540 
Abstract.— Stable nitrogen (5 15 N) 
and carbon (5 13 C) isotope measure- 
ments were used to differentiate groups 
of king mackerel, Scomberomorus cav- 
alla, in the northwestern Gulf of Mexico 
and off the southeastern coast of 
Florida, as well as off the coast of 
Mexico. Northwestern (+13.1%e) and 
southeastern (Mexico=+10.8%e and 
Florida=+10.8%<?) groups, as well as the 
Atlantic group, had significantly differ- 
ent stable nitrogen isotope ratios. These 
were attributed to isotopic variations 
at the base of the food chain. Variabil- 
ity in 5 13 C measurements was too large 
and did not corroborate the 8 15 N re- 
sults. The grouping suggested by the 
5 15 N data can be explained by the in- 
fluence of the Mississippi River and the 
Gulf of Mexico Loop Current. 
Manuscript accepted 6 February 1997. 
Fishery Bulletin 95:540-551 (1997). 
Use of stable isotopes to 
assess groups of king mackerel, 
Scomberomorus cavalla, in the Gulf 
of Mexico and southeastern Florida 
Lynn A. Roelke* 
Luis A. Cifuentes 
Department of Oceanography 
Texas A&M University 
College Station, Texas 77843 
*E-mail address: lroelke@nitro.tamu.edu 
King mackerel, Scomberomorus 
cavalla, is one of the most sought 
after migratory pelagic resources in 
waters of the contiguous United 
States (Dwinell and Futch, 1973; 
Manooch et al., 1978; Manooch, 
1979; Finucane et al., 1986). This 
species is strongly exploited by both 
sport and commercial fisheries, and 
fishing pressure may exceed maxi- 
mum sustainable yields of the Gulf 
resource (Gulf of Mexico and South 
Atlantic Fishery Management 
Councils 1 ). In the United States, 
commercial catches of king mack- 
erel were 2,013 metric tons (t) in 
1994 (U.S. Dep. Commer., 1995). 
Recreational catches in the United 
States are thought to be larger than 
commercial landings (Deuel and 
Clark, 1968; Deuel, 1973; Manooch, 
1979; U.S. Dep. Commer., 1985- 
1987). Recreational catches are re- 
ported as individuals rather than as 
weight; however, an estimated to- 
tal weight of recreational catches for 
1991 was 2,713 1 (U.S. Dep. Commer., 
1992). 
Current management plans are 
based on a two-stock model, an At- 
lantic stock and a Gulf of Mexico 
stock (Gulf of Mexico and South At- 
lantic Fishery Management Coun- 
cils 1 ). However, general consensus 
among scientists is that two stocks 
of king mackerel exist within the 
Gulf of Mexico. Evidence for two 
stocks within the Gulf of Mexico 
includes fisherman observations of 
migration (Baughman, 1941), elec- 
trophoretic studies (Grimes et al., 
1987; Johnson et al., 1994; May 2 ), 
catch-per-unit-of-effort data from 
charter boats (Trent et al., 1987), 
and differences in spawning times 
(Grimes et al., 1990). 
Stable isotopes (C, N) have been 
used to study trophic levels and 
feeding strategies of organisms (see 
Macko et al., 1984; Peterson and 
Fry, 1987; Koch et al., 1995). Accord- 
ing to BeNiro and Epstein (1978), 
the carbon isotopic composition of 
a food source is not substantially 
altered during assimilation. DeNiro 
and Epstein (1981) found that ani- 
mals also reflect the nitrogen isoto- 
pic composition of their diet; how- 
ever, there is a trophic-level enrich- 
ment. Regardless of habitat, form 
of nitrogen excreted, and growth 
rate, an isotopic enrichment of +3.4 
1 Gulf of Mexico and South Atlantic Fishery 
Management Councils. 1992. Amend- 
ment 6 to the fishery management plan for 
coastal migratory pelagics in the Gulf of 
Mexico and South Atlantic includes envi- 
ronmental assessment regulatory impact 
review and initial regulatory flexibility 
analysis. Gulf Mex. S. Atl. Fish. Manage. 
Counc., Tampa, FL, var. pagin. 
2 May, B. 1983. Genetic variation in king 
mackerel (Scomberomorus cavalla). 
Final Rep. FL Dep. Nat. Resour. Contract 
C-1434, 20 p. 
