Figure 74. Geographic distribution of bluefin tag release and recapture data from 

 Moroccan and Bay of Biscay tagging studies. 



only to reappear in the Bay of Agadir 

 in November and December. These 

 bluefin which have attained a mean 

 length of 67 cm then undertake a 

 longer migration, passing along the 

 Portuguese coast and penetrating 

 deeply into the Bay of Biscay. 



In certain years, as in 1 973, when 

 the cold waters of the central sector 

 extended far offshore, the tuna may 

 have migrated earlier in the season, 

 bypassing the Agadir area. 



These results of tagging off Por- 

 tugal and Morocco and Lamboeuf s 

 (1975) hypothesis explain Fumestin 

 and Dardignac's (1962) observation 

 that young bluefin become scarce 

 along the Moroccan coast in Novem- 

 ber, when they have attained a length 

 of 64 cm, and confirm Aloncle's 

 (1964) hypothetical migration of 

 young bluefin from Moroccan wa- 

 ters to the Bay of Biscay. 



After feeding in the Bay of 

 Biscay through the remainder of the 

 warm season, the age 2 fish depart, 

 along with fish a year or two older, 

 mainly in October. Their next desti- 

 nation is their wintering area, which 

 is little known. There is a wintering 



zone between the Canary Islands and 

 Morocco, but the total winter habitat 

 is probably much larger, with the fish 

 spreading over greater areas as they 

 attain older ages. 



Age 3 and 4, and older, bluefin 

 move northward again in the spring; 

 probably the majority of them even- 

 tually enter the Bay of Biscay. Many 

 of the older individuals in this group 

 are mature. Some of these may spawn 

 during this northward migration, and 

 others may split off from it and spawn 

 with the larger fish in the Ibero-Mo- 

 roccan Bay. During late summer and 

 fall, considerable aggregations of 

 small and medium bluefin occur off 

 the central Moroccan coast, as noted 

 previously. Some of these fish may 

 have been trapped there b> a warm 

 water mass during their northward 

 movement and others may have mi- 

 grated there after spawning in the 

 Ibero-Moroccan Bay. Most of the 

 small bluefin which have spawned 

 there, however, probably proceed to 

 the Bay of Biscay, entering it during 

 the last half of July or early August. 

 Thus the hypothetical two-part mi- 

 gratory pattern of this group may be 



interrupted by local hydrological con- 

 ditions, and the mature individuals 

 are inclined toward the three-part mi- 

 gratory pattern proposed for the older 

 age groups. 



iii. Very Small Fish 



Eastern Atlantic bluefin (apply- 

 ing this term to individuals which are 

 recruited to stocks in the eastern At- 

 lantic as juveniles) are hatched mainly 

 in the Mediterranean Sea and/or the 

 Ibero-Moroccan Bay. Those born in 

 the former area, however, would have 

 to pass through the latter to reach 

 their nursery areas. Supplementary 

 hatching may extend to the area be- 

 tween the Canary Islands and Mo- 

 rocco, and also into the Bay of Biscay. 

 The principal hatching occurs from 

 about the middle of June to about the 

 middle of July, with reproduction by 

 the smaller individuals occurring up 

 to the end of July or later. By mid- 

 November, when they are about 40 

 cm long, the bulk of the new-born 

 tuna have converged from their vari- 

 ous bii1h places to concentrate off 

 the Atlantic coast of Morocco, where 

 they complete their first year of life 

 in the following June or July. 



2. Western Atlantic 



a. Introduction 



Tag returns, data on the fisher- 

 ies, and knowledge of oceanographic 

 conditions must be used in combina- 

 tion to establish hypothetical migra- 

 tion patterns for bluefin tuna. Since 

 1954, about 2,760 giant, 490 me- 

 dium, and 14,630 small bluefin have 

 been tagged in the western Atlantic 

 and 63, 9, and 3,000 returns, respec- 

 tively, have resulted. These returns 

 have not only provided important in- 

 formation on movements within the 

 area, but have also revealed surpris- 

 ing migrations to distant regions. 

 Extensive data on the coastal and oce- 

 an ic fisheries are available (see Sec- 

 tions IVC2 and 1VC3) to comple- 

 ment the pattern disclosed by tag- 

 ging. Also, comprehensive works on 

 the oceanography of the Atlantic, 

 with details of its circulatory system, 

 have been published (Sverdrup et al. 

 1942). Those concerning the Gulf 

 Stream system (Fuglister and 

 Worthington 1951, Von Arx et al. 

 1955) are of special interest in rela- 

 tion to the distribution and migra- 



112 



