 9 month period, while the number of juveniles within mussel shell decreased considerably
Fig. 3.
2
An initial density of 35 lobsters m was recorded in mussel shell after month 1 of the
2
study. This was significantly higher P , 0.01 than the density of 19 lobster m
Fig. 2. Size-frequency distribution of surviving lobsters found on all substrata after a 1 month period. This data is pooled from the 3 trials in Table 1.
A . Linnane et al. J. Exp. Mar. Biol. Ecol. 249 2000 51 –64
57
Fig. 3. Comparison of mean lobster densities 61 S.D. on cobble, mussel shell and coralline algae over the 9
2
month study period. No lobsters were found on sand. Means were calculated from 10 3 1 m quadrats of each. Table 2
Results of t-tests performed on the number of lobsters on cobble and mussel shell substrata over the 9 month study period. Data are presented in Fig. 3
Time months df
t P
1 18
2 3.36 0.00
2 18
2 0.52 0.30
3 18
2 0.31 0.38
4 18
3.01 0.00
5 18
2.03 0.02
6 18
2.25 0.01
9 18
5.54 0.00
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. Linnane et al. J. Exp. Mar. Biol. Ecol. 249 2000 51 –64
observed in cobble Table 2. Within months 2 and 3 the density of juveniles within mussel shell substratum decreased to levels comparable with that of cobble 18–19
2
lobsters m and these levels continued to drop significantly in the following counts until the conclusion of the study. Few lobsters were located in coralline algae and after 3
months individuals were no longer sighted in this substratum. No lobsters were recorded in sand.
3.3. Substratum–size relationship The size composition of EBP lobsters varied markedly on the two substrata Fig.
4a–g. Lobsters found on cobble became increasingly variable in size as the study
Fig. 4. a–g. Size frequency distributions of lobsters on cobble and mussel shell substrata as sampled monthly throughout the study period.
A . Linnane et al. J. Exp. Mar. Biol. Ecol. 249 2000 51 –64
59
Fig. 5. The mean monthly temperature 6S.D. as recorded on the surface of the pond throughout the 9 month study period.
progressed. Up to a period of 3 months, lobsters in both substrata showed similar growth trends Fig. 4a–c. However, after this time period, lobsters in mussel shell were
observed to maintain a size mode of 8–10 mm CL within a narrow size range of 6–14 mm Fig. 4d–f. In month 9, the mode did increase to 10–12 mm but by then the sample
number was reduced to 13 individuals Fig. 4g. By the conclusion of the trial, lobsters in cobble tended to have a central mode of 10–12 mm CL within a range of 8–24 mm
CL Fig. 4g.
3.4. Temperature Average monthly temperatures as recorded on the surface of the experimental pond
followed a clear seasonal thermal regime, with an Autumn cooling leading to a Winter low followed by a Spring warming Fig. 5.
3.5. Cheliped loss Data on cheliped loss as an indication of fighting and aggression suggest that agonistic
Table 3 Mean percentage cheliped loss 61 S.D. on cobble and mussel shell substrata over the 9 month study period.
2
Means were calculated from the number of lobsters missing one or both chelipeds from 10 3 1 m plots of each substratum. Month 1 is excluded from the analyses as the initial number of postlarvae missing chelipeds
was unknown Time months
2 3
4 5
6 9
Cobble 15.8610
19.9616. 2567
21.169 18.8611
19.469 Mussel
11.369 18.9610
27611 19.8615
18.7615 19.1619
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encounters were common on both cobble and mussel shell substrata Table 3. Monthly censuses revealed that 11.3–27 of lobsters recovered were missing either one or both
chelipeds. There was no significant difference t-test, P . 0.05 in cheliped loss between substrata.
4. Discussion
