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Fort Pierce-Palm 
Beach, FL 
Panama City, FL 
Gulf Port, MS 
Grand Isle, LA 
Galveston, TX 
Port Aransas, TX 
Veracruz Mexico 
Celestun Mexico 
Significantly different for nitrogen 
Significantly different for carbon 
Significantly different for both nitrogen and carbon 
Figure 4 
Results of MANCOVA, LSmeans, and pairwise comparison for all mack- 
erel sites. Fork length is a covariate. 
observed similar differences for mean 8 15 N 
values of king mackerel spines collected 
in these regions. The 8 15 N data of Macko 
et al. (1984), Fry (1983), and this study all 
showed an 15 N-enriched in the northwest- 
ern Gulf of Mexico relative to samples col- 
lected in Florida and Mexico. Enriched 
nitrogen values are often observed off the 
mouths of estuaries (e.g. Cifuentes et al., 
1989). This enrichment often reflects the 
assimilation of isotopically altered inor- 
ganic nitrogen from riverine sources by 
algae. The influence of the Mississippi 
River could account for the more positive 
5 15 N values (Lopez-Veneroni 4 ) detected in 
the northwestern Gulf of Mexico. 
In contrast to the 5 15 N data for the king 
mackerel, 8 13 C measurements were not as 
discriminating between sites (Fig. 4) or 
regions (Table 1). Although not significant, 
the king mackerel 5 13 C values for the 
northwestern Gulf of Mexico region were 
more negative than those for the Mexico 
region. More negative 8 13 C values were 
also detected at the base of the food chain 
in the northwestern Gulf of Mexico region 
in comparison with those for Florida (Table 
1). The influence of the Mississippi River 
on the northwestern Gulf of Mexico area 
is most likely the primary reason for 8 13 C 
values being more negative. Although C0 2 
depletion resulting from enhanced pri- 
mary production can increase 8 13 C values 
(Raven et al., 1993), the primary impact 
of the Mississippi River is the large terrestrial input 
of particulate organic matter (Trefry et al., 1994) lead- 
ing to more negative 8 13 C values. 
Commonly, less variability is observed in carbon 
isotopes than with nitrogen. This trend was not ob- 
served in this study. Our results, however, are con- 
sistent with some previous studies that reported that 
8 15 N data could be more discriminating than 8 13 C 
data. For example, Sholto-Douglas et al. (1991) used 
carbon and nitrogen isotopes to study food web rela- 
tions among plankton and pelagic fish and found 
greater variability in 8 13 C data than in 8 15 N mea- 
surements. Perhaps these systems have numerous 
carbon sources that create greater than expected 
variation in stable carbon isotope values, thereby 
rendering them ineffective. 
4 Lopez-Veneroni, D. 1997. Oceanography Department, Texas 
A&M University, College Station, TX 77843. Manuscript in 
prep. 
Numerous studies have observed seasonal migra- 
tions of king mackerel. King mackerel migrate along 
the eastern coast of the Gulf of Mexico and into the 
northern Gulf of Mexico from southeastern Florida 
(wintering grounds) in the summer (Trent et al., 
1987; Sutter et al., 1991; see also Johnson et al., 1994). 
Migrations may extend as far as Galveston and Port 
Aransas, TX (Williams and Sutherland, 1978). A re- 
turn migration from the northern Gulf of Mexico into 
southeast Florida occurs in late summer and early 
fall (Williams and Sutherland, 1978). While the king 
mackerel that winter in southeast Florida are mi- 
grating into the northern Gulf of Mexico, a simulta- 
neous migration from the Yucatan area (wintering 
grounds) occurs along the western coast of the Gulf 
of Mexico into the northern Gulf of Mexico (Trent et 
al., 1987; see also Johnson et al., 1994). 
Wind circulation along the Mexican and south 
Texas coast during the late spring and early sum- 
mer may cause upwelling off the Texas-Mexico bor- 
der (Dagg et al., 1991). Consequently, coastal bound- 
