the state of the gonads, determined the 

 movements of the species during the 

 spawning season. 



Sar&(1964, 1973) also maintained 

 that the entrance of bluefm tuna into 

 the traps depended greatly on the posi- 

 tion of the Atlantic currents, which in 

 turn was influenced by the prevailing 

 winds before and during the spawning 

 season. He believed, however, that most 

 of the large maturing bluefin taken in 

 the traps had followed the Atlantic cur- 

 rent from the ocean into the Mediterra- 

 nean. He found that layers of water 

 with temperatures of 16°C to 19°C, at 

 suitable depths, were most favorable 

 for the approach of maturing bluefin 

 tuna to the traps. The depth of the suit- 

 able layer varied, in different parts of 

 western Sicily, from 5 to 25 m. Sar^ 

 felt that the vertical thickness of the 

 17°C layer was a determining factor. 

 He agreed with Scordia that the 1 5°C 

 isotherm, which usually occurred at 

 depths of 50-60 m in the areas which 

 he studied, constituted a barrier to the 

 maturing fish. He further noted that the 

 bluefin in different groups, usually con- 

 sisting of individuals of the same, or 

 neariy the same, size, tended to remain 

 together in the traps, with each group 

 staying at a depth where a favorable 

 environment existed. He believed that 

 the conditions selected by each group 

 depended on the conditions in the area 

 from which it had come to the trap, and 

 the degree of acclimatization it had un- 

 dergone during this migration and while 

 being held in the trap. Sara also studied 

 the fransparency of the water in the 

 vicinity of the traps. He found no rela- 

 tionship between this factor and the 

 occurrences of bluefin tuna, despite the 

 firm belief of the local fishermen that 

 clear water was a prerequisite to their 

 presence. 



Arena (1964) also made detailed 

 studies of the behavior of bluefin tuna 

 in and around the traps, and of related 

 environmental factors. His research was 

 conducted at traps at Milazzo (Gulf of 

 Patti, northeastern Sicily) in 1 96 1 - 1 962, 

 and at San Cusumano (near Trapani, 

 western Sicily) in 1958. He found that 

 the fish entered the traps at various 

 depths (40 meters to the surface) and in 

 waters of various temperatures, rang- 

 ing from 1 5.5 °C to 2 1 .5°C. In general, 

 however, his findings supported Selia's 

 (1929a) view that the larger fish pre- 



ferred the cooler waters. Like Sara 

 (1964, 1973), he noted that groups of 

 tuna which entered the traps at differ- 

 ent times sought different depths and 

 temperatures. There was some ten- 

 dency, however, for the groups to ac- 

 climate to warmer waters nearer the 

 surface as their period of confinement 

 increased. He did not note any relation- 

 ship between the occurrences of the 

 bluefin with tlie salinity, density' in situ, 

 or transparency of the water. 



As noted above, little research on 

 the important subject of the effects of 

 the environment on eggs and early 

 stages of bluefin tuna has been carried 

 out. The difficulty in collecting these 

 early stages at the tuna traps led Roule 

 (1923) to conclude that they were pho- 

 tophobic and occupied abyssal waters. 

 He extended his hypothesis to the pos- 

 sible passive transport of larval bluefin 

 from the Mediterranean into the Atlan- 

 tic by the deep outflowing current. Sella 

 (1929a) refuted this idea, pointing out 

 that larval and Juvenile bluefin were 

 photophilic and that he had collected 

 thousands of them at the surface with 

 the aid of lights. He believed that, if 

 there was any passive transport of early 

 stages of bluefin through the Strait of 

 Gibraltar, it would be from the Atlantic 

 to the Mediterranean, with the inflowing 

 surface current. Sparta ( 1 933) and Sar^ 

 (1973) supported Sella's opinion that 

 juvenile bluefin were attracted to light. 



Scaccini et al. (1975) concluded 

 that the eariy stages of the bluefin tuna 

 could withstand a wide range of hydro- 

 logical conditions, even when changes 

 occurred rapidly. They believed that an 

 abundant supply of food, consisting of 

 zooplankton, was the prime requisite 

 for the survival of the larvae and small 

 juveniles. 



3. Eastern Atlantic 



a. Distribution, Objectives and 

 Nature of Research 



Environmental research related to 

 the spawning behavior of bluefin tuna 

 in the eastern Atlantic has been inten- 

 sive in the Ibero-Moroccan Bay, espe- 

 cially at the sites of tuna traps, and in 

 the Strait of Gibraltar, but sporadic in 

 the remainder of this large area. 



Some of these studies were of a 

 local nature, in the vicinity of particu- 

 lar traps, but others covered most or all 

 of the Ibero-Moroccan Bay, or even 



extended eastward into the Mediterra- 

 nean Sea or southward to the Canary 

 Islands. 



The immediate purpose of these 

 investigations was to determine the en- 

 vironment in which the tuna matured 

 and presumably spawned, and the ef- 

 fects of the various factors on the spawn- 

 ing behavior and related movements of 

 the tuna. The ultimate objective was to 

 improve the fisheries, or at least to per- 

 mit better estimates of their potential 

 and predictions of their fluctuations. 

 The eflects of the environment on the 

 early stages of the bluefin tuna were 

 also investigated. 



Frade and Vilela (1962) summa- 

 rized the environmental factors consid- 

 ered by various authors as follows: 



Intrinsic factors: Temperature of the 

 water at various depths and 

 themioclinal topography, density 

 of the water, salinity, pH, transpar- 

 ency, currents, etc. 



Extrinsic factors: Air temperaUire, 

 atmospheric pressure, irradiation, 

 light intensity, wind, rain, etc. 



Other factors considered include 

 dissolved oxygen and tides (Lozano 

 Cabo 1957, 1958, 1970) and chloro- 

 phyll productivity (Rodriguez-Roda 

 1963). 



b. Portugal 



Don Carlos de Bragan^a, king of 

 Portugal, conducted what was prob- 

 ably the first methodical study of envi- 

 ronmental conditions, the spawning be- 

 havior of the bluefin tuna, and the 

 catches of tuna traps in the eastern At- 

 lantic. We have not seen his report (de 

 Bragan9a, 1 899) but have found quota- 

 tions from it, and discussions of its 

 contents, in Pavesi (1889), Roule 

 (1914b, 1917), Parona (1919). F. de 

 Buen ( 1 925) and other sources. 



During the fishing season of 1 898, 

 de Braganfa collected hydrological and 

 meteorological data, and attempted to 

 observe the behavior of the tuna them- 

 selves, from his yacht "Amelia" in the 

 Ibero-Moroccan Bay. He also had de- 

 tailed data on the catches of the Portu- 

 guese tuna traps collected concurrently. 

 These traps were located in the prov- 

 ince of Algarve on the country's south 

 coast, which forms the northwestern 

 boundary of the Ibero-Moroccan Bay 

 (Figure 44). He examined the com- 



87 



