This section focuses on aspects of life history of Atlantic bluefin tuna that may be relevant to management. All known aspects of bluefin tuna life history have been discussed in detail by Clay (1990) and will not be reviewed here. The committee holds the view that there are aspects of life history that may influence catch per unit effort (CPUE) or other indices used in stock assessment models. We also argue that there are important aspects of life history that are not considered in ICCAT's Standing Committee on Research and Statistics (SCRS) deliberations. Further, much of the life history of Atlantic bluefin tuna is not known.

Life history variables such as age composition, growth, age at maturity, and mortality have been used to infer the population structure of several fish (Ihssen et al., 1981). When used as evidence for two populations, these measures are indirect indicators of possible genetic differences. They also can reflect individual responses to environmental differences among localities, so that conclusions concerning population structure based on these data are suspect since they cannot differentiate between genetic and environmental influences. If differences in life history variables result entirely from environmental factors, the two-population hypothesis cannot be tested with these values. In theory, natural selection, genetic drift, and migration between or among localities determine the degree that life history variables change from one locality to the next: high levels of migration tend to minimize differences among localities, whereas extremely low levels of migration could allow population differences to appear in only a few generations.

Although bluefin tuna have been found as far north as Newfoundland in the western Atlantic Ocean and as far north as Norway in the eastern Atlantic Ocean, and a fishery existed for a short time as far south as Brazil, extensive searching has detected only two spawning localities: the Gulf of Mexico and the Mediterranean Sea (Figure 2-1). Each of these localities is large relative to the spawning areas of many other fish species, but small relative to the spawning areas of tropical tunas. Individual females in both the east and the west produce about 30,000,000 eggs each (Clay, 1990). There is no evidence that the large geographic separation of the spawning localities represents reproductive separation.

Richards has reviewed evidence of spawning in the Gulf of Mexico (Richards, 1976) and summarized results of ichthyoplankton surveys in the western Atlantic Ocean (Richards, 1987). Larvae and juveniles are found primarily in the northern region of the Gulf of Mexico, with sporadic occurrences in the Florida Straits and off the Texas coast. Larvae have been sampled off the Carolina coast in the western Atlantic Ocean, but their presence there may result from advection by currents from the Florida Straits and not from local spawning

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)