412 
Fishery Bulletin 95(3), 1997 
1.0 
>> 0.8 
— (a) 
(b) 
*j3 
O 0.6 
a. 
<D 
- // 
1 °' 4 
g 
U 0.2 
;// 
012345676 
Identified difference in mean size (mm) 
Figure 8 
Cumulative distribution of the probability of correctly reject- 
ing the null hypothesis of no difference between (A) two esti- 
mates, both measured with error and (B) an estimated mean 
size and a predefined value, measured without error. The dis- 
tribution is based on 1,000 simulation runs with a sampling 
intensity of 50 abalone per day in 100 diver-days in a fishery of 
4,000 diver-days. 
of abalone at harvest may provide a false sense of 
reliability if higher level sources of variation are ig- 
nored. If variation above that found among abalone 
within a diver-day is not included, false conclusions 
may be drawn about the statistical significance of 
trends through time, both because of unrepresenta- 
tive sampling and imprecise estimates of variability. 
In the present study, the relatively large differences 
in sizes of abalone among diver-days meant that even 
relatively high sampling fractions (>10%) would have 
a relatively low probability of detecting changes in 
mean size less than 3 mm. This conclusion appeared 
to be robust over a realistic range in the number of 
diver-days sampled. Given the broad similarities 
between the sampling scheme described for this fish- 
ery and those in many commercial fisheries, concerns 
may be raised about the reliability of samples taken 
from processing plants in the absence of an under- 
standing of the contributions of higher level sources 
of variation. The simulation framework described 
may be directly expanded to accommodate more com- 
plex situations. 
Acknowledgments 
We are grateful to Geoff Gordon and Duncan 
Worthington for discussions, and Penny Brett and 
Ron Avery for help with the manuscript. We thank the 
abalone divers and staff from NSW Abalone, S.O.S. 
(Southern Oceans Seafoods), A.S.E. (Abalone 
Shellfish Enterprises), and Interon for assistance. 
This work was funded in part by grants from the 
Australian Fisheries Research and Development 
Corporation and the New South Wales commercial 
abalone industry for which we are grateful. 
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