sitivity of the fish to these properties of 

 the water were heightened during the 

 period of gonadal maturation. He con- 

 cluded that, in this period, the bluefm 

 sought the warmest and most saline 

 waters of the western Mediterranean. 

 This conclusion is somewhat inconsis- 

 tent with his use of the terms 

 "stenothermic" and "stenohaline." 



In support of his "halo-thermic" 

 theory, Roule (1924) showed that the 

 direct route from a wintering area for 

 bluefin tuna off the Mediterranean coast 

 of France to a spawning area off Tuni- 

 sia followed the path of maximum ther- 

 mal increment, crossing isotherms of 

 successively higher temperatures at 

 right angles. The isotherms at a depth 

 of lOm had been traced during an 

 oceanographic survey made in the 

 spring of 1923. 



Roule ( 1 924) also studied the rela- 

 tionship between the bluefin catches of 

 the trap at Sidi Daoud, Tunisia, and the 

 rainfall in spring at Bizerte, over a 35 

 year period. He found that tlie trap 

 catches generally declined in years of 

 heavy rainfall. He attributed this to the 

 decrease in the salinity of the waters 

 near the trap caused by the increased 

 outflow of fresh water from the large 

 Lake of Bizerte. He used these two 

 examples to demonstrate the 

 stenothermic and stenohaline charac- 

 teristics of the bluefin. 



Roule ( 1 9 1 4a, 1 9 1 7) believed that 

 the relatively cold waters of the Strait 

 of Gibraltar prevented the passage of 

 any significant numbers of bluefin tuna, 

 particularly during the spawning sea- 

 son when their sensitivity was increased, 

 from the Atlantic into the Mediterra- 

 nean. He therefore strongly supported 

 Pavesi's (1887) and de Bragan^a's 

 (1899) opinion that the Mediterranean 

 bluefin constituted an autochthonous 

 stock, completely independent of the 

 Atlantic bluefin which spawned very 

 near the Strait of Gibraltar. 



The influence of winds on the 

 movements of the fish near the 

 Sardinian traps was slight, according to 

 Roule (1914a). He concluded from his 

 observations there, however, that in- 

 creases in the density of the water fa- 

 vored increased catches. Roule (191 4a) 

 stated that all of the traps for spawning 

 bluefin were designed to capture fish 

 which were swimming against the fish- 

 able current. 



Sella (1927, 1029a, 1929b, 1932a, 

 1 932b) disputed the almost unanimous 

 views on the movements and effects of 

 environmental factors by the scientists 

 who studied the biology of the eastern 

 Atlantic and Mediterranean bluefin 

 tuna. Most of these supported the views 

 of Pavesi (1887), de Bragan^a (1899), 

 Sanzo, (19 10a) and Roule (19 14a, 1917, 

 1924). Sella traced migrations of blue- 

 fin tuna by a new method. He collected 

 many hooks and lures which had been 

 found in fish caught in traps. He then 

 determined the areas where the hooks 

 and lures, which were then handmade 

 in distinctive local patterns, were in 

 use. His findings indicated that several 

 bluefin tuna had migrated from the At- 

 lantic into the Mediterranean, and that 

 others had made extensive migrations 

 within that sea. 



Sella also disagreed with Roule's 

 view that the species was absolutely 

 stenothermic and stenohaline. He 

 pointed out that feeding bluefin tuna 

 occurred in numbers in many localities 

 whose waters included a very wide 

 range of temperature and salinity. He 

 agreed with Roule in regard to the in- 

 creased sensitivity to environmental 

 factors of maturing bluefin tuna, but 

 showed that even these fish did not 

 seek maxima of temperature and salin- 

 ity. As an example, he pointed out that 

 no spawning occurred in the eastern 

 Mediterranean, where the salinity was 

 the highest. He supported this state- 

 ment by pointing out that all of the 

 "arrival" traps, whose catch consisted 

 almost entirely of maturing fish, were 

 west of the 38 o/oo isohaline, where 

 the salinity was lower, whereas only 

 "return" traps, whose catch consisted 

 mainly of large spent fish, were east of 

 it, where the salinity was higher. He 

 felt that, instead of maxima, the matur- 

 ing bluefin tuna sought specific hydro- 

 logical conditions. He maintained that 

 maturation occurred in the period of 

 the most rapid thennai increment. He 

 also showed that the larger maturing 

 bluefin evidently favored somewhat 

 colder and less saline water than the 

 smaller ones. The important Favignana 

 trap off the western end of Sicily, and 

 the group of three equally productive 

 traps off southwestern Sardinia, where 

 the water temperature (about 18.0- 

 1 8.8°C) and salinity are relatively low, 

 took mainly large maturing fish. The 



traps off Calabria (southern Italy) and 

 Tripolitania, where the water is warmer 

 (about 20.8°C - 21.8°C) and more sa- 

 line, caught mainly small and medium- 

 sized fish. 



Sella (1027) refuted Roule's 

 (1924) hypothetical migration of blue- 

 fin tuna along the line of maximum 

 thermal increment from off southern 

 France to off Tunisia. He showed that 

 the tuna taken in the former area aver- 

 aged less than 20 kg, whereas those 

 taken in the Tunisian traps averaged 70 

 kg. Sella also differed with Roule 

 (1914a) in regard to the influence of 

 currents. Sella ( 1 932b) showed that suc- 

 cessful fishing of the very productive 

 traps off southwestern Sardinia de- 

 pended on having their leaders reach 

 beyond the local northeasterly counter- 

 current into the main southwesterly cur- 

 rent, which was farther offshore. This 

 main current was deflected upward by 

 the bottom topography, resulting in 

 colder water at the traps than elsewhere 

 in the vicinity. Sella offered the hy- 

 pothesis that the tuna followed this deep 

 current from a distant area, and favored 

 its water over the local waters with 

 different characteristics. He emphasized 

 the importance of studying currents in 

 research on the behavior of the species. 



Scordia(1937, 1938, 1942) con- 

 ducted extensive studies at the traps off 

 eastern Sicily and Calabria, and also of 

 the hook and line fishery in the Strait of 

 Messina (Scordia 1932, 1934). She 

 maintained that density in situ was an 

 important factor in controlling the 

 movements of spawning bluefin tuna. 

 She also found that atmospheric de- 

 pressions and "zoo currents" (currents 

 containing large quantities of zooplank- 

 ton) and water temperature strongly 

 influenced local movements of matur- 

 ing fish in the vicinity of the traps. A 

 requirement for successflil fishing of 

 the traps in the Tyrrhenian Sea was the 

 presence of a "hypothermia barrier" of 

 water with a temperature of I4°C at 

 depths of 35-40 m. Although she did 

 not believe that bluefin from the Atlan- 

 tic spawned in the Mediterranean, she 

 concluded (1938) that the arrival 

 ("transgression") of Atlantic current 

 waters was the deteiTnining cause of 

 the reproductive migration of the blue- 

 fin tuna, and consequently of their en- 

 try into the traps. In Scordia's opinion 

 environmental conditions, rather than 



86 



