which have been used within recent 

 years were distributed from Cabo de 

 Santa Maria in Portugal to Tarifa, Spain, 

 and along the Moroccan coast from 

 Kenitra to Cape Spartel, Morocco (Fig- 

 ure 44). It should be noted that the two 

 latter localities are at the very entrance 

 of the Strait of Gibraltar. Some of the 

 Portuguese and Spanish traps were re- 

 arranged at the end of June to fish in the 

 "return" period. 



The gonad studies showed that the 

 "arrival" fish were maturing and that 

 their indices of maturity became lower 

 (indicating more advanced maturity) 

 as the season progressed. On the other 

 hand, the "return" fish were generally 

 spent, with their indices of maturity 

 increasing (indicating shrinking of the 

 gonads) toward the end of the season. 

 Since the more mature ovaries were an 

 important by-product of the fishery (F. 

 de Buen 1 928), these facts were known 

 to the fishermen for centuries before 

 the scientists arrived on the scene. In 

 fact, they may have played a role in the 

 development of Aristotle's migratory 

 theory. 



This simple combination of cir- 

 cumstances was interpreted in opposite 

 ways by the proponents of the "Atlan- 

 tic-Mediterranean migration" or "mi- 

 gratory" theory and the advocates of 

 "separate Atlantic and Mediterranean 

 populations" or "sedentary" theory. The 

 former maintained that these facts indi- 

 cated that many of the "arrival" tuna 

 entered the Mediterranean to spawn, 

 and that the "retum" run included many 

 individuals which had spawned in the 

 Mediterranean and were returning to 

 feeding areas in the Atlantic. The latter 

 believed that few, if any, of the "ar- 

 rival" fish passed through the Strait of 

 Gibraltar and that most of them 

 spawned in the Ibero-Moroccan Bay. 



Despite the many decades of in- 

 tensive research on bluefin tuna in the 

 area, however, no identifiable eggs, lar- 

 vae or juveniles (as defined here) of 

 bluefin tuna have been collected in tlie 

 Ibero-Moroccan Bay. 



In addition to the gonad studies, a 

 great deal of other biological informa- 

 tion pertinent to the possible spawning 

 of the species in this area has been 

 published. Extensive data on the prop- 

 erties of the environment are also avail- 

 able. 



Evidence of reproduction of the 

 species in other parts of the eastern 

 Atlantic, and in the central and the South 

 Atlantic (Figure 65), is ver> meager. 

 Some larvae have been collected in the 

 eastern equatorial Atlantic. A few oc- 

 currences of ripe bluefin in the Bay of 

 Biscay have been reported. Spawning 

 in the vicinity of the Azores has been 

 suggested. A spawning area between 

 Lanzarote, one of the Canary Islands, 

 Conception Bank, and the Moroccan 

 coast has been hypothesized, but not 

 investigated. 



b. Specific Occurrences 



From numerous studies of the con- 

 dition of the gonads of bluefin tuna 

 caught in the Ibero-Moroccan Bay, we 

 have selected for discussion those of 

 Frade and Manai;as (1933) and Frade 

 ( 1 937a) for Portugal, F. de Buen ( 1 927) 

 and Rodriguez-Roda ( 1 964a) for Spain, 

 and Lozano Cabo (1957, 1958) and 

 Fumestin and Dardignac (1962) for 

 Morocco. 



Frade and Mana^as (1933) pre- 

 sented one of the few histological stud- 

 ies on the stages of maturity of the 

 gonads of bluefin tuna which are avail- 

 able. Their observations confirmed the 

 fractional nature of the spawning of the 

 species. This accounts for the lack of 

 completely ripe fish in the traps, be- 

 cause all the eggs destined to be emit- 

 ted in a given season do not mature 

 simultaneously. A large number of the 

 individuals entering the traps in the 

 "arrival" period may have already emit- 

 ted part of their spawning products. In 

 addition the small bluefin I m long 

 (about 3 years of age) behave differ- 

 ently according to their sex. The males 

 are in active or terminated spermato- 

 genesis, whereas the females are in very 

 retarded ovulation. 



Later Frade (1937a) described a 

 simple method for determining stages 

 of maturity consistent with the abso- 

 lute ones established by his histologi- 

 cal studies (Frade and Mana^as 1933). 

 To offset the effect of seasonal varia- 

 tions in the length-weight ratio, his in- 

 dex of maturity was the quotient of the 

 weight of the head, rather than the total 

 weight ofthe fish, divided by the weight 

 of the ovaries. He also used a factor K 

 (the ratio ofthe weight of ovaries to the 

 weight of testes at the same stage, as 

 determined by histological studies, usu- 



ally about 1.5, to make the indices for 

 males comparable to those for females 

 at the same stage of maturity. Even 

 though these corrections did not pre- 

 vent the occurrence of similar indices, 

 in the intermediate stages, for pre- 

 spawning and post-spawning fish, he 

 felt that this procedure clearly showed 

 the progressive emptying of the go- 

 nads, or the fractional emission ofthe 

 spawning products. His stages are 

 shown in Table 17. 



Frade (1937a) presented the indi- 

 ces obtained for 171 bluefin tuna ex- 

 amined at Vila Real de Santo Antonio, 

 Portugal, from May 31 to August 18, 

 1933 (Table 18). These same values 

 are shown graphically in Figure 66. 

 Frade leached the following conclu- 

 sions: 



1) Ripe and intermediate stages (A- 



D) of both sexes are represented in 

 June, but in decreasing frequen- 

 cies. 



2) In July intermediate stages (C and 



E) predom inate among the females, 

 and in the males, late intermediate 

 and spent stages (E and F) appear. 



3) In August intermediate stages (D 

 and E) are predominant for the fe- 

 males while ripe and intermediate 

 stages (B and C) are still pre- 

 dominant among the males. Some 

 fish of both sexes have attained the 

 fully spent stage (G) and females 

 in the fiilly ripe stage (A) have 

 disappeared from the catches. 



F. de Buen ( 1 927) noted that blue- 

 fin tuna which had arrived to spawn 

 were caught in the southern Spanish 

 Atlantic traps in April, May and June, 

 and that they also entered these traps 

 beginning in July, after spawning. Af- 

 ter examining a large number of speci- 

 mens from these traps in 1923, he con- 

 cluded from the data that in 1923 the 

 bluefin spawned in the Ibero-Moroc- 

 can Bay in the months of June and July, 

 and that the males had attained sexual 

 maturity before the females (Table 1 9). 



In one of his major works on the 

 biology ofthe bluefin tuna taken in the 

 traps off the southern coast of Spain, 

 Rodriguez-Roda (1964a) provided 

 much infonnation on the variations in 

 gonad condition through the spawning 

 season. He set up the following scale 



73 



