840 
Fishery Bulletin 96(4), 1998 
from a source population in the northwest Pacific 
region or Hawaii. Colonization of the Revillagigedo 
Islands may have occurred through the migration of 
adults. Gene flow between other isolated populations 
( J-IVC and J- ARG; A-NZL and A-AUS ) was also sug- 
gested by the mtDNA data and may be facilitated by 
the high vagility of species of Scomber. 
Despite the genetic similarity observed among 
widely separated samples of S. australasicus, ex- 
change in the north-south direction is restricted. 
Haplotypes of the unique New Zealand-Australia lin- 
eage were not observed in samples from Japan or 
Mexico. These data suggest that the equatorial re- 
gion of the Pacific is a barrier to gene flow in S. 
australasicus, and together with the significant dif- 
ference observed between western Atlantic S. 
japonicus samples, strongly emphasizes the impact 
of physical oceanographic conditions in shaping 
phylogeographic patterns. Scomber japonicus from 
the eastern North and South Pacific were also dif- 
ferentiated by allozyme analysis which revealed three 
differentiated loci of fourteen that were variable (Nei’s 
D= 0.033, F st =0.20) (Stepien and Rosenblatt, 1996). 
These observations support the hypothesis that physi- 
cal oceanographic characteristics of equatorial regions 
may limit gene flow in species of Scomber, and possi- 
bly a variety of other temperate pelagic species. 
These data demonstrate that levels of heterogene- 
ity reflect species vagilities and are shaped by physi- 
cal oceanographic processes. Other studies have 
demonstrated that population structure in several 
species is generally lower between populations sepa- 
rated by vast ocean basins than between those sepa- 
rated by land masses, in some cases suggesting con- 
tinued gene flow (Vawter et al., 1980; Rosenblatt and 
Waples, 1986; Shulman and Bermingham, 1995). But 
not all genetic data support a simple relationship 
between the degree of geographic barriers, dispersal, 
and genetic relatedness. In one study, five species 
with various life history features that limit dispersal 
showed no intraspecific differentiation, whereas one, 
Gnatholepis thompsoni, which had nonpelagic eggs 
and a long planktonic life, showed population struc- 
ture across the Caribbean Sea (Shulman and Berm- 
ingham, 1995). Sinclair (1988) reviewed the effects 
of oceanographic characteristics on population struc- 
ture and concluded that it is defined at the plank- 
tonic phase of the life history, rather than at the ju- 
venile or adult phases, and is largely regulated by the 
physical environment. However, Thresher and Broth- 
ers (1985) were unable to relate larval stage duration 
or adult size to dispersal in several angelfish species 
(Pomacanthidae) and suggested that geological features 
are better predictors of dispersal. Clearly, the processes 
by which population structure emerges in marine 
fishes are complex. Our data support the hypothesis 
that oceanographic characteristics are significant in 
limiting dispersal in Scomber, especially in some 
equatorial regions. The exchange of adult individu- 
als among populations appears to have a significant 
effect on limiting the divergence between some popu- 
lations, possibly reflecting adaptations in Scomber 
for survival in the pelagic environment. 
Acknowledgments 
We thank the following who assisted with specimen 
collections: Kurt Schaefer, Juan M. Diaz de Astarloa, 
Yu-Yun Chen, Lounes Chikhi, Ed Everett, A. 
Frettsome, Daniel Golani, Liz Hoenson, P. Alexander 
Hulley, J. Brian Jones, Emile G. Marchal, Chris 
Martin, Hin-Kiu Mok, Eugene L. Nakamura, Bar- 
bara J. Palko, Julian Pepperell, G. A. Potts, Clive 
Roberts, Sachiko Tsuji, Yoshiro Watanabe, Mitsu 
Yesaki. We are indebted to Peter Burn and Ray 
Webster of Air New Zealand for providing air cargo 
services. We thank D. Swofford for allowing the use 
of a test version of PAUP* Bert Ely and Stefan -M. 
Pulst provided resources for parts of the study. 
Antony Lewis provided regional literature. Eldridge 
Bermingham, Brian Bowen and Kent Carpenter criti- 
cally reviewed the manuscript. David Carlini assisted 
with data analysis. 
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