An Assessment of Atlantic Bluefin Tuna (1994)

Tuna populations have been studied with immunologically detectable blood markers, protein electrophoresis, and restriction enzyme analysis and direct sequencing of mtDNA. The first genetic study of tuna (Cushing, 1956) demonstrated variability in red blood cell antigens that are analogous to human blood types. Subsequently, several workers (Suzuki, 1962; Fujino, 1970; Suzuki et al., 1958, 1959) attempted to assay blood group variability with immunological methods to study population structure but were largely unsuccessful in resolving intraocean population structure. More recently, protein electrophoresis and mtDNA restriction enzyme analysis have been used to search for population differences. The results for three tunas, yellowfin (Thunnus albacares), albacore (Thunnus alalunga), and skipjack (Katsuwonus pelamis), are reviewed here because of the similarity of these species to bluefin tuna, and because the results may give insight into the possible population genetic structure of bluefin tuna.

Scoles and Graves (1993) examined RFLPs of mtDNA from 20 yellowfin tuna sampled from each of five widely spread Pacific Ocean localities (n = 100) and one Atlantic Ocean locality (n = 20). Although they found high levels of

genetic diversity, there was no evidence of genetic differentiation among samples; common haplotypes occurred in similar frequencies in all samples. These results are consistent with high levels of gene flow among localities throughout the Pacific Ocean and between Pacific and Atlantic Ocean localities. Ward et al. (1994) examined four polymorphic loci encoding allozymes and mtDNA with two informative restriction enzymes among seven samples from the western, central, and eastern Pacific Ocean, and a fifth polymorphic enzyme in eastern and central Pacific samples. They found no significant frequency differences among localities in mtDNA haplotypes or allozymes at four loci. The frequencies for GPI-F, however, were not significantly different between two eastern Pacific samples (southern California and southern Mexico), but were significantly different between these samples and samples from the central and western Pacific Ocean (Coral Sea, Philippines, Kiribati) and two samples taken near Hawaii. Their results were consistent with an earlier study (Sharp, 1978) in identifying heterogeneity between these areas. Ward et al. (1994) concluded that gene flow between eastern and western Pacific yellowfin tunas was severely restricted, with only a few individuals per generation moving between the two regions. The lack of concordance of the other four polymorphic loci and the mtDNA haplotypes with the PGI-F locus, suggests minimally that additional study of mtDNA in yellowfin tuna is warranted. The study of Ward et al. (1994) does emphasize the need for multiple molecular genetic techniques for examining the stock structure of a given species (i.e., the same conclusion may not have been reached with the use of any one technique).

The combined results of Suzuki et al. (1958, 1959), Suzuki (1962), and Fujino (1970) for the Tg blood group of albacore tuna showed little allele-frequency heterogeneity between albacore tuna sampled from the north and south Pacific Ocean, suggesting that fish in this area consist of a single, genetically homogeneous population. There were, however, allele-frequency shifts between samples from the Atlantic and Indian Oceans, indicating population-level differentiation. In another study of albacore tuna, Keyvanfar (1962) found significant frequency differences in blood group alleles between samples from the Atlantic Ocean and the Mediterranean Sea. He also found qualitative immunodiffusion differences between albacore tuna from the Atlantic Ocean and Mediterranean Sea; Atlantic fish had an antigen that was apparently lacking in Mediterranean fish. The genetic basis of this difference is unknown. Graves and Dizon (1989) analyzed mtDNA between albacore tuna from southern Africa (n = 11) and San Diego (n = 12). They found six fragment length variants in individual fish but virtually no differentiation between samples from the Atlantic and Pacific Oceans. The high proportion of shared haplotypes is strong evidence for recent or ongoing gene flow between oceans. Similar results have recently been obtained

with other highly migratory fishes that inhabit temperate seas (Graves and McDowell, 1994). However, given the small sample size of the study, the results are not definitive.

Skipjack tunas (Katsuwonus pelamis) are more tropical and have a more restricted temperature range than bluefin tuna. An analysis of the allelic frequencies of transferrin in skipjack tuna (Fujino and Kang 1968b) revealed a significant difference (G = 22.12, degrees of freedom = 2, P < 0.01) between samples from Hawaii (n = 2,257) and the eastern Pacific Ocean (n = 175). There were no significant differences, however, between samples from the eastern (n = 2,432) and western (n = 3,792) Pacific Ocean or between samples from the Atlantic (n = 213) and Pacific (n = 4,328) Oceans. Richardson (1983) made an extensive study of 42 isozyme loci in 70 skipjack samples collected throughout the Pacific Ocean and found a longitudinal cline across the central and southwestern Pacific Ocean for an esterase locus and a locus encoding guanine deaminase. The distribution of allelic frequencies appeared to fit an isolation-by-distance model of migration, and estimates of genetic neighborhood size were about 2,000km. This estimate was similar to neighborhood sizes estimated from tagging data. These results indicate that the dispersal range of skipjack is restricted enough to allow genetic differences to accumulate among regions in the Pacific Ocean.

Two studies have searched for differences between Pacific and Atlantic skipjack populations. Fujino (1969) concluded from the analysis of serum esterase frequencies and three blood group systems that skipjack tuna from the Pacific Ocean (n = 1,080) were distinct genetically from those in the Atlantic Ocean (n = 127). More recently, Graves et al. (1984) used nine restriction enzymes to search for mtDNA sequence variability in skipjack tuna from Hawaii (n = 9), Brazil (n = 6), and Puerto Rico (n = 1). Although they found polymorphisms within each ocean, sequence divergence between samples from the Atlantic and Pacific Oceans was 0.0% (i.e., there were no demonstrable mtDNA differences). However, the number of individuals examined by Graves et al. (1984) was limited.

Recent studies on the population genetics of billfishes (Xiphiidae and Istiophoridae) provide a basis for comparison, given that many species of billfishes are highly migratory and occupy pelagic habitats similar to those occupied by tunas. Finnerty and Block (1992) sequenced 612 bp of the cytochrome b gene from 26 blue marlin (Makaira nigricans ) from the Pacific Ocean (n = 14) and the Atlantic Ocean (n = 12) and found two distinct lineages (Pacific Ocean or ubiquitous and Atlantic Ocean only) that differed by at least nine substitutions. Maximum-parsimony

analysis of blue marlin cytochrome b variants revealed the occurrence of two major evolutionary lines. The frequencies of the haplotypes in the Atlantic Ocean and Pacific Ocean samples were significantly different (P <0.05). Direct sequencing of the control region of mtDNA, a region of higher variability, provided a similar result: a ubiquitous blue marlin mtDNA lineage found in both ocean basins and a unique mtDNA lineage specific to the Atlantic Ocean (n = 40 [Meyer et al., unpublished results]). Graves and McDowell (1994) examined mtDNA sequence variability using RFLPs among Atlantic (n = 56) and Pacific (n = 58) samples. A single group of closely related haplotypes was found among samples from different oceans, and a discrete mtDNA haplotype that differed by several restriction sites changes was identified in a subset of the Atlantic samples. A similar result was found in another species of warm temperate to tropical billfish, the sailfish (Istiophorus platypterus). In blue marlin, two distinct evolutionary lineages with historical roots in the separate ocean basins (Pacific Ocean versus Atlantic Ocean) are hypothesized to be associated with formation of the land bridge between ocean basins and the temperature distribution of the species which limits intermixing to South African waters (Finnerty and Block, 1992).

The striped marlin (Tetrapturus audax), a cold temperate marlin thought to be restricted to the Pacific Ocean basin, has been examined in three studies (Block et al., 1993; Finnerty and Block, 1994, Graves and McDowell,² unpublished data). A surprising result has been the inability to genetically distinguish the striped marlin from the morphologically distinct white marlin (Tetrapturus albidus), a temperate-water marlin of the Atlantic Ocean. Direct sequencing of 612 bp of the cytochrome b gene in striped marlin (n = 2) from the Pacific Ocean and white marlin (n = 2) from the Atlantic Ocean revealed a sequence divergence of <0.1%. In a larger study, Graves and McDowell (1994) used mtDNA RFLP analysis of white and striped marlin and found no mtDNA differences between the two species. Within the Pacific Ocean basin, Graves and McDowell (1994) examined samples taken from four localities in the Pacific Ocean (near Mexico, Ecuador, Australia, and Hawaii) and found significant differences among the frequencies of composite mtDNA haplotypes. The significant heterogeneity evident in the striped marlin sample may be due to differences in the absolute number of migrants exchanged between ocean populations or an underlying behavioral difference associated with fidelity to a spawning ground in this species.

Swordfish (Xiphias gladius) are ecologically distinct from istiophorid billfishes and have a cosmopolitan distribution and wider temperature tolerance due to a unique endothermic strategy (Carey, 1982; Block et al., 1993). Tag and

release programs have not revealed extensive large-scale transoceanic migratory movements, but instead have demonstrated predominantly latitudinal north and south movements. Recent studies have attempted to examine the genetic structure of swordfish. Magoulas et al. (1992) used two informative restriction enzymes to examine mtDNA variation among three samples of swordfish from the Mediterranean Sea (near Spain, n = 94; near Greece, n = 73; and near Italy, n = 75), two samples from the eastern Atlantic Ocean (near Gibralter, n = 40), and one sample from the Gulf of Guinea (n = 95). The Mediterranean samples did not differ significantly from one another in mtDNA genotype frequency. The sample from the Gulf of Guinea differed significantly in haplotype frequencies from all other samples, minimally suggesting the existence of two discrete swordfish populations. Grijalva-Chon et al. (in press) surveyed mtDNA RFLP variability in samples from the western (n = 42), central (n = 42), and eastern (n = 59) Pacific Ocean and found no significant difference among haplotypic frequencies. Rosel³ and Block (unpublished results) sequenced the D-loop region from a worldwide sample of 150 swordfish and found evidence for both global and interocean mixing of populations as well as discrete clades unique to ocean basins.

Molecular genetic studies of istiophorid billfishes (marlin and sailfish) have revealed more genetic structure than for tunas. In contrast, swordfish throughout the Atlantic and Pacific Oceans have a more homogeneous genetic stock structure. This observed difference in population structuring among istiophorid billfishes, swordfish, and tunas may reflect differences in exchange rates between ocean basins that ultimately may be associated with the thermal ecology/physiology of each species (many of the tuna species and swordfish have endothermic capabilities that allow them to move through wider temperature gradients). In the case of the swordfish, Xiphias gladius, controversy exists as to whether there are one or two stocks in the Atlantic Ocean. In contrast to bluefin tuna, however, several molecular genetic studies have been conducted in the past three years (see preceding paragraph).

Alvarado-Bremer, J.R. 1992. Stock differentiation of Atlantic swordfish using mitochondrial DNA analysis. ICCAT Coll. Vol. Sci. Pap. XXXIX(2):607-614 (SCRS/91/48) (Rev).

Baglin, Jr., R.E. 1982. Reproductive biology of western Atlantic bluefin tuna. Fish. Bull. 80: 121-134.

Bartlett, S.E., and W.S. Davidson. 1991. Identification of Thunnus tuna species by the polymerase chain reaction and direct sequence analysis of their mitochondrial cytochrome b genes. Can. J. Fish. Aquat. Sci. 48: 309-317.

Block, B.A., J.R. Finnerty, A.F.R. Stewart, and J. Kidd. 1993. Evolution of endothermy in fish: Mapping physiological traits on a molecular phylogeny. Science 260: 210-214.

Brunemeister, S. 1980. A summary and discussion of technical information pertaining to the geographical discreetness of Atlantic bluefin tuna resources. ICCAT Coll. Vol. Sci. Pap. IX:506-527 (SCRS/79/95).

Calaprice, J.R. 1986. Chemical variability and stock variation in northern Atlantic bluefin tuna. ICCAT Coll. Vol. Sci. Pap. XXIV(2):222-254 (SCRS/85/36).

Calaprice, J.R., H.M. McShefrey, and L.A. Lapi. 1971. Radioisotope X-ray fluorescence spectrometry in aquatic biology: a review. J. Fish. Res. Board Can. 28: 1583-1594.

Carey, F.G. 1982. A brain heater in Swordfish. Science 216, p.1327(3).

Chow, S., and S. Inoue. 1993. Intra-and interspecific restriction fragment length polymorphism in mitochondrial genes of Thunnus tuna species. Bull. Nat. Res. Inst. Far Seas Fish. 30: 207-225.

Clay, D., ed. 1990. Atlantic bluefin tuna (Thunnus thynnus (L.)): A review. World Bluefin Meeting, May 25-31, La Jolla, Calif. pp. 89-180.

CLIMAP. 1976. The surface of the ice-age earth. Science 191: 1131-1136.

Committee on Fisheries. 1994. Improving the management of U.S. marine fisheries. National Academy Press. 82 pp.

Cort, J.J., and B. Liorzou. 1990a. Larval biology-Eastern Atlantic and Mediterranean. In: World Bluefin Meeting, May 25-31. (ed. D. Clay). LaJolla, Calif. p. 95

Cort, J.J., and B. Liorzou. 1990b. Reproduction-Eastern Atlantic and Mediterranean. In: World Bluefin Meeting, May 25-31. (ed. D. Clay). La Jolla, Calif. pp. 99-101.

Cort, J.J., and B. Liorzou. 1990c. Tagging interpretation-Eastern Atlantic and Mediterranean. In: World Bluefin Meeting, May 25-31. (ed. D. Clay). La Jolla, Calif. pp. 110-127.

Cort, J.L., and J.M. de la Serna. 1993. Revision de los datos de marcado/recaptura de atun roho (Thunnus thynnus L.) en el Atlantico Este y Mediterraneo. ICCAT.

Cushing, J.E. 1956. Observations on serology of tuna. U.S. fish Wildl. Serv., Spec. Sci. Rept. Fish. 183. 14 pp.

Dicenta, A., C. Piccinetti, et al. 1980. Comparison between the estimated reproductive stocks of bluefin tuna (T. thynnus) of the Gulf of Mexico and western Mediterranean. ICCAT Coll. Vol. Sci. Pap. IX:442-448 (SCRS/79/45).

Dizon, A.E., C. Cockyer, W.F. Perrin, D.P. Demaster, and J. Sisson. 1992. Rethinking the stock concept: a phylogenetic approach. Conserv. Biol. 6: 24-36.

Edmunds, P.H. and J.I. Sammons, 111. 1971. Genic polymorphism of tetrazolium oxidase in bluefin tuna, Thunnus thynnus, from the western North Atlantic. J. Fish. Res. Board Can. 28: 1053-1055.

Edmunds, P.H. and J.I. Sammons, 111. 1973. Similarity of genic polymorphisms of tetrazolium oxidase in bluefin tuna, Thunnus thynnus from the Atlantic Coast of France and the western North Atlantic. J. Fish. Res. Board Can. 30: 1031-1032.

Finnerty, J.R. and B.A. Block. 1992. Direct sequencing of mitochondrial DNA detects highly divergent haplotypes in blue marlin (Makaira nigricans ) Mol. Mar. Biol. Biotechnol. 1: 206-214.

Fujino, K. 1969. Atlantic skipjack tuna genetically distinct from the Pacific specimens. Copeia 1969(3): 626-629.

Fujino, K. 1970. Skipjack subpopulation identified by genetic characteristics in the western Pacific. Proc. CSK Symp., East-West center, Honolulu, Hawaii, April 29-May 2, 1968.

Fujino, K. 1970. Immunological and biochemical genetics of tunas. Trans. Am. Fish. Soc. 99(1): 152-178.

Fujino, K., and T. Kang. 1968a. Serum esterase groups of Pacific and Atlantic tunas. Copeia 1968(1): 56-63.

Fujino, K., and T. Kang. 1968b. Transferrin groups of tunas. Genetics 59: 79-91.

Grant, W.S. 1984. Biochemical population genetics of Atlantic herring, Clupea harengus, Copeia 1984: 355-362.

Grant, W.S. and G. Stahl. 1988. Evolution of Atlantic and Pacific cod: Loss of genetic variation and gene expression in Pacific cod. Evolution 42: 138-146.

Graves, J.E., S.D. Ferris, and A.E. Dizon. 1984. Close genetic dimilarity of Atlantic and Pacific skipjack tuna (Katsuwonus pelamis) demonstrated with restriction endonuclease analysis of mitochondrial DNA. Mar. Biol. 79: 315-319.

Graves, J.E., and A.E. Dizon. 1989. Mitochondrial DNA sequence similarity of Atlantic and Pacific albacore tuna (Thunnus alalunga) Can. J. Fish. Aquat. Sci. 46: 870-873.

Graves, J.E. and J.R. McDowell. 1994. Genetic analysis of striped marlin Tetrapturus audax population structure in the Pacific Ocean. Can. J. Fish. Aquat. Sci. In press.

Graves, J.E. and J.R. McDowell. submitted. Inter-Ocean genetic divergence of istiophorid billfishes.

Grijalva-Chon, J.M., K. Numachi, 0. Sosa-Nishizaki, and J. de la Rosa-Velez. 1994. Mitochondrial DNA analysis of north Pacific swordfish (Xiphias gladius) population structure. Mar. Ecol. Prog. Ser., in press.

Gunderson, T.P., R.G. Fairbanks, and J.L. Rubenstone. 1994. Tropical temperature variations since 20,000 years ago: modulating interhemispheric climate change. Science 263:663-665.

Gutierrez. M. 1967. Estudio hematologicos en el atun, Thunnus thynnus (L.). Immunologia: Grupos sanguineos, eritrocitos y fitoaglutininas. Invest. Pesq. 31:137-143.

Ihssen, P.E., H.E. Booke, J.M. Casselman, J.M. McGlade, N.R. Payne, and F.M. Utter. 1981. Stock identification: Materials and Methods. Can. J. Fish, Aquat. Sci. 38:1838-1855.

Keyvanfar, A. 1962. Serologie et immunoligie de deux especes de thonides (Germo alalunga Gmelin et Thunnus thynnus Linne) de I'atlantique et del la Mediterranee. Rev. Trav. Inst. Peches. Mar. 26(4): 407-456.

Kimura, M. 1955. Solution of a process of random genetic drift with a continuous model. Proc. Natl. Acad. Sci. USA 41: 144-150.

Lee, J.Y. 1965. Observations sur la serologie et l'immunologie des thons rouges (Thunnus thynnus Linne) de Mediterranee. Rapp. Proc.-Ver. Reunions Comm. Int. Explor. Sci. Mer Mediter. 18: 225-228.

Magoulas, A., G. Kotoulas, J.M. de la Sema, G. de Metrio, N. Tsimendides, and E. Zouros. 1992. Genetic structure of swordfish (Xiphias gladius ) populations of the Mediterranean and the eastern side of the Atlantic: Analysis by mitochondrial DNA markers. ICCAT Coll. Vol. Sci. Pap. XL(1):126-136 (SCRS/92/84).

Mather, F.J. 1980. A preliminary note on migratory tendencies and distributional patterns of Atlantic bluefin tuna based on recently acquired and cumulative tagging results. ICCAT Coll. Vol. Sci. Pap. XL (2):478-491 (SCRS/79/76).

Mather, F.J., J.M. Mason, and A.C. Jones. 1974. Distribution, fisheries and life history data relevant to identification of Atlantic bluefin tuna stocks. ICCAT Coll. Vol. Sci. Pap. II:234-258 (SCRS/73/54).

Murphy, G.I. 1990. A review of the evidence of stock structure in Atlantic bluefin tuna with an altimate stock hypothesis. Unpbl. MS, Div. Environ. Stud., Univ. California, Davis, California, USA.

Parrack, M.L. 1990. Tagging applications-Western Atlantic. In: World Bluefin Meeting, May 25-31. (ed. D. Clay). La Jolla, Calif. pp. 104-109.

Phipps, M. 1980. Preliminary studies of population structure and mortality of bluefin tuna (Thunnus thynnus) in Nova Scotia, Canada. MSc. thesis, University of Guelph.

Richards, W.J. 1976. Spawning of bluefin tuna (Thunnus thynnus) in the Atlantic Ocean and adjacent seas. ICCAT Coll. Vol. Sci. Pap. V(2):267-278 (SCRS/75/97).

Richards, W.J. 1987. Mexus-Gulf ichthyoplankton research, 1977-1984. Mar. Fish. Rev. 49: 39-41.

Richards, W.J. 1990. Early life history-Western Atlantic. In: World Bluefin Meeting, May 25-31, (ed. D. Clay). La Jolla, Calif. pp. 93-94.

Richardson, B.J. 1983. Distribution of protein variation in skipjack tuna (Katsuwonus pelamis) from the central and southwestern Pacific. Australian Journal of Marine and Freshwater Research. 34:231-251.

Rodriquez-Roda, J. 1967. Fecundidad del atun, Thunnus thynnus (L.), de la costa sudatlantica de Espana. Invest. Pesq. 31: 33-52.

Rodriquez-Roda, J. 1971, Investigations of tuna (Thunnus thynnus) in Spain. ICCAT Report for biennial period, 1970-1971, Part 11: 110-113.

SCRS. 1992. Report of the ICCAT Standing Committee on Research and Statistics, Selected pages on bluefin tuna.

SCRS. 1993. Report of the ICCAT Standing Committee on Research and Statistics, Selected pages on bluefin tuna.

Scoles, D.R., and J.E. Graves. 1993. Genetic analysis of the population structure of yellowfin tuna, Thunnus albacares, from the Pacific Ocean. Fish. Bull. 91: 690-698.

Sharp, G.D., and A.E. Dizon. 1978. The physiological ecology of tunas proceedings of the tuna physiology workshop. Jan. 10-15, 1977 . La Jolla, CA. 485pp.

Smith, A. C. 1962. The electrophoretic characteristics of albacore, bluefin tuna, and kelpbass eye-lens proteins. Calif. Fish Game 48(3): 199-201.

Smith, A. C. 1965. Intraspecific eye-lens protein differences in yellowfin tuna, Thunnus albacares. Calif. Fish Game 51(3): 163-167.

Smith, A. C. 1966. Electrophoretic studies of solubleprotein from lens-nuclei of bluefin tuna, Thunnus thynnus, from California and Australia. Amer. Zool. 6: 333 (Abstr.)

Sprague, L. M. 1962. Blood group studies of albacore (Germo alualunga ) tuna from the Pacific Ocean, p. 37. In: Pacific Tuna Biology Conference, Aug. 14-19, 1961. Honolulu, Hawaii (ed. J.C. Marr). U.S. Fish Wildl. Serv., Spec. Sci. Rept. Fish. 415. (Abstr.) p. 37.

Suzuki, A. 1962. Serological studies of the races of tuna VI. Bigeye-3 antigens occurred in the albacore. Rep. Nankai Reg. Fish. Res. Lab. 16: 67-70.

Suzuki, A., T. Morio, and K. Mimoto. 1959. Serological studies of the races of tuna 11. Blood group frequencies of the albacore in Tg system. Part 1. Comparison of the Indian and the northwestern Pacific Ocean. Rep. Nankai Reg. Fish. Res. Lab. 11: 17-23.

Suzuki, A., Y. Shimizu, and T. Morio. 1958. Serological studies of the races of tuna 1. The fundamental investigations and the blood groups of albacore. Rep. Nankai Reg. Fish. Res. Lab. 8: 104-116.

Suzuki, Z. 1990. Reproduction-Western Atlantic. In: World Bluefin Meeting, May 25-31. (ed. D. Clay). La Jolla, Calif. pp. 97-99.

Tiews, K. 1957. Biologische Untersuchungen am Roten Thun (Thunnus thynnus, Linnaeus) in der Nordsee. Ber. Dtsch. Wiss. Komm. Meeresforsch. 14(3): 192-220.

Tiews, K. 1963. Synopsis of biological data on bluefin tuna Thunnus thunnus (Linnaeus) 1758 (Atlantic and Mediterranean). FAO Fish. Biol. Synop. No. 56, pp. 422-481.

Utter, F. 1981. Biological criteria for definition of species and distinct intraspecific populations of anadromous salmonids under the U.S. Endangered Species Act of 1973. Can. J. Fish. Aquat. Sci. 38: 1626-1635.

Walters, V. Ectoparasites of eastern and western Atlantic bluefin tunas . ICCAT Coll. Vol. Sci. Pap. IX(2):491-498 (SCRS/79/82).

Waples, R. S. 1991. Pacific salmon, Oncorhynchus spp., and the definition of ''species" under the endangered species act. Mar. Fish. Rev. 53: 11-22.

Ward, R. D., N. G. Elliott, P. M. Grewe, and A. J. Smolenski. 1994. Allozyme and mitochondrial DNA variation in yellowfin tuna (Thunnus albacares) from the Pacific Ocean. Mar. Biol. 118:531-539.