bined information in an effort to deter- 

 mine the spawning beiiavior and the 

 related movements of the tuna, and the 

 influences of environmental factors on 

 them. 



De Bragan^a concluded that varia- 

 tions in meteorological conditions- 

 waves, strength and direction of wind, 

 barometric pressure, and air tempera- 

 ture-had no effect on the movements of 

 the tuna during the trap fishing season 

 (May-August). He believed that varia- 

 tions in their displacements in this pe- 

 riod were caused by changes in the 

 marine environment itself, mainly in 

 its temperature. He observed that the 

 bluefm tuna was not found in waters 

 with temperatures of less than 13°C, 

 and believed that the tunas' choice of 

 limited areas in the trap fishing and 

 spawning season was explained by 

 variations in the temperature of the 

 water. 



He also concluded that the same 

 groups of tuna which had passed the 

 Algarve coast in the "arrival" fishery 

 re-appeared there from 50 to 32 days 

 later in the "return" fishery. In the mean- 

 while, he believed, they had spawned 

 in the eastern part of the Ibero-Moroc- 

 can Bay, with only insignificant num- 

 bers of them entering the Mediterra- 

 nean. He noted that, although some 

 bluefln tuna occurred in Portuguese 

 waters throughout the year, the 

 Algarvian trap fishery was the only one 

 in which these fish were abundant and 

 followed a regular migratory pattern. 



Vilela and Pinto (1958) presented 

 monthly maximum, minimum and av- 

 erage sea surface temperatures and sa- 

 linities taken near the tuna trap off Cabo 

 de Santa Maria, on the southern or 

 Algarve coast of Portugal during the 

 trap fishing season, from April 24 

 through August. Although this was not 

 specified, we presume from the con- 

 text in which they were presented that 

 these determinations were made in 

 1958. They also listed the portion of 

 the 1958 catches of the Algarvian traps 

 which was used by the canneries (about 

 80 percent of the total) by months in 

 numbers and weight of fish in each of 

 the traditional Portuguese size groups. 

 These data are shown in Table 26. 



c. Spain 



From April to August 1923, an 

 extensive dual survey of the oceanog- 



raphy and the tuna fisheries of the Ibero- 

 Moroccan Bay (westward to Cape St. 

 Vincent and southward to Casablanca, 

 Morocco) and the western Mediterra- 

 nean, or Alboran Sea (eastward to Cape 

 de Gata, Spain, and the Mulaya River, 

 Morocco) was carried out (F. de Buen 

 1 924, 1 925, 1 927; O. de Buen 1 924; R. 

 de Buen 1927). The most important of 

 these reports, from the biological point 

 of view, is that of F. de Buen (1925). 

 This work was conducted under the 

 terms of intemational accords reached 

 at the meetings of the International 

 Commission forthe Scientific Explora- 

 tion of the Mediterranean Sea in Paris, 

 January 1 1, 1923 (O. de Buen 1924, F. 

 deBuen 1925). 



A general oceanographic survey 

 of the area, including 540 operations, 

 was conducted under the direction of 

 Dr. O. de Buen on the Spanish naval 

 transport "Almirante Lobo." A party 

 on the smaller vessel "Principe Alberto 

 de Monaco" concurrently collected bio- 

 logical and statistical data from the tuna 

 fisheries in the area. Dr. F. de Buen w as 

 in charge of the biological work of the 

 entire campaign. 



To determine the causes of the 

 arrival of the tuna on the Spanish coasts, 

 data were gathered on the winds, the 

 surface currents, and the transparency 

 of the water. F. de Buen (1925) main- 

 tained that the general oceanographic 

 conditions were the unique local causes 

 of the spawning concentration of tunas 

 off the Atlantic coast of southern Spain. 

 Water transparency was a ver>' impor- 

 tant factor, as was demonstrated by the 

 catches of two traps near the mouth of 

 the Guadiana River at the Portuguese 

 border. About 90 percent of the fish 

 were caught in clear water. The re- 

 mainder were taken in "regular" or 

 "dirty" water, without a consistent dif- 

 ference between the catches in these 

 two types. The clarity of the water, 

 however, seemed more critical during 

 the "return" (westward) movement than 

 during the "arrival" (eastward) transit. 

 It was difficult to find a definite rela- 

 tionship between winds, or wind-driven 

 currents, and the catches of the traps. 

 De Buen concluded that the direction, 

 intensity and frequency of the winds 

 might act as a secondarj' factor influ- 

 encing the local movements of the tuna 

 by altering the temperature of the sur- 

 face waters. 



In discussing the results, F. de Buen 

 (1925) stated that the bluefin tuna was 

 a typically stenothermic fish, whose 

 disffibution was exactly limited by tem- 

 perature. In the spawning season, its 

 sensitivit) to temperature increased. In 

 his opinion, the tuna was a surface- 

 dwelling fish, submerging occasionally 

 to depths of only a few meters. He 

 rejected the abyssal winter habitat pro- 

 posed by Pavesi ( 1 887) and supported 

 by Sanzo ( 1 910a). He believed that the 

 adult tuna which inhabited the western 

 Mediterranean basin were altogether 

 independent of those occupying the 

 adjacent parts of the Atlantic. He pos- 

 tulated that the colder waters of the 

 Strait of Gibraltar separated the two 

 basins oceanographically and prevented 

 the interniinghng of these two groups 

 of tuna. 



De Buen concluded, from his own 

 observations, that the water tempera- 

 ture was the main factor infiuencing 

 the movements of the tuna, and pro- 

 posed a thermic theory, as opposed to 

 the hydrodynamic theory of Bounhiol 

 (191 1) and the halothermic theory of 

 Roule (1914a). 



F. de Buen (1925) found, by com- 

 paring the Spanish trap catches of May 

 1923 with oceanographic observations 

 made concurrently in the same area, 

 that the maturing "arrival" tuna pre- 

 ferred the warmer and less saline wa- 

 ters near Cadiz to the adjacent colder 

 and more saline waters, which are typi- 

 cal of the open ocean to the west and 

 the Strait of Gibraltar to the east. 



After the peak of the "arrival" fish- 

 er) along the coast, the fisher> dimin- 

 ished. This coincided with the greatest 

 intensity of spawning In general, the 

 conditions causing variations in the wa- 

 ter temperature, such as the persistence 

 of certain winds and the influence of 

 coastal currents, were secondary fac- 

 tors atTecting the local movements of 

 tuna. 



Fresh waters carrying suspended 

 materials might have caused the tuna to 

 leave the coast and temporarily pre- 

 vented their capture by certain traps. 

 The spawning areas coincided with the 

 centers of high temperature, varying 

 constantly under the effect of coastal 

 factors on one side, and the high seas 

 on the other. 



F. de Buen stated that affer spawn- 

 ing, the tuna, no longer subject to such 



88 



