Kupchik and Shaw: Effects of recruitment through a coastal boundary layer on growth of larval Brevoortia patronus 
207 
on the basis of a decrease in population fecundity over 
the last decade (Vaughan et ah, 2007). 
Gulf menhaden are estuarine dependent and report¬ 
edly spawn from October through February (White- 
head, 1985; Nelson and Ahrenholz, 1986; Vaughan et 
ah, 2000) and the peak estuarine recruitment occurs in 
late January and early February (Lewis and Roithmayr, 
1981; Shaw et ah, 1988). Spawning depth for Gulf men¬ 
haden is usually 90 m and shallower (Whitehead, 1985; 
Powell, 1994), and spawning locations occur farther 
offshore as the season progresses, suggesting shorter 
“recruitment corridors” (Cushing, 1975) during fall re¬ 
cruitment (Vaughan et ah, 2007). Mean egg diameter 
has been reported to be 1.61 mm, and length at hatch¬ 
ing to be approximately 3 mm total length (Dahlberg, 
1970; Lewis and Roithmayr, 1981; Shaw et al., 1985). 
The pelagic eggs take 2-3 days to hatch and another 
2-3 days until yolk absorption is complete, with the 
result that first feeding and first otolith increment 
formation occur approximately 5 days after spawning 
(Warlen, 1988). Offshore larval drift and cross shelf 
transport have been reported to take between 4 and 10 
weeks (Shaw et al., 1988). The variability in transport 
times is tied to the limited swimming capacity of larval 
fish (Shanks and Eckert, 2005); successful estuarine 
recruitment is therefore driven more by oceanograph¬ 
ic fiows (Guillory et al., 1983; Epifanio and Garvine, 
2001; Gillanders et al., 2003). Recruitment from more 
oligotrophic inner continental shelf spawning grounds 
through the hydrodynamically variable oceanographic 
coastal boundary layer, which is produced by atmo¬ 
spheric effects, into tidal passes, and ultimately more 
productive estuarine waters (Raynie and Shaw, 1994) 
corresponds with the time period when Atlantic men¬ 
haden {Brevoortia tyrannus) and Gulf menhaden lar¬ 
vae transform from selective particulate feeding to om¬ 
nivorous filter-feeding juveniles (Stoecker and Govoni, 
1984; Deegan, 1990; Chen et al., 1992; Lozano et al., 
2012). This transformation begins at approximately 20 
mm standard length (SL) and is completed by approxi¬ 
mately 30 mm SL (Hettler, 1981; Warlen, 1988), with 
a corresponding increase in gill raker counts. For ex¬ 
ample, gill raker counts were recorded to increase from 
5 +14 to 13 +25 for larvae between 19 and 22 mm SL, 
as they were beginning to undergo metamorphosis in 
Lake Pontchartrain, Louisiana (Suttkus, 1956). 
Studies of larval Gulf menhaden age and growth 
in Louisiana have focused on both the offshore (Shaw 
et al., 1985, 1988; Warlen, 1988; Raynie and Shaw, 
1994) and inshore components of the recruitment cor¬ 
ridor (Deegan and Thompson, 1987; Marotz et al. 1990; 
Raynie and Shaw, 1994). These studies have reported 
growth rates between 0.28 and 0.42 mm/day for the 
smaller larvae typically encountered on the continental 
shelf (Deegan and Thompson, 1987; Raynie and Shaw, 
1994) and between 0.11 and 0.12 mm/day for larvae 
collected within Sabine Pass and Fourleague Bay, Loui¬ 
siana (Warlen, 1988; Raynie, 1991). 
Daily growth increments in otoliths have been con¬ 
firmed in larval Gulf menhaden in laboratory studies 
(Warlen, 1988). The daily rings in otoliths of larval 
fish can provide growth rates and can act as a proxy 
for identification of changes in developmental stages 
and for environmental stress reflected in the variabil¬ 
ity in otolith ring width (Maillet and Checkley, 1990, 
1991; Chambers and Miller, 1995). Analysis of larval 
otolith structure was initially done by visual inspec¬ 
tion; however, video and digital methods have become 
prevalent with an increase in computing resolution and 
digital imaging (Ralston and Williams, 1989; Campa- 
na, 1992; Morales-Nin et ah, 1998). Regardless of what 
ring counting method is being used, the ring structure 
must be verified because the shape and relative size of 
otoliths are species specific and genetically controlled 
(Schmidt, 1969; Gaemers, 1976; Nolf, 1985; Lombarte 
and Morales-Nin, 1995; Morales-Nin et ah, 1998). 
The objectives of our study were as follows. First, 
to determine the length at age of Gulf menhaden for 
the sampling period. Second, to determine at what age 
there is a shift in growth rate consistent with the ex¬ 
pected shift in feeding strategy from a selective par¬ 
ticulate feeder to an omnivorous filter feeder. Third, to 
compare otolith microstructure with length at age mod¬ 
els for confirmation of growth rate and shift in feeding 
strategy upon entering the coastal boundary layer and 
the transition from oceanic to estuarine waters. Fourth, 
to determine the distribution of the spawning period 
by using back calculation of spawning dates from age 
frequency keys. Fifth, to determine the duration of the 
recruitment corridor from offshore spawning grounds 
across the coastal boundary layer, and into the estua¬ 
rine nursery grounds. 
Materials and methods 
Sampling location 
Samples of ichthyoplankton were collected near the 
Port of Fourchon, in Bayou Tartellan, Louisiana 
(Fig. 1). This sampling location is connected to the 
GOM at Belle Pass (29°5'53.9"N, 90°13'17.8"W) and is 
one of the first major inland bifurcations of the tidal 
pass. The tidal pass and Bayou Tartellan are season¬ 
ally well mixed and have limited temperature, salinity, 
or dissolved oxygen stratification. Bayou Tartellan is 
also characterized by high turbidity, and a low volume 
of freshwater input owing to a limited drainage basin. 
The sampling location (29°6'49"N, 90°11'4''W) was de¬ 
termined to maximize flow rates for passive sampling 
of the tidal pass. The passive sampling was conducted 
from the end of a 3.7-m-long dock on the northern bank 
of the tidal pass, which had a sampling depth of 10 m 
and an overall tidal pass width of approximately 73 m. 
Field methods 
Individual samples of ichthyoplankton were collected 
passively with a 60-cm ring net (with 333-pm mesh, 
and 2 m in length) that was dyed dark green to mini- 
