92 



Fishery Bulletin 92(1). 1994 



dependent exploitation or management and contain- 

 ing as much of an interbreeding unit or as few re- 

 productively isolated units as possible (Royce, 1972). 

 An additional term that has been used to define the 

 stock concept used in fishery management is "unit 

 stock" which was referred to by Kutkuhn (1981) as 

 "one consisting of randomly interbreeding members 

 whose genetic integrity persists whether they re- 

 main spatially and temporally isolated as a group, 

 or whether they alternately segregate for breeding 

 and otherwise mix freely with members of other unit 

 stocks of the same species." This term is more func- 

 tional for application to many marine resources 

 which have identifiable components but for which 

 reproductive isolation has not been demonstrated. 

 We consider stock and unit stock to be identical with 

 regard to king mackerel resources at the present 

 time. 



Using Kutkuhn's (1981) definition, this report 

 presents evidence of two stocks of king mackerel 

 existing in the Gulf of Mexico (the Gulf), an east- 

 ern and a western stock which winter off south 

 Florida and off the Yucatan peninsula (Mexico), re- 

 spectively. In the spring these fish migrate along 

 their respective coasts to summer areas in the 

 northern Gulf. The concept of two Gulf of Mexico 

 stocks was first presented by Baughman ( 1941). He 

 based his hypothesis on observations by fishermen 

 of simultaneous migrations along the eastern and 

 western sides of the Gulf. More recently, May 

 (1983) 3 reported electrophoretic differences in king 

 mackerel between the eastern and western Gulf. 

 Using more recent tagging data and electrophoretic 

 information, Grimes et al. (1987) reintroduced the 

 hypothesis. 



Additional evidence for a two-stock hypothesis is 

 the following: 



1 Fish movements along the coast, as indicated by 

 mark-recapture studies (Fable et al., 1990 4 ). 



2 The simultaneous migration along the eastern 

 and western coasts of the Gulf in spring and 

 early summer as detected by analysis of 

 charterboat CPU data (Trent et al., 1987b). 



3 The difference in spawning times of king mack- 

 erel in the northern and southern areas of the 

 Gulf (Grimes et al., 1990). 



3 May, B. 1983. Genetic variation in king mackerel 

 (Scomberomorus cavalla). Final Rep. Fla. Dep. Natl. Resour. 

 Contract C-14-34, 20 p. 



4 Fable, Jr., W.A., J. Vasconcelos P., K. M. Burns, H. R. Osburn, 

 L. Schultz R., and S. Sanchez G. (1990). King mackerel, 

 Scomberomorus cavalla, movements and migrations in the Gulf 

 of Mexico. Natl. Mar. Fish. Serv., Panama City Lab., Panama 

 City, FL (unpubl. ms.l. 



We report the results from electrophoretic inves- 

 tigations and summarize current information from 

 tagging, migration, and spawning time studies. We 

 also propose a possible mechanism to explain the 

 observed results with regard to the water circula- 

 tion of the area. 



Methods and materials 



Samples of muscle tissue, along with fork length 

 (mm) and sex, were collected during 1985 through 

 1990 from fish obtained in recreational and commer- 

 cial fisheries from North Carolina to Yucatan 

 (Table 1). The samples were frozen as soon as pos- 

 sible in the field and then shipped frozen to the Na- 

 tional Marine Fisheries Service's Panama City Labo- 

 ratory. Muscle tissue (about 10 grams) was excised 

 from each sample and stored in a freezer (in 1985 at 

 -5° to -10°C and from 1986 through 1990 at -100°C). 



Tissue extracts were prepared by mixing equal 

 volumes of muscle tissue and distilled water and 

 grinding with glass rods to uniform pastes. Extracts 

 were centrifuged at 3,400 rpm (1,000 x G) for five 

 minutes, then supernatants were drawn onto 4 mm 

 x 8 mm filter paper inserts (Whatman 1). 



Starch gel electrophoretic separation of the ex- 

 tracts was performed following the methods of 

 Kristjansson (1963). Electrophoretic buffers were 

 those of A) Markert and Faulhaber (1965), and B) 

 N-(3-aminopropyl)-morpholine-citrate (pH 6.1) 

 buffer of Clayton and Tretiak (1972). The gel con- 

 sisted of 35 g of starch (Sigma Chemical Co. lots 

 123F-0591, 35K-0383, and 94F-0536) plus 250 mL 

 of buffer. Amperage during electrophoresis was kept 

 below 50 MA, and voltage varied between 100 and 

 400 V, depending on the buffer. Temperature was 

 maintained at 2°C by using a refrigerated cooling 

 system (see Aebersold et al., 1987, for description). 

 After electrophoresis, the gels were sliced into four 

 horizontal sections and stained for dipeptidase (EN 

 3.4.-.-). In 1985 (1,223 fish) and 1988 (879 fish), 27 

 additional enzymes were examined. Methods fol- 

 lowed May (1983) 3 and Aebersold et al. (1987). 



We conducted statistical analyses using Biosys-1 

 (Swofford and Selander, 1981) to test for conform- 

 ance to Hardy-Weinberg expectations and spatially 

 related differences in allele frequencies compared to 

 distance and physical feature subdivisions. The 

 Kolmogorov-Smirnov goodness-of-fit test was used 

 for comparing allele distributions by size of fish 

 (100-mm-FL intervals), while the chi-square contin- 

 gency test was used for comparing allele distribu- 

 tions by sex (see Sokol and Rohlf (1981) for proce- 

 dures i. 



