542 
Fishery Bulletin 95(3), 1997 
Methods and materials 
King mackerel were obtained from the Southeast 
Fisheries Center, National Marine Fisheries Service, 
Panama City, FL, and laboratory of John R. Gold, 
Department of Wildlife and Fisheries Sciences, Texas 
A&M University. These samples were collected 
within the Gulf of Mexico and Atlantic Ocean (Fig. 
1) during November and December 1991; February, 
May, and July 1992; and March, May, and June 1993 
(Table 2). Two to twenty fish were analyzed per site 
(average of seven per locality). Locality, date col- 
lected, fork length (standard measure of size), weight 
and sex of the fish specimens were recorded for most 
samples. According to the length-at-age study of 
DeVries and Grimes, 3 all fish were between 1 and 19 
years of age. 
Stable carbon and nitrogen isotope measurements 
were performed on dorsal fin spines. Dorsal fin spines 
were extracted and frozen prior to laboratory pro- 
cessing. After thawing, fin spines were cleaned of 
epidermal and dermal tissue, soaked in a dilute so- 
lution of HC10 3 ~ (bleach) to remove excess tissue, 
and then washed thoroughly with double-distilled 
3 DeVries, D. A., and C. B. Grimes. 1991. Spatial and tempo- 
ral variation in age composition and growth of king mackerel 
Scomberomorus cavalla from the southeastern U.S., 1986-1989; 
implications for stock structure and recruitment varia- 
bility. U.S. Dep. Commer., NOAA, NMFS, 3500 Delwood Beach 
Rd., Panama City, FL 32408-7403. Unpubl. manuscript, 41 p. 
water (no significant difference was observed with 
the dilute bleach method of cleaning and simply 
scraping the spine clean). Collagen was extracted 
according to the method of Tuross et al. (1988), who 
found that collagen extractions obtained with 
ethylenediaminetetraacetic acid (EDTA) yielded 
higher demineralization than those obtained with 
hydrochloric acid. Contamination of EDTA had been 
detected at less than 1 ng EDTA per mg of dry pro- 
tein (Tuross et al., 1988). Spines of each individual 
were soaked separately in 50 mL of 0.5M EDTA, pH 
7.2, at 4°C and shaken on a laboratory shaker for 
five days to remove mineralized bone. Mineralized 
bone was considered removed by evidence of a trans- 
lucent, pale yellow appearance (Tuross et al., 1988). 
The remaining collagen was washed with dilute 
NaOH, rinsed thoroughly with double distilled wa- 
ter, and freeze-dried. 
An investigation of sample preparation techniques 
was conducted to ensure accurate data collection. 
Incomplete removal of the mineral phase of the dor- 
sal fin spine would cause erroneous 13 C-enriched 
values. In turn, poor conversion of collagen carbon 
to C0 0 would result in CO production and inaccu- 
rate 15 N-enriched values owing to 13 C 16 0, which in- 
terferes with the 15 N 14 N signal on the mass spec- 
trometer. These sources of contamination were most 
likely to occur in large samples, reflecting a relation 
between sample size and isotope value. 
Owing to the large size of these dorsal fin spines, 
multiple sections were taken from all samples to 
ensure that the whole spine was measured 
isotopically. Spines were divided into ap- 
proximately 3-mg sections; therefore, 
spines from larger mackerel had more sec- 
tions than did spines from smaller mack- 
erel. Each section of the dorsal fin spines 
was placed in a separate quartz tube with 
elemental copper and cupric oxide and 
sealed under vacuum. These sections were 
converted to C0 2 and N 2 gas with modi- 
fied Dumas combustion (850°C for two 
hours ) (Macko, 1981). The C0 2 and N 2 
were then isolated cryogenically and ana- 
lyzed on Finnigan MAT 251 and Nuclide 
3-60-RMS isotope ratio mass spectrom- 
eters. The reproducibility of the measure- 
ments for 8 13 C was ±0.2 %c and ±0.3 %c for 
N 2 . Minimum sample size was 50 pg for 
both 5 13 C and 5 15 N. 
Stable carbon and nitrogen isotope mea- 
surements were performed on 65 and 64 
dorsal fin spines, respectively. Stable iso- 
tope ratios, denoted in parts per mil, were 
calculated in terms of 8 as follows: 
100 W 95 90 85 80W 
King mackerel study sites. Dotted line is mean position of the Loop 
Current. Arrow heads on the dotted line denote direction of flow. Solid 
arrow indicates convergence zone at Brownsville, TX. 
