Johnson et al.: Evidence for distinct stocks of Scomberomorus cavalla 



99 



gest one plausible scenario with regard to king 

 mackerel stocks in the Gulf of Mexico. A western 

 population exists that winters and spawns in the 

 Gulf of Campeche. The Mexican Current serves as 

 an entrainment system for its young. As these young 

 become older and larger, they are able to cross the 

 region of offshore advection and utilize the north- 

 ern Gulf area (Texas to Florida) for summer feed- 

 ing. This stock of fish has a high PEPA-2*a fre- 

 quency and spawns earlier in the year than fish in 

 the northern and eastern Gulf of Mexico. No infor- 

 mation (tagging, electrophoretic, or reproductive) is 

 available on fish of the Yucatan Straits area and the 

 Caribbean Sea to evaluate their relation to the west- 

 ern Gulf of Mexico fish. An eastern population of 

 king mackerel uses the eastern and northern Gulf 

 of Mexico area as entrainment systems for its young 

 and the northern Gulf (Florida-Texas) as summer 

 feeding grounds. The spawning area extends from 

 Texas to northwest Florida between April and Oc- 

 tober; the majority of spawning probably occurs in 

 the northwest Florida-Louisiana area. Tagging stud- 

 ies suggest that this stock uses south Florida and the 

 southeast coast of Florida as its wintering grounds. 



The Louisiana area is somewhat of an enigma. 

 Tagging studies indicate that the area is used by fish 

 from both sides of the Gulf, fish are in the area year- 

 round, PEPA-2'a frequencies are between the ex- 

 tremes of the east and west Gulf, and tag recover- 

 ies from winter tagging in Louisiana have been from 

 Louisiana and westward, whereas recoveries from 

 summer tagging were both east and west of Louisi- 

 ana. Additionally, Finucane et al. (1986) suggested 

 an earlier distinct peak in gonadal development 

 (May) for Louisiana-Mississippi than in northwest 

 Florida (August) and in Texas (August). The ques- 

 tion still remains: Does the Louisiana area have an 

 independent spawning population that utilizes the 

 northern Gulf currents for its life cycle? The exist- 

 ing evidence (especially tagging) suggests the area 

 is not independent; however, information comes 

 from larger fish. Thus, the area may be occupied by 

 individuals from both sides of the Gulf which may 

 or may not reproduce in the area. Further investi- 

 gation especially on the younger life stages using other 

 methods of analyses may answer this question. 



Another group (stock) of king mackerel that im- 

 pinges upon the Gulf of Mexico resources (officially 

 recognized by Fishery Management Councils) is the 

 Atlantic Migratory Group. This group has a vary- 

 ing range from Virginia to southwest Florida de- 

 pending on the time of the year (Gulf of Mexico and 

 South Atlantic Fishery Management Councils, 

 1985). The stock is considered to winter in South 

 Florida and ranges along the Atlantic coast to North 



Carolina and South Carolina during the summer. 

 The fish probably spawn from May to October with 

 a peak in July (Finucane et al., 1986). These fish are 

 currently regulated as a group with seasonal south- 

 ern boundaries of lat. 25°48'N (the Collier/Monroe 

 County line, FL) from 1 April to 31 October and lat. 

 29° 25'N (the Volusia/Flagler County line, FL) from 

 1 November to 31 March. Tagging information sup- 

 ports this separation (Gulf of Mexico and South 

 Atlantic Fishery Management Councils, 1985). 



PEPA-2' a allele frequencies are generally low 

 (0.00-0.10) along the Atlantic coast as in the east- 

 ern Gulf of Mexico. The higher PEPA-2*a values 

 (>0.10) occasionally encountered may be the result 

 of fish entrapped in water masses coming up the 

 coast from outside the east coast of Florida. This 

 possibility is suggested by the recovery along this 

 coast of drift bottles that were released in the 

 Yucatan Straits area (Salsman and Tolbert, 1963 7 ). 



All these stocks need to be further investigated in 

 order to be elevated to the status of genetic stocks 

 (i.e., completely isolated reproductive populations of 

 the same species). 



Conclusion 



Four lines of evidence for a two stock hypothesis for 

 the Gulf of Mexico king mackerel have been pre- 

 sented. The two stock hypothesis states that the 

 Gulf contains a western stock of king mackerel, 

 which winters in Mexico and migrates in spring and 

 early summer to the northern Gulf (Texas-Alabama), 

 and an eastern Gulf stock which winters in south 

 Florida and migrates in spring and early summer 

 to the northern Gulf. The two stocks mix in the 

 northern Gulf during the summer. 



The four lines of evidence are the following: 



1 Dipeptidase (PEPA-2' ) data showing western 

 Gulf fish high in *a allele and eastern fish low 

 in *a allele. 



2 Mark-recapture data showing movement along 

 both sides of the Gulf from south to north. 



3 Catch data indicating simultaneous migrations 

 northward on each side of the Gulf in early 

 spring and summer. 



4 Estimates of spawning dates suggesting pos- 

 sible temporal and spatial differences between 

 the northern and southern Gulf. 



Acknowledgments 



Especially helpful in collecting specimens and data 

 were staff members of the following organizations: 



