knots (6.5 km/hr). These facts refute 

 the argument that no lag between the 

 catches in the respective localities 

 existed, or that it was so short that 

 impossible speeds would have been 

 required for a tuna to have accom- 

 plished the implied migration (Pavesi 

 1887, 1889, Roule 1925) This infor- 

 mation, combined with Sella's 

 (1929a) findings of Atlantic hooks 

 and lures in tuna caught in the Medi- 

 terranean and the tag returns show- 

 ing similar migrations by Rodriguez- 

 Roda (1969a), strongly supports the 

 old concept that numerous large blue- 

 fin migrate from the Atlantic into the 

 Mediterranean in May and June. The 

 departure of large bluefin from the 

 Mediterranean in the "return" period, 

 July and August, has been indicated 

 by rather small catches in the "re- 

 turn" traps near the eastern end of the 

 Strait (see Section I VC6a). The mod- 

 est size of these catches in compari- 

 son with those of the Ibero-Moroc- 

 can Bay traps may be explained by 

 Sari's (1973) hypothesis in regard to 

 the depths at which the bluefin travel 

 in the respective migratory periods. 

 He maintained that the "arrival" fish 

 swam in the surface layers, follow- 

 ing the inflowing Atlantic current. 

 This is in accord with the findings of 

 Lozano Cabo (1959b) during his so- 

 nar survey of the Strait. Sara (1973) 

 assumed that, on the other hand, the 

 "return" fish swam at deeper levels, 

 following the outflowing Mediter- 

 ranean current. He cited a new fish- 

 ing technique used in the Sicilian 

 Channel as evidence of this (see Sec- 

 tion VIC). Sari believed that this ten- 

 dency to follow the deep current per- 

 sisted until the gonads had shrunk 

 and the feeding urge became pre- 

 dominant. This might occur more 

 readily after the fish had emerged 

 from the Strait into the Atlantic, 

 where the velocity of the Mediterra- 

 nean current diminishes rapidly. The 

 Mediterranean "return" trap at La 

 Linea, on the European side of the 

 Strait, usually caught more large blue- 

 fin than those on the African side. 

 This is in accord with the tendency 

 of the "return" tuna to follow the 

 European coast of the Ibero-Moroc- 

 can Bay closely, as evidenced by the 

 formerly large "return" catches taken 

 there, in contrast to the lack of "re- 



turn" trap fisheries along its African 

 coast. 



Sari (1973) believed that the 

 smaller bluefin (less than 100-150 

 kg) did not participate in this Atlan- 

 tic-Mediterranean migratory pattern. 

 He felt, as did Sella ( 1 929a), that the 

 immature individuals were relatively 

 sedentary. He concluded that the in- 

 dividuals in this size range made 

 longer migrations after reaching ma- 

 turity but that those in the Mediterra- 

 nean spawned there and those in the 

 Atlantic reproduced in that ocean. At 

 some period in their development as 

 their weight approaches 150 kg, 

 groups of these fish tend to join the 

 groups of larger ones in their Atlan- 

 tic-Mediterranean migrations. Sara 

 ( 1 973) speculated that this change in 

 life style is related to some change in 

 the physiology of the animal, most 

 probably the attainment of the full 

 development of the swim bladder. 



The sizes of the four fish tagged 

 by Rodriguez-Roda (1969a) near 

 Cadiz and recaptured in the western 

 Mediterranean do not fully support 

 Sara's view in regard to the sizes of 

 fish which usually follow this route. 

 One of them weighed 120 kg, but the 

 weights of the other three ranged from 

 41 to 90 kg. Only one of these smaller 

 fish, however, could have accom- 

 plished this migration during the "ar- 

 rival" period which Sara (1973) was 

 discussing. TTie results indicate that 

 casual migrations through the Strait 

 by bluefin of various sizes may oc- 

 cur at any time, but that the mass 

 periodic movements are carried out 

 mainly by large individuals. 



One fact not explained by Sara's 

 (1973) theory is the continued 

 productivity of the fisheries for small 

 bluefin off the Atlantic coast of Mo- 

 rocco and in the Bay of Biscay (Sec- 

 tion IVC5). Since there is no hard 

 evidence of extensive spawning of 

 bluefin tuna in the Eastern Atlantic 

 (Section VD2), important recruitment 

 to these fisheries from the Mediterra- 

 nean seems logical. F. de Buen ( 1 925) 

 hypothesized a migration of small 

 bluefin fi^om the Mediterranean to 

 the Atlantic in the autumn of their 

 second year of life when they 

 weighed 4-5 kg. We believe that it is 

 more probable that this migration oc- 

 curs mainly in the autumn of their 



first year of life, when they weigh 

 about 1 kg. This ties in with the dis- 

 appearance of age bluefin weigh- 

 ing about 1 kg from the Mediterra- 

 nean coast of Spain in October, and 

 the regular appearance of age blue- 

 fin weighing about 1 .5 kg off the 

 Atlantic Moroccan coast in the sec- 

 ond half of November (Section 

 1VC5). The massive catches of age 

 bluefin in the fall by the "return" 

 traps and purse seiners at the western 

 end of the Mediterranean (Rodriguez- 

 Roda 1964b, 1969d; Crespo and Rey 

 1976) also suggest such a migration. 

 In 1963, captures around Ceuta in- 

 creased sharply in mid-September 

 and remained high until late Octo- 

 ber, when they declined. 



c. Migrations and Stocks 

 Within the Mediterranean 



Much has been written about the 

 migrations and stocks of bluefin tuna 

 in the Mediterranean Sea, but the tag- 

 ging results required to reach defini- 

 tive conclusions are still lacking. De- 

 ductive studies have lead to widely 

 diverging opinions, but a reasonable 

 working hypothesis has emerged. 



Since many of the pertinent stud- 

 ies have already been discussed in 

 parts a and b of this section, they will 

 be recapitulated briefly here, stress- 

 ing the aspects which are relevant to 

 migrations within the Mediterranean. 

 According to Aristotle's (circa 325 

 B.C.) theory, the bluefin was essen- 

 tially an Atlantic fish, but spawned 

 in the Black Sea. Therefore it made 

 the circuit of the Mediterranean by 

 passing through it once, en route to 

 the spawning ground, and again to 

 return to the ocean. 



Little new information was 

 added until Cetti (1777) made two 

 very important discoveries. One was 

 that bluefin spawned near Sardinia. 

 He surmised correctly, moreover, that 

 more of them spawned in the central 

 Mediterranean than in the Black Sea. 

 His other major finding was that me- 

 dium sized bluefin occurred in the 

 Gulf of Sardinia throughout the year. 

 He named these fish "golfitani", a 

 term which is still in common use. 

 He adhered to Aristotle's basic con- 

 cept, however, and described several 

 distinct routes by which he believed 

 that the larger bluefin reached the 



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