Ogburn and Forward: Effect of sampling interval on estimates of larval supply 
455 
Table 2 
Coefficients of variation for the September-November and June-November data sets on nightly blue crab ( Callinectes sapidus) 
megalopa settlement. Data presented are for subsampling of the data sets of daily larval supply at intervals of 2, 3, 4, 5, and 7 
days. Means and standard deviations (in parentheses) are included separately for the 2004-06 period for comparison between 
the September-November and June-November sampling periods; data were collected in the longer period from June to Novem- 
ber only in 2004-06. 
Sampling interval 
Sept.-Nov. 
2 days 
3 days 
4 days 
5 days 
7 days 
1993 
0.23 
0.06 
0.38 
0.27 
0.50 
1994 
0.04 
0.21 
0.16 
0.10 
0.24 
1995 
0.16 
0.17 
0.18 
0.37 
0.49 
1996 
0.35 
0.45 
0.75 
0.77 
0.88 
1998 
0.02 
0.11 
0.16 
0.30 
0.34 
2000 
0.43 
0.43 
0.61 
0.54 
0.99 
2002 
0.24 
0.21 
0.47 
0.41 
0.37 
2003 
0.17 
0.19 
0.25 
0.38 
0.52 
2004 
0.10 
0.12 
0.11 
0.14 
0.26 
2005 
0.16 
0.30 
0.39 
0.48 
0.40 
2006 
0.09 
0.13 
0.10 
0.22 
0.15 
1993-2006 Mean 
0.18(0.13) 
0.22 (0.13) 
0.32 (0.22) 
0.36(0.19) 
0.47(0.26) 
2004-06 Mean 
0.12 (0.04) 
0.18 (0.10) 
0.20 (0.16) 
0.28 (0.18) 
0.27 (0.13) 
June-Nov. 
2 days 
3 days 
4 days 
5 days 
7 days 
2004 
0.03 
0.14 
0.04 
0.05 
0.20 
2005 
0.06 
0.28 
0.14 
0.43 
0.31 
2006 
0.04 
0.01 
0.06 
0.06 
0.14 
2004-06 Mean 
0.04 (0.02) 
0.14 (0.14) 
0.08(0.06) 
0.18(0.22) 
0.22 (0.09) 
by chance as the range of correlation coefficients in- 
creased with an increasing number of possible data 
combinations (Table 1). This increase in the likelihood 
of detecting a significant recruit-stock relationship by 
chance at longer sampling intervals (nearly 14% at the 
7-day interval) represents an additional concern for 
the design of larval-supply studies. Determining the 
level of variation that is appropriate for a particular 
sampling effort is not the subject of this article. How- 
ever, if we set the maximum acceptable CV at 0.25, 
equivalent to 25% of the actual annual mean based on 
daily sampling, the 2-day sampling interval performed 
within this threshold in 9 of 11 years, but the 7-day 
interval provided a sufficient estimate of larval supply 
in only 2 of 11 years (Table 1). Assuming that extending 
the sampling period from 3 to 6 months would reduce 
the CV by 20% in all years, as observed in this study 
for 2004-06, we predicted there would have been no 
change in the number of years for which the CV ex- 
ceeded 0.25. Therefore, scientists or managers must 
consider carefully the goals of multiyear larval surveys 
before they establish the appropriate sampling interval. 
For instance, weekly surveys probably are sufficient for 
estimation of seasonal trends in the supply, size, or age 
of larvae (Hettler et ah, 1997). If the goal of a survey 
is to determine annual year-class strength, however, 
weekly sampling can result in widely ranging estimates 
of annual larval supply, and sampling daily or every 
other day may be necessary. It should be noted that 
we assumed that larvae do not remain on collectors for 
more than one day. Longer sampling intervals may be 
appropriate if larvae are retained on or in collectors for 
the entire sampling interval and if there are no density- 
dependent effects of additional larvae. 
Conclusions 
Determination of the acceptable level of variability in 
estimates of larval supply is the responsibility of the sci- 
entist or manager who is to conduct a particular survey. 
An increase in the sampling interval may reduce costs 
but can result in highly inaccurate estimates of abun- 
dance in some years. The episodic nature of blue crab 
larval supply appears to be consistent with other marine 
species (e.g., Warlen, 1994; Shanks and Roegner, 2007). 
Therefore, the results of this study are likely to apply 
across a wide range of marine species. If recruit-stock 
relationships are the goal of a larval-supply survey, we 
recommend that supply be sampled daily or every other 
day to maximize the likelihood of detection of significant 
relationships. 
