A . Linnane et al. J. Exp. Mar. Biol. Ecol. 249 2000 51 –64
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2. Materials and methods
2.1. Experimental animals All lobsters used in the experiment were hatched at the National University of Ireland,
Galway, Shellfish Research Laboratory, Carna, County Galway, Ireland, using the methods described by Mercer and Brown 1994. Within 24 h of reaching stage IV,
postlarvae were transferred directly from the hatchery to the experimental unit.
2.2. Experimental unit The trials were undertaken in an outdoor rectangular 10 3 4 3 1 m concrete pond
which was lined with black polythene Fig. 1. The bottom of the unit was covered with
2
a layer of coarse sand approximately 50 mm deep. The pond was divided into 40 3 1 m plots, each of which was filled with one of the following substrata: coarse sand 1–1.5
mm as classified by Wentworth 1936, coralline algae Lithothamnion sp ., mussel
shell Mytilus edulis or cobble 64–256 mm; Wentworth, 1936. Thus, each sub- stratum was represented by ten plots. The sand, coralline algae and mussel shell plots
were approximately 3–4 cm in depth while each cobble plot contained two layers of cobble stone.
By positioning the plots as shown in Fig. 1, the experiment endeavoured to make each substratum equally available to every lobster. While realising that choice is dependent on
Fig. 1. Plan view of experimental pond showing the exact location of each natural substratum. C: Cobble, L: Lithothamnion coralline algae, M: Mussel Shell, S: Sand.
54 A
. Linnane et al. J. Exp. Mar. Biol. Ecol. 249 2000 51 –64
the sequence in which substrata are encountered, the large sample size and the long duration of the study contributed to counteract effects associated with biased settlement
pattern related to plot distribution. In addition, research has shown that postlarval lobsters are capable of rapid, directional swimming Ennis, 1986; Hudon et al., 1986;
Cobb et al., 1989 with the function of allowing animals to move into areas suitable for settling. Combined with this capability, the experimental design endeavoured to allow
individuals to behaviourally determine their distribution and final settling location.
Seawater entered the pond via an inflow pipe positioned at one corner of the unit. Throughout the experiment the inflow was maintained at approximately 10 l min. Water
exited the system via a central standpipe 70 mm diameter which was covered with a 2 mm mesh screen to prevent juveniles escaping from the pond.
2.3. Releases Stage IV juveniles were removed from the rearing unit using a hand net and
transferred into a 10 l plastic container filled with seawater. Lobsters were released into the pond from the central standpipe at a rate of 250 individuals every 15 min. For each
trial, a total of 4000 postlarvae were used thereby giving an initial density of 100
2
animals m . Prior to release, the water volume was lowered to approximately 5 cm below the outflow level.
2.4. Sampling protocol Lobster distribution in different substrata was examined on both a short- and
long-term basis. ‘Short-term’ consisted of three separate trials in which postlarvae were released into the pond, allowed to settle and then sampled 30 days later. Sampling
consisted of draining the system and manually searching each plot for lobsters. Discovered lobsters were removed from the unit and the number of individuals on the
2
respective plots was recorded. A x test was used to test the null hypothesis that the
observed distribution of settled lobsters would not differ significantly from equal proportions among substrata. The carapace length CL, measured using a vernier
calipers and cheliped number of each individual was also recorded. Between trials, the system was drained for a period of 5 days to ensure that undiscovered lobsters did not
invalidate the results.
The ‘long-term’ trial consisted of returning the juveniles to their original substratum following the third, short-term census. The pond was refilled and periodic sampling was
undertaken at 30 day intervals for the next consecutive five months. A final census was taken a further three months later, nine months after the postlarvae had been introduced
into the system. When sampling, care was taken to minimise disturbance. Recovered juveniles were maintained in seawater and on return to their selected habitat were placed
under cover of the substratum to avoid desiccation. The surface water temperature within the system was recorded daily.
A . Linnane et al. J. Exp. Mar. Biol. Ecol. 249 2000 51 –64
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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
