(Richards, 1990). No bluefin tuna larvae have been found in the eastern Atlantic Ocean (Cort and Loirzou, 1990a), and it has been assumed that bluefin tuna do not spawn there.

The western Mediterranean Sea and the Adriatic Sea appear to constitute the second major spawning location for bluefin tuna, but the highest concentrations of bluefin larvae are found in the central part of the western basin of the Mediterranean Sea between southern Italy and Sardinia and around the Balearic Islands of Spain (Cort and Loirzou, 1990a).

There are some key questions regarding these spawning locations. What are the biological requirements for spawning, and what environmental factors trigger spawning? How much spawning occurs in one location relative to another, and does it vary from year to year because of changes in environmental conditions? A scientific effort should be made to learn the biological and environmental requirements of spawning. Some scientific effort should also be directed toward estimating the relative amounts of spawning in the two locations, by using identical survey methods in both locations.

Most aspects of spawning in Atlantic bluefin tuna are still unknown because spawning has not been observed. It is not known if bluefin tuna spawn once or many times per season or whether an individual spawns yearly. It is also not known whether individuals can spawn in the east and then in the west at different times. Spawning in the Gulf of Mexico reportedly occurs from mid-April to mid-June (Richards, 1990; Dicenta et al., 1980). Spawning in the Mediterranean Sea is thought to occur from June to August (Rodriguez-Roda, 1971; Dicenta et al., 1980; Cort and Loirzou, 1990b). It is not known, however, whether later spawning times for the Mediterranean fish are based on genetic differentiation or whether they are in response to environmental differences between the two locations. Differences in spawning times do not necessarily indicate that bluefin tuna produced in the Gulf of Mexico and maturing there or in the western Atlantic Ocean would be precluded from spawning, as adults, in the Mediterranean Sea, or vice versa. Tagging experiments demonstrated that fish can cross the Atlantic Ocean in less than 60 days. It is possible for a fish to spawn in the west in April, migrate to the east, and arrive in time to spawn in the east the same year.

Earlier studies providing information on water temperature during spawning of bluefin tuna are referred to in Tiews' (1963) review of biological data on bluefin tuna. A range of 24.9°C to 29.5°C is reported for the Straits of Florida, from Havana to Bimini (Rivas 1954). In the central Mediterranean Sea, the reported range is 19°C to 21.6°C (Roule 1924). Also, for the Mediterranean Sea, large and small sized, sexually mature bluefin tuna spawn at different water

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)