178 S
.S. Montgomery J. Exp. Mar. Biol. Ecol. 255 2000 175 –186
hypothesis that catches of J . verreauxi recruits will be greater on collectors set far from
natural reef than on collectors set near reef. Collectors set far from reef would be the only piece of preferred habitat in the vicinity, available to the recruits. So, those animals
that encounter the collector and choose to stay would have no other area of preferred habitat available to them. In contrast, collectors set near reef would be part of a greater
area of habitat for recruits to settle and perhaps move within. It follows from this reasoning that the number of recruits on collectors set far from reef would be expected
to be greater than on those near reef. I also present cost–benefit analyses on data from the experiment to determine the optimal numbers of collectors replicates and sites at
each location to use in future surveys.
2. Materials and methods
2.1. Study sites The experiment was done at two exposed Coalcliff and Era and two sheltered
Wattomalla and Curracurrang sites near the New South Wales Fisheries Research Institute, the base for this study Fig. 1. At each site, collectors were set around 5 m
from vegetated reef Near Position or in waters around 200 m from reef and over bare sand Far Position.
2.2. Procedure The sea-weed-type collectors used in this study were similar to those described by
Phillips 1972 for collecting pueruli of Panulirus cygnus George, 1962. The collectors consisted of three PVC 61 3 35 3 0.4 mm panels supported by an aluminium frame with
four polystyrene buoys through its centre Fig. 2. Attached to each panel were ‘tufts’ of polyethylene split rope fibre. A tuft was comprised of 80 g of 500 mm lengths of 125
text polyethylene fibre. The middle of the tuft was tied with a plastic tie, passed through a hole in the PVC panel and secured by monel wire. The collectors were moored to the
substratum by attaching 20 m of 14 mm polypropylene rope to 3 m of 6 mm ‘black chain’ and a car tyre filled with cement 70 kg laying on the substratum.
Factors in the experiment were level of Exposure fixed, Sites nested in Exposure and Position fixed. At each Position, seven replicate sea-weed-type collectors were set
approximately 10 m apart and about 10 cm below the surface at each Position i.e. a total of 56 collectors.
Collectors were sampled fortnightly on board a vessel from September to December 1993 inclusive. Each panel of the collector was shaken 30 times into a 320 l container.
Lobsters were sorted from the contents of the container, released into a second container of sea-water, taken to the laboratory, identified to species, staged into one of six
categories of development and measured CL. The stages of development were:
S .S. Montgomery J. Exp. Mar. Biol. Ecol. 255 2000 175 –186
179
Fig. 2. A member of staff shows the make up of the sea-weed-type collector used in this study to catch J .
verreauxi recruits.
180 S
.S. Montgomery J. Exp. Mar. Biol. Ecol. 255 2000 175 –186
Stage 1 puerulus: transparent;
Stage 2 puerulus: transparent, but with digestive gland evident;
Stage 3 puerulus: cephalothorax cream to brown in colour, digestive
gland no longer visible; Stage 4 puerulus:
whole body brown, but no spines on the cephalothorax; Stage 5 first instar juvenile:
whole body dark brown, spines on the cephalothorax and pleopods smaller than in stages 1–4; and
Stage 6 early juvenile: whole body green, spines on the cephalothorax and at a
size less than 30 mm CL that indicated that the animal had recruited this year
For the purposes of this paper recruits are defined as lobsters in stages 1–6, inclusive. 2.3. Cost–benefit analyses
After the experiment was completed, cost–benefit analyses were done to determine the optimal numbers of sites, and collectors within sites, needed to measure the relative
abundance of recruits at a location. Analyses were done on a subset of data made up of the catch-rates of recruits for collectors set at the Far Position, because it became evident
during the course of this experiment that it was easier to sample collectors at the Far Position and catch-rates of recruits at Far Positions were greater than those at Near
Positions. These data were analysed in a two-factor nested analysis of variance to collect information on variances. The standard cost–benefit analysis procedure was followed
Snedecor and Cochran, 1967; Underwood, 1981. The product of the variance of the estimated mean of each sampling period and the total cost of each sampling period was
minimised. The variances of the means of sites and collectors were calculated by dividing the appropriate mean square by the number of readings in each mean. Variance
V for the mean number of recruits per location is given by
2 2
V 5 s 1 n s n n 3.1
e e
s e
s 2
2
where n , n are the numbers and s , s , the variances among collectors and sites,
e s
e s
respectively. The total cost C of sampling at one time in one location is given by
C 5 C n 1 C n n 3.2
s s
e e
s
where n and n are as above and C and C are the costs of each collector or site,
e s
e s
respectively. The most efficient sampling is that which reduces V and C to minima. The minimum
value of the product, VC, for different sample sizes can be found by differentiating VC with respect to n and setting the differential to zero to find a minimum. This results in
e
the following solution:
2 2
n 5 œC s C s 3.3
e s
e e
s
The restricting cost in this study was the amount of time available to sample the
S .S. Montgomery J. Exp. Mar. Biol. Ecol. 255 2000 175 –186
181 Table 1
Analyses of numbers of i recruits, ii pueruli and iii juvenile stages of J . verreauxi on collectors between
a
type of Location exposed or sheltered to sea-swell and Position of collectors with respect to reef Source of
df Recruits
Pueruli Juveniles stages
variation MS
P MS
P MS
P C 5 0.23 ns
C 5 0.26 ns C 5 0.31 ns
Exposure, E 1
0.04 ns
0.01 ns
0.01 ns
Sites, SE 2
0.02 ,ns
0.01 ,ns
0.001 ns
Position, P 1
0.10 ns,
0.05 ns,
0.01 ns
E3P 1
0.01 ns
0.01 ns
0.0001 ns
P3SE 2
0.01 ns,E
0.004 ns,E
0.003 ns
Residual 29
0.01 0.01
0.00 Total
55
a
The degrees of freedom for the residual have been adjusted for missing replicates. Higher-order effects have been pooled P . 0.25. ns, non-significant P . 0.05; significant P , 0.05; significant P , 0.01,
ns,E, term was non-significant in original model but pooled with residual; ,ns, term was significant in original model but non-significant after pooling procedures; ns,, term was non-significant in original model but
significant after pooling procedures. Cochran’s test, C.
collectors at sea on 1 day 300 min. It cost 40 min to do each site. This was determined by measuring the time it took to prepare and travel to each site during this sampling.
Similarly, I determined that it cost 10 min to sample each collector.
3. Results
