Mediterranean bluefin tuna; (2) possible differences in spawning productivity exist; and (3) the Gulf Stream, via its extension in the North Atlantic Current, could transport larval and postlarval bluefin tuna into the middle of the Atlantic Ocean.

Molecular techniques that assay genetic variability have an obvious advantage over techniques that measure life history traits where the genetic component is unknown. Such techniques can be used to quantify genetic divergence and gene flow among populations and to estimate breeding structure within populations. There are four criteria for using a molecular method to find genetic markers that may be used to define fish management units: (1) expression of the genetic markers does not change during the life of an individual; (2) barring mutation, markers are inherited unchanged from one generation to the next; (3) it is possible to assay a large number of individuals from a large number of localities to adequately resolve the genetic structure of one population; and (4) there is sufficient within-population variability to make make robust statistical tests of geographic structure.

Data resulting from methods satisfying these criteria can be analyzed in several ways. If genotypic data from nuclear genes (allozymes) of randomly sampled individuals surveyed with protein electrophoresis are available, a contingency table analysis of gene frequencies can be used to test for homogeneity among different sampling localities. If significant differences are found, and one can assume migration-drift equilibrium, then one can infer that the samples were drawn from genetically discrete populations. A caveat is that large samples must be used to detect small but significant gene-frequency differences between or among areas. Another test for geographic structure is to compare the observed numbers of genotypes (AA, AB, BB, etc.) in a pooled sample with the number expected from random mating (Hardy-Weinberg proportions),

where p is the frequency of the A allele and q is the frequency of the B allele. If there are regional genetic differences, the pooled sample will show a significant deficit of heterozygotes (Wahlund's effect) owing to mixing of individuals from discrete populations. This test, however, lacks power to detect small but significant genetic differences between populations. This significant deficit is taken as evidence for genetic differentiation between or among populations.

More recently, methods for detecting nucleotide sequence variability have been used to study fish populations. Most of this effort has been directed toward the analysis of mitochondrial DNA (mtDNA), a circular piece of DNA found in

Front Matter (R1-R18)

Summary (1-4)

1 Background (5-8)

2 Historical Evidence for Stock Structure (9-38)

3 Transatlantic Movement of Atlantic Bluefin Tuna (39-78)

4 Fish Stock Assessment (79-108)

5 Conclusions and Recommendations (109-112)

References (113-118)

Appendix A: Biographical Sketches of Committee Members (119-122)

Appendix B: Bibliography (123-126)

Appendix C: Genetic Variation in Other Tunas and Related Fish (127-134)

Appendix D: Archival Tag Technology (135-138)

Appendix E: Evidence for Mixing Based on Parasites (139-146)

Appendix F: Microconstituent Analysis (147-148)