196 
Fishery Bulletin 108(2) 
average for water temperature was calculated for each 
month with data from a coastal observing buoy (NOAA 
National Data Buoy Center Station 42007) located 
approximately 54 km west of our sampling station at a 
water depth of approximately 15 m (Fig. 1). Although the 
temperature values from the buoy were measured near 
the surface (0.6-m depth), these observations serve as 
good indicators of seasonal shifts in water-column ther- 
mal structure, as indicated by our own CTD comparisons 
of sea surface temperature and depth-integrated tem- 
perature (correlation coefficient, r 2 =0.98; slope, m=0.90; 
P<0.0001). Together, these data were used to define 
ecologically relevant “seasons” (rather than calendar 
date) for multivariate analyses. 
Preparation of ichthyoplankton data 
Ichthyoplankton samples were sorted and larval fish 
were identified to the lowest possible taxonomic level 
at the Plankton Sorting and Identification Center (Szc- 
zecin, Poland) and at the Dauphin Island Sea Laboratory 
(Dauphin Island, Alabama). Many larval fishes were not 
identified to the species level, owing to the relatively 
small sizes of larvae collected in the 202-pm mesh nets 
and the overall diversity of larval forms present in the 
western central Atlantic region, which includes the 
Gulf of Mexico (Marancik et ah, 2005). Most identifica- 
tions were at the family level (52%), followed by species 
(22%), order (14%), and genus (7%) level identifications. 
Five percent of the larvae collected were damaged or 
unidentified. 
Unidentified clupeiforms (engraulids and clupeids) 
were excluded from further analyses because their ex- 
treme concentrations and taxonomic ambiguity can 
often mask abundance and assemblage trends (Tolan et 
al., 1997; Hernandez et al., 2003). Order-level taxa and 
unidentified larvae were removed from consideration for 
similar reasons. Further taxonomic analyses, therefore, 
were limited to taxa that represented at least 1% of the 
total catch during any individual sampling event, where 
the proportion of the total catch for each taxonomic 
group was determined after removing unidentified lar- 
vae, order-level larvae, and all unidentified clupeiforms. 
Following Marancik et al. (2005), we further modified 
the data sets to exclude genus-level groupings in in- 
stances where many congeners could potentially mask 
any seasonal trends. The following genus-level group- 
ings were retained because each represented relatively 
few congeners with likely similar early life histories in 
the northern Gulf of Mexico: Auxis spp. (A. rochei and 
A. thazard), Centropristis spp. (C. philadelphica, C. 
ocyurus, and C. striata), Diplectrum spp. ( D . bivattatum 
and D. formosum), Microdesmus spp. (M. lanceolatus 
and M. longipinnis), and Paralichthys spp. (P. albigutta, 
P. lethostigma, and P. squamilentus) . Similarly, all fam- 
ily-level groups were removed except Gerreidae (most 
likely Eucinostomus gula or E. argentus) and Labridae 
(most likely Xyrichtys novacula ). In all, 30 taxa were 
considered for analyses (Table 2). Because the objective 
of this study was to examine the seasonal variability of 
larval fish occurrence and relative larval fish concentra- 
tions and not size-selectivity or vertical distribution, 
our analyses included ichthyoplankton data collected 
from all surveys (monthly and quarterly diel), mesh 
sizes (202 pm and 333 pm), and depth bins. Depth 
stratification and gear selectivity will be addressed in 
separate analyses in forthcoming publications. 
Analyses 
All fish egg and larval fish abundances were standard- 
ized by the volume filtered to determine concentration 
estimates (no./m 3 ). Taxonomic diversity was calculated 
for each sample by taking the exponential of Shannon 
entropy, exp (H), following the method of Jost (2006). 
Monthly mean observations of total fish eggs, total fish 
larvae, and taxonomic diversity were compared to mean 
temperature and salinity data by using least squares 
regressions. Two approaches were used to examine 
larval fish seasonality. First, monthly mean concentra- 
tions (no./lOO m 3 ) were calculated for the dominant 
taxa to examine monthly trends in abundance. Second, 
observed and historic water temperature observations 
were used to define distinct seasons for the sampling 
region. Seasonality in fish egg concentrations, total 
larval fish concentrations, and taxonomic diversity was 
examined (after log+1 transformation) by using one-way 
ANOVAs with season as a factor and Tukey’s honesty 
significant difference (HSD) tests. Lastly, larval con- 
centrations for dominant taxa were square-root trans- 
formed and analyzed by using Bray Curtis similarity 
and cluster analysis with the PRIMER statistical pack- 
age (PRIMER, vers. 6, Plymouth Marine Laboratory, 
Plymouth, U.K.). 
Results 
Mean monthly water temperature varied seasonally 
over the two year period, with a low of 16.5°C (January 
2005) and a high of 30.2°C (August 2006) (Fig. 2). The 
general pattern of our monthly temperature observations 
was similar (±2°C) to that of recent historical values 
(Fig. 3). Notable deviations were relatively cooler tem- 
perature observations in May during our study (mean 
differences of 3.2°C and 2.4°C during 2005 and 2006, 
respectively) and warmer temperatures in October (mean 
differences of 2.6°C and 3.0°C during 2005 and 2006, 
respectively) and December (mean difference of 3.0°C 
in 2004). Even with these disparities, both data sets 
were in agreement to define seasonal breaks in water 
temperature. (Fig. 3). Sampling periods with mean water 
temperature values <18°C were classified as winter, and 
those with mean water temperatures above 26°C were 
classified as summer. The transitional periods of spring 
and fall had mean water temperatures between 18°C 
and 26°C. In general, the observed seasonal pattern 
comprised three-month winter (December-February) 
and spring (March-May) seasons, a relatively long five- 
month summer period ( July-October), and a relatively 
