Ogburn and Forward: Effect of sampling interval on estimates of larval supply 
453 
2500 
2000 
3 
Q. 
1500 >< 
o 
Q. 
l/t 
T3 
a> 
1000 S 
o' 
- 500 
0 
1993 1995 1997 1999 2001 2003 2005 
Figure t 
Variability of daily blue crab (Callinectes sapidus) larval supply, measured as 
number of megalopae collected in each hog’s hair collector during each day 
(megalopae collector -1 day -1 ) for each year of an 11-year record (1993-2006) 
used in this study to evaluate the effect of sampling at intervals of 2, 3, 4, 5, 
and 7 days on the variability of estimates of annual larval supply of blue crab 
megalopal settlement in the Newport River estuary, North Carolina. Data are 
the mean and standard error of the mean of larval supply (black diamonds and 
error bars) and the index of dispersion (gray vertical bars). No sampling was 
conducted and no data collected in 1997, 1999, and 2001. 
ouu 
250 - 
200 
= 150 
3 100 - 
50 - 
resulted in a 20% decrease in the CV (from a mean of 
0.27 to 0.22) at the 7-day interval (Table 2). The CV was 
typically small in years, such as 1998 (CV=0.34 at the 
7-day interval), when supply was spread over many days 
(Fig. 3A) and large in years, such as 2000 (CV=0.99 at 
the 7-day interval), in which supply was dominated by 
a single pulse (Fig. 3B). 
Subsampled estimates of annual larval supply were 
rarely correlated with CPUE of the crab fishery at sam- 
pling intervals >2 days. At a 2-day sampling interval, 
larval supply was significantly correlated with land- 
ings for 64.5% of possible comparisons (Table 1). There 
were no possible combinations of estimates of larval 
supply that were correlated with CPUE at the 3-day 
sampling interval. A small proportion of combinations 
(<15%) were significantly correlated with CPUE for 
longer sampling intervals, primarily because of an in- 
crease in the range of correlation coefficients with an 
increasing number of comparisons even as the mode 
decreased (Table 1). 
Discussion 
Monitoring the supply of larval shellfishes and finfishes 
is a costly and time-consuming endeavor. Researchers 
often use sampling intervals of a week or more to reduce 
the cost and effort associated with data collection (e.g., 
Warlen, 1994), but it is important to fully understand the 
Table 1 
Sampling intervals, numbers of possible comparisons 
between annual estimates of blue crab ( Callinectes sapi- 
dus) larval supply and catch per unit of effort in the North 
Carolina blue crab fishery (A), percentages of compari- 
sons with significant correlations (type-1 error [a] = 0.05 
coefficient of correlation [r]>0.69), modes of correlation 
coefficients, and differences between the lowest and high- 
est values of correlation coefficients in this study to evalu- 
ate the effect of sampling at intervals of 1-7 days on the 
variability of estimates of annual larval supply derived 
from an 11-year record of blue crab megalopal settlement. 
Interval 
N 
% significant 
Mode 
Difference 
1 
1 
100 
0.88 
— 
2 
256 
64.5 
0.68 
0.30 
3 
6561 
0 
0.59 
0.41 
4 
65,536 
2.3 
0.56 
0.87 
5 
390,625 
7.4 
0.52 
1.08 
6 
1,679,616 
7.9 
0.54 
1.10 
7 
5,764,801 
13.8 
0.5 
1.22 
consequences of choosing a particular sampling interval. 
Sampling at intervals longer in duration than the wind 
or tide-driven processes that underlie fluctuations in 
larval supply could result in high variability in esti- 
