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We used a before-after control-impact design to quantify the genetic effects of the large piscivorous Japanese Spanish mackerel (Scomberomorus niphonius) stock enhancement program on wild populations in the Seto Inland Sea. Samples of 1424 wild and 230 hatchery fish collected from 13 sites around Japan were genotyped using five microsatellite markers. A total of 758 wild and 103 hatchery fish were sequenced for the mitochondrial DNA D-loop region. The population structure of Japanese Spanish mackerel was panmictic around Japan. Hatchery fish had significantly lower genetic diversity indices than did wild fish. However, there was no significant change in any of the diversity indices in the Seto Inland Sea, despite the substantial genetic mixing proportion of hatchery-origin genes (7.8%-14.5% from releases in 2001 and 2002), a conclusion supported by simulations. The estimated effective population size was surprisingly small (~430-970) but stable in the Seto Inland Sea compared with the large census size. A Ryman-Laikre effect was not likely in the Japanese Spanish mackerel. Nous avons utilise un schema controle-incidence avant-apres pour quantifier les effets genetiques du programme d'amelioration des stocks de thazard oriental (Scomberomorus niphonius), un grand poisson piscivore, sur les populations sauvages dans la mer interieure de Seto. Des echantillons de 1424 poissons sauvages et 230 poissons d'elevage preleves en 13 endroits differents au Japon ont ete genotypes a l'aide de cinq marqueurs microsatellites. Un total de 758 poissons sauvages et 103 poissons d'elevage ont fait l'objet d'un sequen^age de la region de la boucle D de l'ADN mitochondrial. La structure de la population de thazard oriental etait panmictique dans la region du Japon. Les poissons d'elevage presentaient des indices de diversite genetique significativement plus faibles que les poissons sauvages. Cela dit, aucun indice de diversite ne presentait de variation significative dans la mer interieure de Seto, malgre l'importante proportion de melange genetique de genes issus d'ecloserie (7, 8-14, 5% dans les laches en 2001 et 2002), une conclusion appuyee par les simulations. La taille effective estimee de la population etait etonnamment faible (~430-970), bien que stable, dans la mer interieure de Seto comparativement a la grande taille recensee. La presence d'un effet Ryman-Laikre chez les thazards orientals etait peu probable. [Traduit par la Redaction], Introduction Japanese Spanish mackerel (hereafter JSM; Scomberomorus niphonius) is a large piscivorous fish that is primarily distributed off the western coast of Japan. This species is an important fishery resource, [...]

The genetic structure and history of the Indian scad mackerel, Decapterus russelli are studied to analyze the genetic breaks for marine fishes and invertebrates between the Indian and Pacific oceans. Results indicate that a Pleistocene drop in sea level caused a geographical isolation, secondary contact is the reason for the occurrence of two major mitochondrial lineages and low genetic diversities indicate a population bottleneck and subsequent speedy population expansion.

Round scad mackerel sampled in 1995-1998 were analysed for genetic variation using mitochondrial-DNA and nuclear-DNA markers. Sequence variation for a fragment of the cytochrome b gene (mitochondrial), amplified by polymerase chain reaction, was screened across individuals using single strand conformation polymorphism (SSCP). Sequence analysis of all SSCP haplotypes indicated two mitochondrial clades separated by, on average, 2.3% nucleotide divergence. The geographic distribution of haplotypes was homogeneous (Weir and Cockerham's [theta] = -0.002). Also, no geographic heterogeneity was detected for length polymorphism of Intron 1 of the gene encoding aldolase B ([theta] = 0.005). Although homogeneity in allele frequencies throughout the Indo-Malay archipelago conformed to the expectations for a widely distributed pelagic fish in a highly connected habitat, this was at variance with the sharp geographic structure previously uncovered in Indian scad mackerel, Decapterus russelli, a fish with life-history characteristics similar to D. macrosoma. A remarkable similarity, however, was the occurrence of two similarly distinct clades within each species, suggesting a common history of geographic isolation. Low sea levels in the Pleistocene might have caused the separation and vicariance of populations within both D. macrosoma and D. russelli. Subsequent genetic exchange between populations would then have erased allele-frequency differences at the cytochrome b and aldolase B loci in D. macrosoma while some barrier to gene flow was maintained in D. russelli.

The genetic relationships among South-East Asian populations of the scad mackerels Decapterus macarellus, D. macrosoma and D. russelli (Pisces: Carangidae) were investigated. In 1995 and 1996, 216 fish were sampled in seven localities spanning the seas of Indonesia and were examined for restriction-site polymorphisms using ten restriction enzymes for the mitochondrial (mt) DNA control region, amplified by the polymerase chain-reaction. The inferred phylogeny of haplotypes led to the recognition of three distinct mitochondrial lineages or phylads consistent with the distinctions of current taxonomy. All 15 mtDNA haplotypes found in D. macarellus and all 9 haplotypes found in D. macrosoma were arranged as star-like clusters, suggesting recent evolutionary history. In contrast, the phylad formed by 6 haplotypes in D. russelli from the Sulawesi Sea exhibited diffuse topology, suggesting that ancestral lineages of this species have been retained to the present. Average nucleotide-divergence estimates between haplotypes of different phylads were between 0.042 and 0.135, suggesting ancient separation, in consistency with published allozyme data. High levels of haplotype diversity, but no geographical heterogeneity, was detected within D. macarellus from the Molucca Sea and the Banda Sea. Populations of D. macrosoma exhibited both significant differences between adjacent regions (Sunda Strait and Java Sea), and broadscale genetic homogeneity from the South China Sea to the Sulawesi Sea via the Java Sea and Makassar Strait. The geographic isolation of the D. macrosoma population sampled in the Sunda Strait suggests that this region constitute a sharp transition zone between the Indian Ocean and the Sunda Shelf. Near-monomorphism of haplotypes and low nucleotide diversity ([d.sub.x]) were observed in the samples of D. macrosoma from the continental shelf (haplotype-diversity estimates, h, = 0.00 to 0.25 [+ or -] 0.08 and [d.sub.x] = 0.000 to 0.002). This was in contrast to the comparatively high haplotype and nucleotide diversities observed in other pelagic fish species including D. macarellus (h = 0.82 [+ or -] 0.05, [d.sub.x] = 0.012 to 0.015) and D. russelli (h = 0.63 [+ or -] 0.12, [d.sub.x] = 0.016), and in the oceanic D. macrosoma population sampled in the Sunda Strait (h = 0.67 [+ or -] 0.31, [d.sub.x] = 0.005). We hypothesise that this may be the consequence of recent and perhaps repeated bottleneck events that have affected the D. macrosoma population sampled on the continental shelf.