451 
Abstract— Estimates of larval supply 
can provide information on year-class 
strength that is useful for fisheries 
management. However, larval supply 
is difficult to monitor because long- 
term. high-frequency sampling is 
needed. The purpose of this study 
was to subsample an 11-year record 
of daily larval supply of blue crab 
( Callinectes sapidus ) to determine the 
effect of sampling interval on vari- 
ability in estimates of supply. The 
coefficient of variation in estimates 
of supply varied by 0.39 among years 
at a 2-day sampling interval and 0.84 
at a 7-day sampling interval. For 8 
of the 11 years, there was a signifi- 
cant correlation between mean daily 
larval supply and lagged fishery catch 
per trip (coefficient of correlation 
[/-] = 0.88). When these 8 years were 
subsampled, a 2-day sampling inter- 
val yielded a significant correlation 
with fishery data only 64.5% of the 
time and a 3-day sampling interval 
never yielded a significant correlation. 
Therefore, high-frequency sampling 
(daily or every other day) may be 
needed to characterize interannual 
variability in larval supply. 
Manuscript submitted 27 January 2012. 
Accepted 24 August 2012. 
Fish. Bull. 110:451-457 (2012). 
The views and opinions expressed 
or implied in this article are those of the 
author (or authors) and do not necessarily 
reflect the position of the National Marine 
Fisheries Service, NOAA. 
Effect of sampling interval on estimates 
of larval supply 
Matthew B. Ogburn (contact author ) 1 2 
Richard B. Forward Jr . 1 
Email address for contact author: ogburnm@si.edu 
1 Duke University Marine Laboratory 
135 Duke Marine Lab Road 
Beaufort, North Carolina 28516 
2 Savannah State University 
3219 College Street 
Savannah, Georgia 31404 
Present address: Smithsonian Environmental Research Center 
647 Contees Wharf Road 
Edgewater, Maryland 21037 
Estimation of the supply of larval 
shellfishes and finfishes entering estu- 
arine nursery habitats can provide 
important information on variability 
in year-class strength of many com- 
mercially important coastal species. 
The episodic nature of larval supply 
is a significant challenge to the gen- 
eration of accurate estimates of larval 
supply (e.g., Dixon et al., 1999). Sam- 
pling must be conducted frequently 
enough to capture pulses in larval 
supply, but frequent sampling is costly 
and time consuming. Hettler et al. 
(1997) examined the effect of reduced 
sampling frequency by subsampling 
from a data set of daily supply of larval 
Atlantic menhaden (Brevoortia tyran- 
nus ) and determined that the vari- 
ability of annual estimates of larval 
supply was “acceptable” [their term] 
on the basis of the standard error of 
estimates at sampling intervals of 7 
days or less. Intervals of 14 and 30 
days were deemed to be too long to be 
useful for estimation of annual larval 
abundance. Because their analysis 
was derived from a single year of 
data on daily larval supply, it did not 
account for interannual variations in 
the pattern of daily supply. In a long- 
term study of settlement of postlarvae 
(megalopae) (a proxy for larval supply) 
of blue crab ( Callinectes sapidus), we 
observed a difference of 2 orders of 
magnitude in larval supply among 
years (Ogburn et al., 2012). This vari- 
ability was due to changes in 1) the 
magnitude of daily larval supply, 2) 
the length of the larval settlement 
season, and 3) the number of pulses 
in settlement that occurred each year. 
Such interannual variability has the 
potential to strongly affect estimates 
of larval supply derived at a sampling 
interval of 7 days. The purpose of this 
study was to evaluate the effect of 
sampling at intervals of 1, 2, 3, 4, 5, 
and 7 days on the variability of esti- 
mates of annual larval supply derived 
from an 11-year record of blue crab 
megalopal settlement. 
Adult blue crabs live in estuaries of 
the western Atlantic from Nova Sco- 
tia to Argentina. Mature females mi- 
grate to coastal areas where they re- 
lease several clutches of larvae (Carr 
et al., 2005, Dickenson et al., 2006, 
Darnell et al., 2009). Planktonic lar- 
vae develop in 30-50 days in surface 
waters of the continental shelf (Cost- 
low and Bookhout, 1959) and return 
to estuarine nursery habitats as 
megalopae (Orth and van Montfrans, 
1987). The settlement of megalopae in 
estuaries is higher during nighttime 
flood tides than during daytime flood 
tides or during ebb tides (DeVries et 
al., 1994), but the annual pattern of 
larval settlement depends on the in- 
teraction between atmospheric forcing 
mechanisms and nighttime flood tides 
(Little and Epifanio, 1991; Ogburn et 
al., 2009). 
Interannual variability in blue crab 
larval supply may be an important 
source of variability in annual fish- 
ery landings (Forward et al., 2004; 
