Kupchik and Shaw: Age, growth, and recruitment of larval and early juvenile Micropogonias undulatus 
19 
greater than 30 m, the Atlantic croaker was either the 
second or third bycatch taxon by weight, depending on 
sampling period (Helies and Jamison 1 ). This high level 
of bycatch and directed fishing-induced mortality may 
increase the importance of minor variations in mortal- 
ity rates of the early life history stages to overall stock 
success, variations that greatly affect recruitment rates 
(Norcross, 1983; Diamond et al., 2000). 
A peak in spawning occurs from July through De- 
cember and in larval estuarine recruitment during 
October-November (Cowan, 1988; Ditty et ah, 1988; 
Warlen and Burke, 1990; Barbieri et ah, 1994a). Atlan- 
tic croaker spawn over a wide range of inner continen- 
tal shelf depths (i.e., 54 m or shallower), and a portion 
of the population moves inshore toward estuaries to 
complete spawning during the winter and early spring 
months (Barbieri et al., 1994a, 1994b). Hydrologic vari- 
ability at large and small spatial and temporal scales 
can greatly affect the numbers of Atlantic croaker lar- 
vae able to successfully recruit to estuarine nursery 
grounds (Norcross, 1983; Shaw et al., 1988; Raynie, 
1991; Raynie and Shaw, 1994). Once larvae are in the 
estuary, lower water temperatures in the first winter 
increase their mortality rate as verified in both the 
field (Norcross and Austin, 1988; Hare and Able, 2007) 
and laboratory (Lankford and Targett, 2001a, 2001b). 
Previous studies on age and growth of Atlantic 
croaker generally have focused on the Mid-Atlantic 
Bight (MAB) and South Atlantic Bight (SAB), however, 
limited studies have occurred in the GOM. For these 
studies, linear growth rate models have been created 
from counts of daily rings within otoliths and paired 
readers have found growth rates between 0.16 and 
0.27 mm/d (Warlen, 1982; Thorrold et al., 1997); in the 
GOM, the growth rate for this species has been deter- 
mined to be 0.19 mm/d (Cowan, 1988). The occurrenc- 
es of higher growth rates for larval Atlantic croaker 
during the spawning and estuarine recruitment peak 
in the late summer and early fall and of lower larval 
growth rates during the overwinter period and into the 
spring have led to the notion that different spawning 
subgroups may exist (Warlen, 1982). This hypothesis is 
further supported by differences in growth rates and 
recruitment dynamics that are based on latitude along 
the estuaries of the MAB and SAB (Barbieri et al., 
1994b; Thorrold et al., 1997). 
Daily formation of otolith increments have been con- 
firmed in Micropogonias (Campana, 1984; Cowan, 1988; 
Albuquerque et al., 2009), making such increments a re- 
liable proxy for age. Otolith rings for larval fish, formed 
daily, not only can provide information on growth rates 
but can also be used to estimate approximate times of 
larval estuarine ingress (i.e., transport time from off- 
shore spawning grounds to estuarine nurseries) (Hoover 
et al., 2012). The approximate timing of larval and early 
juvenile ingress of Atlantic croaker to estuarine nurs- 
eries from offshore spawning grounds in the MAB and 
SAB has been estimated to vary between 30 and 60 d 
after hatching. (Warlen, 1980; Warlen and Burke, 1990; 
Hettler and Hare, 1998; Hoskin, 2002; Hoover et al., 
2012). Daily otolith rings have been also used to exam- 
ine environmental parameters that affect growth and 
survivorship (Campana, 1999; Campana and Thorrold, 
2001) and to determine within-season cohorts for At- 
lantic herring ( Clupea harengus), on the basis of vari- 
able growth between seasons in the same year (Brophy 
and Danilowicz, 2002). Larval otolith growth rings and 
microstructure were initially examined through direct 
observation by paired readers using light microscopy; 
however, video and digital methods have become preva- 
lent with the increase in image resolution from high- 
mexapixel digital imaging sensors (Ralston and Wil- 
liams, 1989; Campana, 1992; Morales-Nin et al., 1998). 
For this study, we had 5 primary objectives. The 
first objective was to define and determine an itera- 
tive digital filtering mechanism that can provide a 
more accurate and automated determination of daily 
increments in otoliths of larval Atlantic croaker. The 
second objective was to determine the length at age 
of Atlantic croaker larvae collected in a tidal pass in 
the northern GOM from fall through spring over a 
2-year recruitment period. The third objective was to 
compare observed larval growth rates determined from 
linear and nonlinear growth models with those deter- 
mined from previous studies. The fourth objective was 
to estimate, on the basis of ring counts corrected for 
hatching date and time at first ring formation, times 
for estuarine ingress through the tidal pass from the 
coastal boundary zone. Finally, the fifth objective was 
to determine the effect of hydrodynamic patterns asso- 
ciated with differences between continental shelf and 
estuarine waters on growth of larval and early juvenile 
Atlantic croaker. 
Materials and methods 
Sampling location 
Ichthyoplankton sampling was conducted in Bayou 
Tartellan, near Port Fourchon, Belle Pass, Louisiana 
(Fig. 1A). Bayou Tartellan and Bayou LaFourche are 
the first major inland channel bifurcations from the 
connection with the GOM at Belle Pass (29°5' 53.9"N, 
90°13'17.8"W). The location of sampling was within a 
seasonally well-mixed tidal pass (i.e., tidal pass with 
little stratification in temperature, salinity or dis- 
solved oxygen) that has high turbidity and a relatively 
small drainage basin that contributes a very low vol- 
ume of freshwater input. The site (Fig. IB; 29°6.82'N, 
90°11.07'W) where passive sampling was conducted 
with a plankton net was located at the end of a dock 
that extended 3.7 m into the tidal pass from the north- 
ern bank of Bayou Tartellan, and the site had a water 
depth of approximately 10 m. 
Field sampling method 
Ichthyoplankton sampling was conducted with a fixed 
davit placed at the end of the dock and from which 
