Hernandez et al.: Variability in ichthyoplankton abundance and composition in the northern Gulf of Mexico 
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Month (2004-2006) 
Figure 3 
Mean temperature and salinity, fish egg and larval fish concentrations, 
and diversity indices for larval fish assemblages for October 2004-October 
2006. Temperature and salinity are depth-integrated mean values for each 
month. Egg and larval fish concentrations are standardized by volume of 
water filtered (error bars denote ±1 standard error). Calculation of diversity 
follows Jost (2006) and depicts the exponential function of Shannon entropy, 
H (error bars denote ±1 standard error). 
pumilio, L. campechanus, Gerreidae, 
S. ocellatus, Labridae, Auxis spp., E. 
alletteratus, P. burti, C. spilopter- 
us, Paralichthys spp., and Syacium 
spp. did not exceed 10 larvae/100 
m 3 during any sampling event. 
Other taxa were characterized by 
relatively high concentrations, either 
during a single sampling event (e.g., 
E. teres, C. chrysurus, C. arenarius, 
L. xanthurus, Microdesmus spp., S. 
maculatus, P. alepidotus ) or dur- 
ing a single year (e.g., H. jaguana). 
The remaining taxa ( M . punctatus, 
B. patronus, Centropristis spp., D. 
punctatus, C. nothus, L. fasciatus, 
M. undulatus, E. crossotus) were 
present during multiple years in 
relatively similar concentrations. 
Results from the cluster analysis 
were largely in agreement with the 
observed seasonal patterns previ- 
ously defined by water temperature 
(Fig. 5). Taxonomic assemblages 
from fall and winter periods were 
clustered separately from spring 
and summer periods. All summer 
months ( June-October) were clus- 
tered together with the exception 
of August 2005 and October 2004. 
Larval collections in August 2005 
were characterized by atypically 
high concentrations of a few spe- 
cies, most notably C. chrysurus and 
C. arenarius, which were present 
in concentrations exceeding >500 
larvae/100 m 3 (Fig. 4), resulting 
in relatively low species diversity 
(Fig. 2) for the summer period. The 
October 2004 sampling event was 
included in the summer period, al- 
though the mean temperature was 
marginally below the 26°C criterion 
for the summer period (Fig. 3) and 
indicative of a seasonal transitional 
period. Similarly, the assemblages 
from the May sampling events were 
relatively distinct from the ear- 
lier spring period sampling events 
(March and April). 
Discussion 
Although numerous ichthyoplankton surveys have 
been conducted in the northern Gulf of Mexico, most 
have been conducted off the coasts of Texas, Louisi- 
ana, and Florida (Ditty et al., 1988), and few have been 
conducted with a high level of temporal resolution and 
sample replication. The Alabama shelf region, although 
relatively small, is unique in that it is bounded by two 
major topographic features (Mississippi River Delta to 
the west and DeSoto Canyon to the east) that poten- 
tially inhibit alongshore transport of larvae (Johnson 
et al., 2009). In addition, the Alabama continental shelf 
receives freshwater outflow from the Mobile River system, 
which drains the fourth largest watershed in the United 
States and has the sixth largest freshwater discharge 
on the North American continent (Park et al., 2007). 
As a result, the inner shelf environment off Alabama 
is a highly productive region that supports valuable 
