Roelke and Cifuentes: Use of isotopes to assess groups of Scomberomorus cavalla 
549 
the Gulf of Mexico (Fig. 5). Statistically, the Mexico 
and Florida regions are significantly different in car- 
bon isotopes; however, neither region differs signifi- 
cantly from the northwestern Gulf of Mexico. It is 
conceivable that a separate Mexico and Florida group 
of king mackerel exists. Possibly neither site dif- 
fered significantly from the northwestern Gulf of 
Mexico owing to individuals from both the Mexico 
and Florida regions being contained in the catch from 
the northwestern Gulf of Mexico. Recall, the north- 
western Gulf of Mexico individuals were collected in 
the summer when migrations to the northwestern 
Gulf of Mexico from Florida and Mexico have been 
documented (Trent et al. 1987; Sutter et al. 1991). 
However, the similarity in nitrogen isotopic compo- 
sition indicates that the Florida and Mexico regions 
are related. 
A year-round sustained population in the north- 
west Gulf of Mexico would contribute to their isoto- 
pically different nitrogen values compared with Mexi- 
can and Florida fish. Other studies have surmised 
that Louisiana may have a resident population 
(Fisher, 1980; Fable et al., 1987) along a broad area 
from the Mississippi delta westward to regions off 
Texas, which are adjacent to oil rigs (Trent et al., 
1983). These artificial structures may attract bait 
fish (Wickham et al., 1973). Northwestern Gulf of 
Mexico fish, being significantly 15 N-enriched, might 
be a nonmigrating or a separate group of king mack- 
erel that feed on an isotopically enriched food source 
compared with king mackerel from Mexico and 
Florida. Alternatively, it is conceivable that the indi- 
viduals are migratory and that the isotopic signal is 
due to assimilation of material from the northwest- 
ern Gulf of Mexico region although they are not per- 
manent inhabitants of the region. 
Physical dynamics within the Gulf of Mexico may 
influence mixing between sites and therefore the iso- 
topic values in mackerel found at different sites. The 
primary current in the Gulf of Mexico is the Loop 
Current, which enters the Gulf of Mexico through 
the Yucatan Channel and exits through the Florida 
Straits (Leipper, 1970; Cooper et al., 1990) (Fig. 1). 
This current is formed by waters from the western, 
north, and south Atlantic and the Mediterranean Sea 
that flow into the Caribbean Sea (Koch et al., 1991). 
It has a mean position of 88° and 89°W and 27°N 
(Auer, 1987). Although the Loop Current reaches into 
the northern Gulf of Mexico, its influence is to the 
east of the Mississippi Delta. Thus, Mexican and 
Florida fish could be linked by the Loop Current to 
the extent that they consume isotopically similar food 
sources. In contrast, fish in the northwestern Gulf of 
Mexico are most likely minimally affected by the Loop 
Current. 
Northwestern Gulf of Mexico fish may also be 
strongly influenced by runoff from the Mississippi 
River system (Dagg et al., 1991). The majority of this 
runoff (two thirds) is westward and contains high 
concentrations of dissolved nutrients in relation to 
the open Gulf of Mexico (Dagg et al., 1991). Dagg et 
al. (1991) also suggested that the Mississippi River 
system is the ultimate source of much of the biologi- 
cal productivity on the Louisiana and Texas shelf. 
The flow of the Mississippi River into the northwest- 
ern area influences the isotopic differences within 
these Gulf of Mexico sites ( Lopez- Veneroni 4 ). Al- 
though GulfPort, MS, is east of the Mississippi river, 
specimens collected from this area could conceivably 
be feeding in or near the Mississippi River Plume 
region. Discharge from the Mississippi River is trans- 
ported west along the shore (Dagg et al., 1991), and 
consumption of prey from this region would be 
heavily influenced by the Mississippi River leading 
to 15 N-enriched values found in this study. Further- 
more, Dagg et al. (1991) stated that king mackerel 
from the northern Gulf of Mexico generally consumed 
prey that were estuarine dependent and are, there- 
fore, most likely influenced by runoff. 
Conclusions 
Stable nitrogen isotope values of spines of king mack- 
erel varied geographically. The northwestern Gulf of 
Mexico ( + 13.1%e) was isotopically distinct from the 
Mexican and Florida ( + 10.8%c and +10.8 %c) regions. 
We interpret these results to mean that there are, at 
least, two distinct groups of king mackerel within 
the Gulf of Mexico. Our results contrast with certain 
previous stock-structure assessments that distin- 
guish only between Gulf of Mexico and Atlantic 
stocks. Stable carbon isotopes were able to distin- 
guish between Mexico and Florida regions, although, 
not the northwestern Gulf of Mexico region. Although 
carbon isotopes were expected to be less variable than 
nitrogen, owing to the enrichment from trophic level 
to trophic level, they were found to be more variable 
within individual spines. The variability and perplex- 
ing isotopic trends within individual spines create 
difficulties in drawing conclusions from the data for 
stable carbon isotopes. In addition, fewer significant 
differences were detected between sites for stable 
carbon isotopes than for nitrogen isotopes. Stable 
carbon isotopes may be more useful when the isoto- 
pic discrimination among food resources is greater, 
which may be found when individuals also feed in 
coastal habitats. King mackerel, being of great com- 
mercial and recreational value, need to be managed 
with a clearer understanding of the number of groups 
