410 
Fishery Bulletin 95(3), 1997 
100 200 400 
1000 1500 
Diver-days sampled 
Figure 6 
Errors in estimating the size composition of abalone caught in different sizes of the “fishery” with sampling scenario 3. “Fisheries” 
of 100, 200, 300, and 400 diver-days were considered. Sample sizes indicated on the figure are the total number of abalone measured. 
tion among those classes (see also Horppila and 
Peltonen, 1992). This approach was not appropriate 
for our situation because we were interested only in 
the size structure of landed abalone — Haliotis rubra 
can not be reliably aged by analysis of growth rings 
(McShane and Smith, 1992). The Monte Carlo approach 
that was adopted allowed us to simulate the simulta- 
neous estimation of all size classes in the population. 
The simulations described were based on the as- 
sumption that the size structures of abalone within 
a diver-day were distributed as a negative exponen- 
tial function. If the fishery declined, the mean size 
of abalone would probably decline and the slope of 
the fitted exponential curve would increase. If this 
occurred, then the sampling scheme described would 
be conservative. If, however, the fishery improved 
and larger abalone were caught, then the size-fre- 
quency distribution might depart from the negative 
exponential distribution. If such a departure was sig- 
nificant, then the simulations would need to be 
reparameterized and each sampling scenario reex- 
amined to determine an appropriate sample size for 
estimating the size structure of abalone in catches. 
One of the objectives of this simulation study was 
to estimate the probability of detecting varying 
changes in mean size of abalone at harvest. Advice 
on trends in mean size at harvest may be given at 
two spatial scales: that of the whole fishery and that 
for individual zones or groups of adjacent zones. At 
present, the NSW abalone fishery is managed as one 
stock— size limit regulations and quota allocations 
are made on a statewide basis. Management mea- 
sures, such as closures, are, however, possible at the 
smaller spatial scale if there are declines in indices 
of abundance (including mean size at harvest). In- 
deed, abalone stocks are increasingly being seen as 
comprising metapopulations of relatively discrete 
populations (see references in Shepherd et al., 1992), 
and thus such management measures are likely to 
be effective. 
The simulations suggest that sampling more than 
1,500 abalone spread across 100 diver-days would 
