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Fishery Bulletin 96(4), 1998 
Crosetti et al., 1994) and bluefish (Pomatomus 
saltatrix, Goodbred and Graves, 1996) have demon- 
strated that significant phylogeographic population 
structuring can exist in discontinuously distributed 
marine fishes. The reduced gene flow among isolated 
populations of such species provide excellent oppor- 
tunities for studying the effects of historical marine 
zoogeographical processes and observe incipient 
speciation. 
Mackerels of the genus Scomber, like the striped 
mullet and bluefish, are ideally suited for studies of 
genetic population structure in cosmopolitan fishes 
with fragmented distributions and for comparative 
studies that describe genetic patterns of other more 
vagile scombrids (Graves et al., 1984; Graves and 
Dizon, 1989; Scoles and Graves, 1993). Distributional 
patterns of scombrids vary widely from world-wide 
species such as the yellowfin tuna, albacore, and skip- 
jack tunas to supposedly wide-spread species such 
as the chub mackerel, S. japonicus , which is divided 
into geographically disjunct populations, to those 
with limited ranges such as a Spanish mackerel 
(Scomberomorus munroi) found only in the Gulf of 
Papua between Australia and New Guinea (Collette 
and Nauen, 1983). The three species of Scomber (S. 
japonicus, S. australasicus, and S. scombrus) occur 
in temperate to subtropical waters, and two (S. 
japonicus and S. australasicus ) display antitropical 
distributions. Each species occurs in disjunct popu- 
lations of various sizes, and their distributions over- 
lap in several areas (Fig. 1; Collette and Nauen, 
1983). The cosmopolitan chub mackerel, S. japonicus, 
occurs in coastal regions and adjacent seas of the 
Atlantic, Pacific, and northwest Indian oceans. The 
spotted chub mackerel, S. australasicus, is restricted 
to the Pacific Ocean, southeastern Indian Ocean, and 
the Red Sea. The Atlantic mackerel, S. scombrus, is 
restricted to the North Atlantic Ocean. The present 
study emphasizes S. japonicus, which of the three 
species is the most widely distributed and morpho- 
logically divergent among its isolated populations 
(Matsui, 1967). 
Life histories and 
population structures 
The spawning behavior and duration of the larval 
stage varies among the three species of Scomber. 
Scomber scombrus is capable of spawning serially 
up to 30 times in a spawning season (Watson et al., 
1992) at any time of day (Walsh and Johnstone, 1992), 
where S. japonicus spawns on average only 9 times 
in a season, and does so only at night (Watanabe, 
Figure 1 
Sampling and distributions of Atlantic mackerel, Scomber scombrus (S-MAS and S-ENG), spotted chub mackerel, S. australasicus 
(A-RED, A-JPN, A-AUS, A-NZL, and A-MEX), and chub mackerel, S. japonicus (J-JPN, J-TWN, J-CAL, J-HAW, J-FLA, J-ARG, 
J-ISR, J-IVC, and J-SAF), according to Collette and Nauen (1983). S. japonicus distributions off Brazil and Namibia follow 
Perrotta and Aubone (1991) and Zenken and Lobov (1989), respectively. 
