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Fishery Bulletin 96(4), 1998 
There is at least suggestive evidence for each of 
the above three possibilities in greater amberjack. 
First, measurements of offshore current velocities are 
consistent with increased passive movement of indi- 
viduals from the Florida Keys into the Atlantic rather 
than northward along the west Florida coast. Mean 
current velocities ( 145 m below the surface) eastward 
from the Keys through the Florida Straits are ap- 
proximately 75 cm/sec, and along the southeast 
Florida coast, these currents can be as fast as 95 cm/ 
sec (SAIC ' ). Because current velocities are expected 
to be greater nearer the surface, and because greater 
amberjack have been observed spawning in the 
Florida Keys (Cummings and McClellan 3 ), buoyant 
eggs and larvae would likely be impacted and have 
little opportunity to move northward along the west 
coast of Florida. Second, in terms of habitat, the 
southern half of the Florida peninsula represents a 
transition zone between temperate and tropical 
forms, where the southern ranges of many temper- 
ate species terminate in tropical southern Florida 
(Briggs, 1974). There also is relatively little reef habi- 
tat or sharp topography between the Florida Middle 
Ground and the Keys; most of the area has a rela- 
tively smooth bottom comprising shell, sand, and 
quartz (Rezak et al., 1985; Rezak and Bright 7 8 ). Be- 
cause greater amberjack exhibit a marked preference 
for reefs, rock outcrops, and wrecks (Shipp, 1986; 
Manooch, 1988), it is possible that the paucity of sig- 
nificant reef or other major structure on the Outer 
Continental Shelf off southwestern Florida may in- 
hibit movement of greater amberjack between the 
Florida Middle Ground and the Keys. Finally, al- 
though relatively little is known about the early life 
history of greater amberjack, spawning is thought 
to occur from mid-spring to early summer both along 
the southeastern coast of Florida (including the 
Florida Keys) and in the northern Gulf off Louisiana 
(Cummings and McClellan 3 ; Thompson 9 ). This 
spawning time suggests that warmer water tempera- 
tures may trigger onset of reproductive activity. One 
might then hypothesize that during the late Pleis- 
tocene Epoch, when waters of the northern Gulf were 
much cooler (Rezak et al., 1985), subpopulations of 
7 SAIC (Science Applications International Corporation). 1992. 
Straits of Florida physical oceanographic field study, final in- 
terpretative report, volume II: technical report. OCS Report/ 
MMS 92-0024. U. S. Dep. Interior, Minerals Mgmt. Serv., Gulf 
of Mexico OCS Regional Office, New Orleans, LA, 179 p. 
8 Rezak, R., and. T. J. Bright (eds.). 1981. Final report: north- 
ern Gulf of Mexico topographic features study. Tech. Rep. No. 
81-2-T, Dept. Oceanography, Texas A&M University, College Sta- 
tion, TX, 150 p. 
9 Thompson, B. A. 1997. Coastal Fisheries Institute, Louisi- 
ana State University, Baton Rouge, LA 70803. Personal 
commun. 
greater amberjack were isolated in warm-water refu- 
gia, perhaps off the Florida Keys (or in the Carib- 
bean) and off the Yucatan Peninsula (Campeche Banks) 
where considerable reef habitat exists (Rezak et al., 
1985). Following glacial retreat, the (putatively) iso- 
lated subpopulations could then have returned to the 
northern Gulf. Because the rate of approach to genetic 
homogeneity would be partly a function of time and 
gene flow, genetic differences between present-day sub- 
populations of greater amberjack could simply be his- 
torical artifacts, reflecting insufficient time (or re- 
stricted gene flow) relative to genetic homogenization. 
Our finding that the sample of greater amberjack 
from the Florida Keys was included in a grouping 
(subpopulation) with samples from the Atlantic dif- 
fers from a recent study in king mackerel (Gold et 
al., 1997), where a sample from the Florida Keys was 
placed in a grouping with samples from the Gulf. 
King mackerel are highly migratory, and the sample 
of king mackerel from the Florida Keys was taken 
during late winter when the majority of king mack- 
erel in the Keys are thought to be from the Gulf Mi- 
gratory Unit or stock (Williams and Godcharles 10 * * ). 
The sample of greater amberjack from the Keys, how- 
ever, was obtained during late March-early April, a 
time when the majority of king mackerel in the Keys 
are considered to be from the Atlantic Migratory Unit 
or stock (Williams and Godcharles 10 ). Although greater 
amberjack are not migratory in the same way as king 
mackerel, it would be of more than passing interest to 
examine winter samples of greater amberjack from the 
Florida Keys and ask whether the Florida Keys consti- 
tute a mixing zone in greater amberjack as in king 
mackerel. Along these lines, it also would be important 
to examine both summer and winter samples of greater 
amberjack in the area between the Florida Middle 
Ground and the Florida Keys. A better definition of the 
geographic limits of the two subpopulations is critical 
for assessment and allocation of the greater amberjack 
resources along the west coast of Florida. 
Avise et al. (1988) presented models that allow 
estimation of evolutionary (long-term) effective fe- 
male-population size (N^ g) values) based on mtDNA 
intrapopulational nucleotide sequence diversities. 
Assuming that the generation time in greater am- 
berjack is three years (Wilson 11 ), we estimated N^ e) 
for the two subpopulations (stocks) of greater am- 
berjack to be 90,000 (Gulf of Mexico) and 93,500 (U.S. 
southeastern Atlantic, including the Florida Keys). 
10 Williams, R. O., and M. F. Godcharles. 1984. Completion re- 
port, king mackerel tagging and stock assessment. Project 2- 
341-R. Florida Dep. Natural Resources., St. Petersburg, FL. 
11 Wilson, C. A. 1997. Coastal Fisheries Institute, Louisiana 
State University, Baton Rouge, LA 70803. Personal commun. 
