would have occurred between catches 

 in "arrival" traps located in westerly 

 areas and those in easterly areas, if 

 all were fishing a single group offish 

 moving eastward, did not exist. Roule 

 and F. de Buen used the relatively 

 poor bluefin catches in the Sea of 

 Alboran (just east of Gibraltar) as 

 another argument against a large scale 

 spawning migration of bluefin from 

 the Atlantic into and out of the Medi- 

 terranean. Both of these scientists 

 cited the sensitivity of the bluefin 

 tuna to environmental conditions as 

 evidence that they could not pass 

 through the relatively cold (R. de 

 Buen 1927) waters of the Strait of 

 Gibraltar, especially during the 

 spawning season. Roule (1917) pro- 

 posed the "halothermic" theory, 

 maintaining that the bluefin were 

 "stenotherms" and "stenohalines", 

 and sought the warmest and most 

 saline waters during the spawning 

 season. F. de Buen (1925) opted for 

 the "thermic" theory, considering 

 water temperature to be the domi- 

 nant factor controlling the tunas' 

 movements. Both agreed, as did most 

 students of the subject in the Mediter- 

 ranean and adjacent Atlantic waters, 

 that the sensitivity of the bluefin tuna 

 to the environment increased during 

 the period of maturation. 



Scordia (1938) also believed in 

 the "sedentary" tuna theory and felt 

 that few, if any, bluefin tuna entered 

 the Mediterranean fi-om the Atlantic. 

 Like Roule (1914a, 1914b, 1917) and 

 F. de Buen (1925), she stressed the 

 increased sensitivity of the maturing 

 tuna to its environment. 



Sella ( 1 929a) showed, however, 

 that bluefin tuna could withstand ex- 

 treme changes in temperature and 

 salinity during their feeding period. 

 He maintained that even during the 

 spawning period they sought specific 

 conditions which varied with size of 

 fish, rather than maxima as proposed 

 by Roule (1917). He further argued 

 that the "thermic" or "halothermic" 

 barrier of F. de Buen (1925) and 

 Roule (1917) did not constitute proof 

 that bluefin could not pass through 

 the Strait of Gibraltar, but would be 

 an explanation of such a situation, if 

 its existence were substantiated. 



Sella noted, as Sara (1964, 1973) 

 and Scaccini et al. (1975) did later. 



that there were relativeh few large 

 bluefin in the Mediterranean except 

 during the spawning migrations and 

 that the arrival fishery actually did 

 begin later in the season in propor- 

 tion to how far east the traps were 

 located. 



After this long period of indirect 

 studies, Sella's (1927, 1929a) find- 

 ing of 25 Atlantic hooks and lures in 

 tuna caught in Mediterranean traps 

 revived the "migratory" or "unit 

 stock" theory. The items recovered 

 included 13 from Tarifa, seven cer- 

 tainly and two probably from the Bay 

 of Biscay, two from the Azores, and 

 one from south of Ireland. These star- 

 tling revelations did not pass 

 unchallenged in this period when the 

 "sedentary tuna" theory was deeply 

 rooted in the minds of nearl) all the 

 scientists concerned with the prob- 

 lem. F. de Buen (1931) and Scordia 

 ( 1 934) questioned Sella's deductions, 

 on the grounds that fishermen from 

 the localities where the hooks and 

 lures were made might actually have 

 used them while travelling in other 

 areas. Although they admitted that a 

 few such migrations might occur, 

 they maintained that they probably 

 were not related to the spawning 

 cycle, and were relatively insignifi- 

 cant numerically. Sella (1929a) had 

 already pointed out, in support of his 

 hypothesis, that the findings of the 

 hooks varied according to the inten- 

 sity of fishing in the area of their 

 origin. When the fishery off Tarifa 

 was important, numerous hooks of 

 the very distinctive type used there 

 were recovered in the Mediterranean. 

 After the fishery off Tarifa had de- 

 clined drastically, and the one in the 

 Bay of Biscay had increased greatly, 

 hooks of the former area were no 

 longer found, but hooks of the Biscay 

 type appeared in numbers. Sella also 

 noted that, considering the enormous 

 odds against recovering a lost hook 

 in a tuna, the numbers of findings 

 indicating these migrations were too 

 large to be attributable to casual fish- 

 ing by transients. 



Detailed information on the 

 movements of bluefin tuna in the 

 Strait of Gibraltar obtained by echo 

 sounder was reported by Lozano 

 Cabo ( 1 959a, 1 959b). In 2^000 miles 

 of cruisina in the Strait, and as far as 



Barbate and Cadiz, Spain, and 

 Larache, Morocco, in June and July 

 1 957, he obtained data which he con- 

 sidered sufficient to provide interest- 

 ing conclusions on the biology of the 

 species. 



Lozano Cabo (1958) had sus- 

 pected that bluefin, at least during 

 their spawning migration, might be 

 negatively rheotactic (tending to 

 swim with the currents). The detec- 

 tion of schools confirmed this. The 

 greatest number of "arrival" schools 

 were found in the middle of the Strait, 

 where the current always favored 

 their eastw ard passage. Many schools 

 were also found on the northern side, 

 where the current was usually favor- 

 able, but they were very rare on the 

 Moroccan side, where the currents 

 tended to vary with the tides. The 

 depth at which the schools traveled 

 was difficult to determine, but ap- 

 peared to be somewhat less than 60 

 m. 



The migration route of arrival at 

 the Strait when departing from the 

 Atlantic followed the Spanish coast. 

 Some schools uere located between 

 Cape Spartel and Cape Malabata (on 

 the southwestern side of the Strait), 

 but none were detected south of Cape 

 Spartel during numerous cruises be- 

 tween Tangier and Larache. Perhaps 

 the tuna which came from the Afri- 

 can coast partially crossed the Strait 

 from off Cape Spartel toward Tarifa, 

 and joined those coming from the 

 Spanish coast, or followed routes 

 similar to theirs through the Strait. 

 The schools which had tended to con- 

 centrate in the center of the Strait 

 while traversing it appeared to dis- 

 perse as they entered the Mediterra- 

 nean at the Ceuta-Algeciras line. 



In general, the migrating tunas 

 appeared to prefer temperatures of 

 18X to 21°C, but deviations from 

 the preferred temperature were more 

 frequent in the Strait than at the traps. 

 Lozano Cabo attributed this to the 

 overcoming of temperature sensitiv- 

 ity by the reproductive urge. Nearly 

 all previous scientists (Roule 1914a, 

 Sella 1929a, Scordia 1938) had main- 

 tained that the sensitivity of the blue- 

 fin to temperature and salinity was 

 greatly increased during the spawn- 

 ing period in contrast to the "erratic" 

 (feeding) period. In an\ case, Lozano 



100 



