'■& Major Known Spawning Aieas 



'* Minor, Onavaluatad or Hypothetical 

 Spawning Jlreas 



Figure 64. Bluefin spawning areas in the Mediterranean and Blaclc Seas and tlie 

 eastern Atlantic. 



of length would be much more useful 

 for back-calculating the localities where 

 larvae or juveniles had probably been 

 spawned. 



Collection data for eggs and small 

 larvae which have been reared to iden- 

 tifiable sizes furnish good indications 

 of spawning dates and areas. Estimates 

 based on collection data for larger lar- 

 vae or juvenile stages, however, must 

 be regarded only as approximations. 



Unfortunately, most of the avail- 

 able data on the growth of early stages 

 of bluefin tuna(d'Amico 1816, Bourge 

 1908, Heldt 1930, Piccinetti and 

 Piccinetti Manfrin 1 970) were presented 

 in terms of weight (a characteristic 

 which is seldom reported in this size 

 range), rather than length. Length-age 

 data for early stages of bluefin tuna are 

 needed to reduce the error in estimat- 

 ing the probable date and location of 

 hatching from the collection data of 

 early stages. 



D. SPAWNING AREAS AND 

 SEASONS 



1. Mediterranean and Black Seas 



a. General Information 



The spawning habits of T. ihynnus 

 thynnus have been more extensively 



studied and are better known in the 

 Mediterranean than elsewhere. 



Bluefin tuna spawn over exten- 

 sive areas of the Mediterranean and 

 Black Seas (Figure 64). The larger in- 

 dividuals, to which most of the research 

 has been devoted, spawn mainly in the 

 last half of June and the first half of 

 July. There is considerable evidence 

 that the smaller bluefin which have 

 reached maturity (the older small blue- 

 fin and the medium-sized fish) spawn 

 later, throughout July and into August, 

 and occasionally even into September. 



The principal known spawning ar- 

 eas were in the southern central Mediter- 

 ranean, around Sicily, off western 

 Sardinia, and off northeastern Tunisia. 

 Additional reproduction has occuned 

 around the Balearic Islands, off 

 Tripolitania (western Libya), off Alge- 

 ria, and in the Black Sea. It is believed 

 that additional research would show 

 that the species spawns over a much 

 broader area. 



The relationship between eastern 

 Atlantic and Mediterranean bluefin tuna 

 has been debated for centuries. An an- 

 cient theory, proposed by Aristotle 

 (circa 325 B.C.), has been believed for 

 centuries by the Mediterranean fisher- 

 men and repealed by many authors. 

 His hypothesis was that the bluefin tuna 

 lived in the Atlantic for most of the 



year, but entered the Mediterranean in 

 the spring, travelled along its northern 

 shores to the Black Sea to spawn, then 

 returned to the Atlantic, following the 

 southern coasts of the Mediterranean, 

 in the summer. Scientific findings in 

 the sixteenth and seventeenth centu- 

 ries, however, raised questions about 

 this theory and from 1 875 to 1 925 most 

 authorities believed that spawning blue- 

 fin did not pass through the Strait of 

 Gibralter in numbers. They felt that the 

 Mediterranean bluefin tuna comprised 

 an autochthonous population, which 

 made only limited local movements to 

 and from the spawning areas. Since 

 1925 evidence of migrations from the 

 Atlantic into the Mediterranean has re- 

 opened the question. 



A recent hypothesis (Sara 1964, 

 1973) satisfies many of the arguments 

 used by both sides in this debate. Sara 

 maintains that most of the large bluefin 

 which spawned in the Mediterranean 

 were migrants from the Atlantic and 

 spent the rest of the year in that ocean, 

 but that most of the medium sized and 

 small fish which spawned in the Medi- 

 terranean had been bom in that sea and 

 had remained there until they attained 

 about 1 50 kg. These problems are dis- 

 cussed in detail in Sections IV and VI. 



b. Specific Occurrences 



The first positive indication that 

 bluefin tuna spawned in the Mediterra- 

 nean was presented by Cetti (1777) 

 from observations made along the 

 coasts of Sardinia. He found eggs, 

 which he believed to be those of blue- 

 fin tuna, on the ropes of the traps, and 

 also in the water around the traps. He 

 also stated that the ovarian eggs offish 

 taken in the traps in May were very 

 well developed, but that those of fish 

 taken in June had degenerated. He con- 

 cluded tentatively that more large blue- 

 fin tuna spawned in the Mediterranean 

 than in the Black Sea. Even though his 

 data are questionable, in view of present 

 knowledge, his conclusions were cor- 

 rect. His work eventually led to the first 

 doubts about the Aristotelian theory. 



This was followed by d'Amico's 

 (1816) revelation that bluefin spawned 

 off Sicily. He noted that juveniles of 

 the species weighed about 42 g in June, 

 122 g in August and 840 g in October. 

 These data and those of Sella (1929a) 

 enabled Heldt (1930) to present the 

 first growth curve for the early stages 



70 



