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Analysis of Pacific Blue Marlin and Swordfish Population Structure using Mitochondrial and Nuclear DNA Technologies

The population structure of Pacific blue marlin and swordfish will be investigated via state of the art mitochondrial and nuclear DNA technologies. Through improving the ability to identify distinct populations, monitor recruitment, and determine impacts of fishing on the genetic variability of the stocks, development of effective management strategies will be enhanced.

Funding was received at the University in August 1993.


Principal Investigator:
Dr. John Graves
School of Marine Science
College of William and Mary
P.O. Box 1346
Gloucester Point, Virginia 23062-1346
Phone (804) 642-7352
FAX (804) 642-7186
email: graves@vims.edu
Dr. Barbara Block
Hopkins Marine Station
Stanford University
120 Ocean View Blvd.
Pacific Grove, California 93950-3094
Phone (408) 655-6236
FAX (408) 375-0793
email: bblock@leland.stanford.edu

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Progress Report

Genetic material from more than 60 individual blue marlin from the Atlantic and Pacific oceans has been analyzed. The preliminary results are that the blue marlin in the Pacific Ocean belong to a single genetic population that also extends into the Atlantic Ocean. A second, less widely distributed, genetic stock is found only in the Atlantic. With regard to swordfish, genetic material from more than 125 individuals has been analyzed. The preliminary conclusions contrast sharply with those for blue marlin. There appear to be several genetic stocks in the Pacific Ocean alone. These conclusions, if sustained by further analyses, would imply very different stock management strategies for these two species.
Discrimination between the species of very small larval and post-larval billfish is difficult using visible features only. Preliminary success was achieved on this problem using genetic "fingerprinting".

This figure represents the relationship among mitochondrial DNA (mtDNA) haplotypes occuring in more than two blue marlin. The area of each circle is proportional to the number of individuals with that haplotype. A "ubiquitous" group of mtDNA haplotypes is found in all Pacific blue marlin and just more than half of the Atlantic blue marlin. The remaining Atlantic blue marlin have mtDNA haplotypes that, as a group, are closely related, but are quite distinct from those of the "ubiquitous" group. These results are consistent with a time considerable isolation between the "ubiquitous" and "Atlantic" groups of blue marlin. This isolation could have occurred during a time of tropical compression when gene flow around the Cape of Good Hope was not possible for tropical marine organisms, with subsequent movement (and proliferation) of blue marlin from the Pacific to the Atlantic when water warmed.

Restriction fragment length polymorphism (RFLP) analysis of mitochondrial (mt) DNA was used to investigate population structure of blue marlin in the Pacific and Atlantic Oceans. Composite mtDNA haplotypes representing digestion with 12 restriction endonucleases have been generated for approximately 450 blue marlin from four locations in the Pacific Ocean (Mexico, Ecuador, Hawaii and Australia) and three within the Atlantic Ocean (U.S. mid- Atlantic coast, Jamaica, Brazil). Extremely high levels of mtDNA variation are present in samples from both oceans, with haplotype diversities (the probability of encountering different haplotypes in two draws from a sample) approaching 0.9. Despite the high levels of molecular character variation, significant inter-ocean genetic divergence is apparent. Two major clades of mtDNA haplotypes are present in Atlantic blue marlin, but only one of these is found in Pacific blue marlin. The presence of intra-ocean population genetic structure is also suggested by the spatial partitioning of particular haplotypes. These apparent under- and intra-ocean genetic differences are temporally stable, at least over a period of several years.

The global population structure of swordfish was assessed through sequencing of the 5' end of the highly variable mitochondrial DNA control region from 159 swordfish collected in three ocean basins: the Mediterranean, Atlantic and Pacific. A total of 332 base pairs of sequence were obtained from swordfish collected from the Pacific (n=105), Atlantic (n=23) and Mediterranean (n=20). Phylogenetic analysis of the relationships among the unique DNA haplotypes revealed two deeply divergent clades with differential geographic distributions, similar to what has been found for certain istiophorid billfishes. Phylogeographic concordance among these species suggests a biogeographic explanation for this structure. The extent of global population subdivision was also examined using an analysis of molecular variance (AMOVA) framework and F- statistic analogs. The results revealed significant geographic partitioning of molecular variance among the three ocean basins, indicating that swordfish populations are structured on a global scale. Estimates of genetic exchange rates among populations within an ocean basin were high, suggesting panmixia within ocean basins. However, much larger sample sizes may be necessary to detect subdivision within ocean basins, because the haplotypic diversity exhibited by the swordfish control region is extremely high.

PORF logo Pacific Ocean Research Foundation (PORF), Kona, Hawaii

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This page updated August 16, 2006