Carson et al.: Population structure, long-term connectivity, and effective size of Lutjanus analis 
425 
Croix is of possible concern for several reasons. First, 
the average long-term N e estimate of 341 is below the 
upper bound of the “50/500” rule (Rieman and Allen- 
dorf, 2001), where an effective size of 500 or greater is 
needed to maintain the equilibrium between the loss 
of adaptive genetic variance from genetic drift and its 
replacement by mutation (Franklin, 1980; Schultz and 
Lynch, 1997). A potential consequence of sustained 
low effective size over time would be loss of adap- 
tive genetic variance and reduced capacity to respond 
to perturbation (including exploitation). Second, this 
sample of mutton snapper came from a known spawn- 
ing aggregation site that currently is under a joint 
territorial and federal closure during the spawning 
season (http://fw.dpnr.gov.vi/fish/Docs/Fisheries%20 
Master%20Plan/Sections/Appendix3.pdf, accessed July 
2011). The estimate of effective size certainly indicates 
that the closure is appropriate and timely. Finally, St. 
Croix is near the northeastern edge of the Lesser An- 
tilles and the spawning aggregation site is located on 
the leeward side of the island. Surface currents in the 
area are almost all to the west (Roberts, 1997) and in- 
clude the Anegada Passage, a fairly wide channel that 
connects the Atlantic Ocean with the Caribbean Sea 
and runs westward between St. Thomas and St. Croix 
(Johns et al., 2002). Both the prevailing currents and 
the observation (Swearer et al., 1999) that leeward- 
island sites are prone to larval retention and less af- 
fected by larval immigration than windward locations 
would indicate that immigration into the spawning 
aggregation from locations outside of St. Croix could 
be limited. Limited immigration into St. Croix waters 
from outside potentially could impede recovery if the 
spawning aggregation becomes depleted. These infer- 
ences also are consistent with the findings of Wares 
and Pringle (2008) who found that N e may be reduced 
in populations where there is unidirectional transport 
of individuals away from natal grounds. 
The differences in long-term N e among the samples 
of mutton snapper further indicate that at least his- 
torically there may have been distinct demographic 
stocks within the region. In addition to the low esti- 
mate of N e for the sample from St. Croix, the estimate 
for the sample from St. Thomas (N e = 922) was nearly 
three times as large as the estimate for St. Croix, 
yet the distance between the two localities (60 km) 
is substantially less than the larval-dispersal ranges 
of Roberts (1999) and the ecologically relevant larval 
dispersal distances of Cowen et al. (2000, 2006). Sail- 
lant and Gold (2006) in their study of red snapper 
( Lutjanus carnpechanus ) in the Gulf of Mexico defined 
demographic stocks as geographic samples that dif- 
fered in dynamics that potentially affected N e and 
the number of individuals that produce surviving off- 
spring. In their study, estimates of N e were negatively 
correlated with several critical fishery parameters, 
including size at age, maximum size, proportion of 
smaller and younger fish, and size and age of females 
at sexual maturity, reported by Fischer et al. (2004) 
and Woods et al. (2003), respectively. Similar age and 
growth and reproductive studies on mutton snapper 
in the U.S. Caribbean are clearly warranted. 
At present, mutton snapper in the U.S. Caribbean 
(Puerto Rico, St. Thomas/St. John, and St. Croix) are 
managed as a single management unit, although island- 
specific management is under consideration. Based on 
data on prevailing surface currents, low probability 
of larval input, and restricted movements of adults, 
the life-history subgroup of a recent stock assessment 
(SEDAR, 2007) indicated a two-stock hypothesis, with 
one stock on the Puerto Rican platform (Puerto Rico 
and St. Thomas/St. John) and a second stock around 
St. Croix. The estimates of long-term N e are consistent 
with the hypothesis that mutton snapper off St. Croix 
may represent a different demographic stock. In addi- 
tion, the estimate of long-term N e for mutton snapper of 
the west coast of Puerto Rico ( N e -646 ) is less than the 
estimate for the east coast of Puerto Rico (. N e -828 ) and 
nearly 1.5-fold less than the estimate for St. Thomas. 
This could indicate that there are different demographic 
stocks of mutton snapper on the Puerto Rican platform. 
Further study of mutton snapper off the west coast of 
Puerto Rico is likely justified because our sample local- 
ity is near a known spawning aggregation (Esteves, 
2005). Finally, stock structure of mutton snapper in 
the U.S. Caribbean may follow metapopulation models 
suggested by Kritzer and Sale (2002), Hellberg et al. 
(2002), and Gstergaard et al. (2003) where 1) subpopu- 
lations (stocks) may be asynchronous demographically 
but display homogeneity at selectively neutral (genetic) 
markers, and 2) subpopulations may be independent in 
terms of recruitment events and yet show no genetic 
differences because of sporadic gene flow. 
Conclusions 
Results of our study indicate that mutton snapper across 
the Caribbean Sea to the Florida Keys may be subdi- 
vided into a number of demographic stocks that differ 
in aspects that impact effective size and hence local 
sustainability. These differences could easily be both 
genetic and environmental, and in the future it will be 
of interest to apply new genomic tools (Allendorf et al., 
2010) that allow identification of specific genomic regions 
responding to local adaptation. A second implication of 
our results is that neither larval drift nor inter-regional 
adult movement may be sufficient over time to offset 
these differences. Critical genetic variability in mutton 
snapper at the intraregional level, consequently, is likely 
maintained by aggregate spawning and random mating 
of local populations. It is perhaps ironic that the life- 
history characteristic (aggregate spawning) that makes 
mutton snapper especially vulnerable to overexploitation 
also could be a critical asset in maintaining local genetic 
diversity. This characteristic elevates the importance of 
securing the vitality of spawning aggregations in spe- 
cies such as mutton snapper. Protective measures for 
spawning aggregations, including seasonal closures and 
appropriately placed marine protected areas (MPAs), 
