Marancik et al.: Spatial and temporal distribution of grouper larvae in the Gulf of Mexico and Straits of Florida 
17 
the Spring MS-AL-nFL subregion model and, along 
with year and water depth, explained over 80% of the 
deviance. These two models describe the importance of 
shelf-edge habitat. Low occurrence of grouper larvae in 
SEAMAP collections made it difficult to analyze fine 
spatial (within subregion) or temporal (within year) 
scale interactions with environment. Targeted sampling 
within subregions would be needed to better describe 
the relationship between the physical environment and 
larval occurrence. 
Change in occurrence over time 
The data presented here represent the best existing data 
set for examining long-term trends of larval grouper 
abundance in the southeast United States. We attempted 
to control for inconsistencies in sampling, but the results 
from this study cannot be fully separated from sampling 
bias, consistently low catches, nonspecies-level identifica- 
tions, and missed peak spawning season for many com- 
mercially relevant species. However, our results provide 
evidence of a shift in grouper species composition toward 
fall-spawning populations over the SEAMAP time frame 
(Figs. 5-7). Spring-spawned larvae dominated collec- 
tions before 1995, but in the more recent decade, fall- 
spawned larvae have come to dominate or have gone 
from nonexistent to present in larval collections. The 
relative increase in occurrence of fall-spawned larvae 
was best illustrated by the rapid rise in the number of 
larval graysby collected. In addition, a clear increase 
in the collection of fall-spawned members of a morpho- 
logically indistinguishable group of species (with small 
spinelets and standard tail pigment), including several 
commercially important species (namely, small H. fla- 
volimbatus [<4.5 mm BL], E. itajara, H. niveatus, and 
possibly preflexion Mycteroperca spp. lacking pigment 
at the cleithral symphysis), was also observed (Fig. 7). 
A shift in larval occurrence could result from a shift 
in abundance at the adult population level (e.g., changes 
in population size or spawning stock biomass), changes 
in the survivability of larvae (e.g., changes to mater- 
nal condition, fecundity, food availability, environmen- 
tal regime, etc.), or a change in distribution. There is 
some evidence of changes in adult grouper population 
dynamics. For example, graysby larvae were one of the 
most abundant grouper species in our Straits of Florida 
collections and have become common in SEAMAP col- 
lections since 1995. Similarly, the occurrence of adult 
graysby increased off the coast of North Carolina be- 
tween 1975 and 1992 (Parker and Dixon, 1998). Similar 
data on the abundance of adult graysby from the Flor- 
ida Shelf is limited, but adult graysby are one of only 
three species of grouper that were not being overfished 
in the Florida Keys before 1996 (Ault et ah, 1998) and 
were a dominant species on Florida Keys reefs in the 
early 1990s (Sluka et al., 1998). In addition, a decline in 
the abundance of larger grouper since the early 1990s 
(Bohnsack et ah, 1994) could result in an increase in 
the abundance of smaller grouper species, like graysby 
(Sluka et al., 1998; C-hiappone et ah, 2000). However, 
increases in larval occurrence could also be the result 
of a shift in adult habitat use without an increase in 
population size. In the southern Caribbean, a signifi- 
cant shift in graysby distribution to deeper habitat 
coinciding with a reduction in coral cover has been 
observed (Nagelkerken et ah, 2005). A similar shift 
in adult distribution on the west Florida shelf could 
explain the increase in larval occurrence observed in 
our study. Further examination of the potential causes 
of a shift in species dominance at the adult level and 
additional targeted investigations into larval surviv- 
ability are needed to corroborate our findings of a shift 
in dominance in the northern Gulf of Mexico. However, 
the larval data here indicate that shifts in grouper 
abundance and species composition occurred over the 
last three decades. 
Conclusions 
Analysis of larval grouper distribution patterns provided 
a means of independently corroborating location and 
seasonality of spawning, but also allowed us to identify 
new patterns in grouper distribution and species com- 
position in the Straits of Florida and northern Gulf of 
Mexico. The timing of larval occurrence, and thus the 
timing of spawning, for most species fell into one of two 
seasons, confirming what was already documented on 
spawning season for many species. However, two spe- 
cies, Cephalopholis cruentatus (graysby) and Epinephelus 
morio (red grouper), were collected during longer seasons 
than previously reported. Grouper larvae were collected 
in three distinct subregions of the Gulf of Mexico and 
along the shelf edge in both the gulf and Straits of 
Florida. Analysis of larval occurrence by subregional 
mean water depth, surface temperature, and surface 
salinity further corroborated the importance of shelf- 
edge habitat, particularly on the west Florida shelf in 
fall and the Mississippi-Alabama-north Florida shelf 
in spring. The species composition of grouper larvae in 
the Gulf of Mexico may have changed over the course 
of SEAMAP sampling. The frequency of occurrence of 
fall-spawned species has increased in relation to spring- 
spawned species since 1995. In the Straits of Florida, 
preflexion graysby were collected along the shelf edge, 
but flexion and postflexion larvae of the species were 
collected farther offshore. The distribution of later-stage 
graysby larvae could be evidence of processes directing 
self-recruitment or loss to the population. These data 
provided a first-time look at larval grouper distribution 
patterns over a large spatial and time scale and provided 
evidence of several topics needing further research. 
Acknowledgments 
We would like to thank the following people for their 
significant contributions to this research: L. Bulloc, J. 
Llopiz, C. Guigand, A. Exum, J. VanWye, K. Shulzitski, 
K. Williams, The SEAMAP Plankton Team, and the 
