An Assessment of Bigeye Tuna (Thunnus obesus) Population Structure in the Pacific Ocean based on Mitochondrial DNA and Microsatellite Variation

• Collect a series of approximately 100 fish samples from a number of different locations across the Pacific. Researchers hope to obtain samples from seven locations: the Phillipines, Federated States of Micronesia, Solomon Islands, Kiribati, Hawaii, Tahiti, and the eastern Tropical Pacific.
• Samples would comprise small fish measuring less than 50 cm FL to maximize the probability that the areas sampled represent the natal areas for the sampled fish. All sampling would take place over a three-month time frame.
• Samples would be collected utilizing existing observer and port sampling programs conducted by various agencies. These include the SPC, FFA, Bureau of Fisheries, Phillipine Aquatic Resources, Micronesian Maritime Authority, PFRP (David Itano), and IATTC.
• The samples would consist of small pieces of muscle tissue, along with data on time, place and fish size. Tissues would be preserved in alcohol.
• The samples would be analyzed by Dr. Peter Grewe of the CSIRO Division of Fisheries in Hobart. Both MtDNA and microsatellite techniques would be applied.

A final project report published as part of the SOEST-JIMAR publications series:
"An assessment of bigeye (Thunnus obesus) population structure in the Pacific Ocean, based on mitochondrial DNA and DNA microsatellite analysis", 1998. Peter M. Grewe and John Hampton. SOEST 98-05, JIMAR Contribution 98-320.
See SOEST-JIMAR Publications page for other PFRP reports.

The following excerpt is from the final project report submitted in 1998, entitled,

An Assessment of Bigeye (Thunnus obesus) Population Structure in the Pacific Ocean, Based on Mitochondrial DNA and DNA Microsatellites Analysis, Peter Grewe and John Hampton, 1998. SOEST Publication 98-05, JIMAR Contribution 98-320.
(Adobe PDF version of manuscript available at end of excerpt)

Introduction

Knowledge regarding population subdivision is central to sustainable fishery management. Uncertainty regarding bigeye stock structure seriously restricts the confidence that scientists and. fisheries managers can place in the regional assessments that have been carried out to date. At a national or sub-regional level, fisheries managers need to have a better idea of the broader surrounding population of bigeye from which the fish in their fisheries are drawn.

Examination of mitochondrial DNA markers is now an established technique for elucidating population genetic structure. While there is little differentiation among yellowfin tuna populations for mtDNA variants (Ward et al., 1994; Ward et al., 1997), there are specific mtDNA polymorphisms that are known to differentiate bigeye tuna from the Atlantic and Indian Oceans (Grewe, unpublished data). mtDNA variation can be used for both population structure analysis and confirmation of species identification. Usually around 3-5% (but sometimes as high as 30%) of tuna samples we receive have been mis-identified by collectors, but all can be unequivocally identified by mtDNA examination (Chow and Inoue, 1993).

Until recently, variation in the much larger nuclear genome has been chiefly assessed through allozyme electrophoresis, but more powerful methods are now available. Pre-eminent among these is the detection and analysis of microsatellite variation. Microsatellites show high levels of genetic variation and high mutation rates, meaning that populations are likely to diverge not only by genetic drift but by mutation as well. Microsatellite markers also have the important advantage over allozyme markers in being able to be screened in alcohol-stored tissue or even fin-clip samples. This greatly simplifies sampling logistics.

Microsatellite analysis is a new technique, and while there have as yet been few studies on fish populations, microsatellite screening in cod has revealed. substantially more about population structure than either allozyme or mtDNA analysis (Wright and Bentzen, 1994). Initial results from examination of DNA microsatellites in yellowfin tuna indicate more population subdivision is present in the western Pacific than is apparent &om either allozyme or mtDNA analysis (Grewe and Ward, unpublished data.). DNA microsatellite data were examined among yellowfin tuna samples &om five locations in the Pacific Ocean (Philippines, Coral Sea, Solomon Islands, Fiji, and California). Analysis of data from four DNA microsatellite loci indicated significant heterogeneity, on one locus between samples collected in the western and eastern Pacific, and on a second locus between samples collected in the Philippines and Solomon Islands and those collected in Fiji and the Coral Sea. Although preliminary, these data are the first indication of genetic structure within what has been assumed previously to be a single Western Pacific yellowfin tuna stock. The d.ata collected thus far are most encouraging, and suggest a finer-scale resolution of yellowfin tuna population structure within the Pacific Ocean than has hitherto been achieved with allozyme and mtDNA markers. These same microsatellite polymorphisms from yellowfin tuna can be used to examine bigeye tuna, substantially reducing the time and cost of the development phase associated with microsatellite analysis.

The present study examines variation of mitochondrial DNA and DNA microsatellite markers among bigeye tuna sampled from various regions throughout the Pacific ocean. Comparison of these marker frequencies among sampling sites provides an assessment of bigeye population structure in the Pacific Ocean.

Grewe & Hampton's bigeye tuna DNA manuscript

This page updated August 16, 2006