A divide across the Pacific
Wild Seriola lalandi sampled in Mexico and Chile showed a pronounced genetic divide between the Northern and Southern Hemispheres. The researchers used pairwise differentiation measures, which compare genetic composition between populations, and found values ranging from 0.162 to 0.208 for FST and from 0.401 to 0.565 for Jost's D. The reported inter-hemispheric comparisons were statistically significant at p below 0.05.
An analysis of molecular variation, known as AMOVA, found that 78% to 90% of genetic variation was within populations. Less than 6% was among populations within the same hemispheric group. The reported between-hemisphere differentiation component was 0.1692, compared with 0.0578 among populations within hemispheres, and both results had p below 0.001. The pattern shows a large geographic division alongside substantial variation within the sampled populations.
The northern side was not one block
Within the Temperate Northern Pacific, the Mexican wild samples labeled BT and BA showed moderate but significant differentiation even after atypical loci were excluded. Their reported FST was 0.078 and Jost's D was 0.111. No genetic differentiation was observed among the sampled wild Chilean populations.
Hierarchical STRUCTURE analysis, a clustering method that groups samples by shared genetic patterns, separated the fish into two main genetic clusters corresponding to the Northern and Southern Hemispheres. The authors interpreted the overall pattern as support for two independent evolutionary lineages and for region-specific management, challenging the idea that the Temperate Northern Pacific is a single northern stock.
Hatchery stocks varied as well
Captive-reared stocks also differed from some wild groups, but the pattern was not uniform. A stock labeled F1CL was most divergent from wild BM among the reported comparisons, with FST of 0.268 and Jost's D of 0.703. It was least divergent from wild CH, with values of 0.021 and 0.020. A stock labeled F1BM also differed from wild BT, with FST of 0.120 and Jost's D of 0.212.
One metric-specific qualification is important: the analysis notes that F1BT was not significantly different from BT by Jost's D. The results therefore point to particular stock-to-wild contrasts, not one uniform genetic difference between every captive and wild group.
Wild Mexican samples also had higher heterozygosity, a measure of genetic variation, than wild Chilean samples. Observed heterozygosity was 0.733 in Mexico versus 0.645 in Chile, while expected heterozygosity was 0.787 versus 0.715. The result was reported without a confidence interval or p-value.
What the findings can show
The researchers began with 336 wild adult fish from four Mexican and four Chilean locations, plus 313 captive-reared F1 juveniles from hatcheries in Mexico, the United States and Chile. After quality-control exclusions, the final dataset contained 497 individuals from eight wild locations and four captive stocks. The exclusions included 96 putative full-sibs with reported relatedness above 0.45 and 56 genotypes with null alleles at more than two loci, as well as the Sdu21 marker, which had 12% missing data.
The team used nine microsatellite markers, six specific to S. lalandi and three isolated from S. dumerili, to compare the fish. Population structure was assessed with FST and Jost's D, with 95% confidence intervals calculated from 10,000 bootstrap replicates. That approach can reveal differences in genetic composition among sampled groups, but it does not directly measure fish movement or identify the cause of those differences.
A separate relative-migration analysis estimated weak but statistically significant directional connectivity from Chile to Mexico at 0.009, mainly toward BM, under the reported threshold below 0.05. Within-region estimates were not statistically supported and were treated as descriptive, ranging from 0.019 to 0.492 in the Northern Pacific and from 0.286 to 1.00 in Temperate South America. These are allele-frequency-based connectivity estimates, not direct demographic counts of fish moving between regions.
The authors' conclusion is narrower than a claim that the fish are definitively separate species or that one environmental or aquaculture factor caused the split. They say the evidence supports two independent evolutionary lineages, northern substructure and region-specific management. The findings apply to the sampled populations and stocks, while broader sampling and additional markers would be needed to test how widely the pattern extends.
Paper data and sources
Original title: Population Genetics ofConfirm Hemispheric Divergence and Reveal Genetic Structure in the Temperate Northern Pacific and Between Wild and Captive-Reared Fish.
Authors: Eduardo Martínez-Matus, Felipe Aguilera, Raquel Muñiz-Salazar et al.
Journal/Repository: Ecology and evolution
Status: Peer-reviewed
First online: 2026-08-20
DOI: 10.1002/ece3.74200
Original paper