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Genetic Analysis of Population Structure in Pacific Swordfish (Xiphias gladius) using Microsatellite DNA Techniques

Progress Reports (PDF): FY 2001, FY 2000, FY 1999, FY 1998 (see below)

Project Overview
This projects extends previous work on mitochondrial DNA which showed that Pacific swordfish comprise a single genetic stock. Analysis of the short, repetitive, and highly variable sequences of DNA known as microsatellites has accurately linked individual to geographic location in some animals. This project will examine the use of microsatellite DNA for identifying the geographic location of individual swordfish.

Project findings published in the following:
Ward, R.D., C.A. Reeb and B.A. Block, 2001. Population structure of Australian swordfish Xiphias gladius. Final Report to Australian Fisheries Management Authority, Canberra.

PDF file of Australian swordfish report

Block, B. A., J. E. Keen, B. Castillo, H. Dewar, E. V. Freund, D. J. Marcinek, R. W. Brill, and C. Farwell, 1997: Environmental preferences of yellowfin tuna (Thunnus albacares) at the northern extent of its range. Marine Biology, 130: 119-132.

Block, Barbara A., Heidi Dewar, Charles Farwell, and Eric D. Prince, 1998: A new satellite technology for tracking the movements of Atlantic bluefin tuna. Proceedings of the National Academy of Sciences, 95:9384-9389, August 1998.

Rosel, Patty E., and Barbara A. Block, 1996: Mitochondrial control region variability and global population structure in the swordfish (Xiphias gladius). Marine Biology, 125, 11-22.

See Journal Publications page and Other Publications for other publications by Block and PFRP investigators.


Principal Investigator:
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
Dr. Carol Reeb
c/o Hopkins Marine Station
Stanford University
120 Ocean View Blvd.
Pacific Grove, California 93950-3094
Phone (408) 655-6200
FAX (408) 375-0793
email: creeb@stanford.edu

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Progress Report - July 1998

PIs: Barbara A. Block and Carol Reeb Hopkins Marine Station, Stanford University

Progress of the Project

We have analyzed mitochondrial control region sequences from 370 swordfish and allele size information for eleven microsatellite loci from 480 swordfish from the Mediterranean, Atlantic and Pacific basins. Results from both types of genetic markers reject the idea that the Pacific is composed of a single panmictic population. However, complications arising from recent common ancestry, population expansion, and contemporary population overlap make it difficult to tell a simple story of stock structure. In addition, differential evolutionary histories of mitochondrial and nuclear genomes add an extra layer of complexity to the analysis. Nonetheless, significant differences exist between some populations. Mitochondrial DNA (mtDNA) shows that the western Pacific is subdivided into northern and southern populations while microsatellites fail to support this divergence. Because mtDNA is maternally inherited, this discrepancy may be best explained by greater philopatric behavior in females than males in the western Pacific which would be reflected in the mitochondrial but not nuclear genome. In the southeastern Pacific, microsatellite loci provide evidence that populations of Chile and Ecuador are different from the rest of the Pacific. Our results further suggest that California and Mexico samples contain a mixture of these two genetic groups. MtDNA does not provide support for subdivision in this region but this genome evolves 10-100 times slower than microsatellites. It may be that population subdivision in the southeastern Pacific is a recent event in which case a lack of mutation in a slower evolving genome has prevented differences that truly exist from being detected. Finally, comparison of Pacific populations to those of the Atlantic and Mediterranean indicate that through time most of the genetic diversity in this species has been maintained in the Atlantic. In addition, the Atlantic is more highly structured than the Pacific. Although genetic nuances characteristic of nuclear and mitochondrial markers can complicate the interpretation of results between the two genomes, when used together these markers provide a powerful to tackle difficult questions about population structure in a highly migratory pelagic fish species.

Plans for Future

We are applying for more funding from JIMAR to analyze regions of the Pacific where swordfish mixing occurs. In particular we think it would be valuable to examine California populations in the context of the results indicating that they are most likely mixtures of migrating fish from the centera1 Pacific and the southeastern Pacific. To do this project we propose to collect swordfish samples from California to Chile over a two year period, with analysis using our newly developed microsatellite markers. The project should discern at one times the catch within the US EEZ is made up of western migrants and at what time the catch is made up of southeastern Pacific swordfish migrants.


Dr. Reeb, the postdoctoral associate on the project is currently writing up several for submission to peer reviewed journals at this time. We anticipate 2 publications from the research.

Other Papers

Reeb, C. and B. A. Block. (1997) The usefulness of mitochondrial DNA studies to define management units of the swordfish, Xiphias gladius: A review of the current literature. International Commission for the Conservation of Atlantic Tunas. Collective Volume of Scientific Papers V. XLVI (3) 390-392.

PORF logo Pacific Ocean Research Foundation (PORF)

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This page updated August 16, 2006