Consequently, feeding and gen- 

 eral activity are presumed to be 

 greatly reduced. The migratory 

 behavior of mature individuals 

 in this group often resembles that 

 of the medium fish. 



Medium Fish (32-122 kg. Ages 5-8, 

 Mature) — These fish make an- 

 nually repeated three-phase mi- 

 grations between warm season 

 feeding areas, deep water win- 

 tering areas, and late spring 

 spawning areas. The warm sea- 

 son distribution is more exten- 

 sive than that of the small fish, 

 and not as limited to coastal wa- 

 ters. This occurrence is again fre- 

 quently in the surface layers, but 

 not as predominantly so as that 

 of the small fish. Feeding is 

 heavy. Seasonal linear growth has 

 not been determined, but is pre- 

 sumably rapid. The length-weight 

 ratio decreases considerably dur- 

 ing this period. The cold season 

 distribution is much wider than 

 that of the small fish, extending 

 far into oceanic waters. These fish 

 then remain in the subsurface lay- 

 ers and it is assumed, but has not 

 been demonstrated, that their 

 feeding, activity and growth are 

 reduced. In late spring and early 

 summer, many of these fish con- 

 centrate in spawning areas which 

 are little known. After spawning 

 they return to the warm water 

 season feeding areas. 



Large or Giant Fish (Over 122 kg. 

 Age 9 or Older, Mature) — The 



fish make three-phase migrations 

 basically similar to those of the 

 medium size group, but their dis- 

 tribution and migrations are much 

 more extensive. Their spawning 

 and migratory periods, as well as 

 the areas concerned, while dif- 

 fering somewhat from those of 

 the medium group, overlap or co- 

 incide with them to a consider- 

 able degree. The seasonal linear 

 growth of these fish is not known, 

 but their length-weight ratio de- 

 creases greatly during the feed- 

 ing season, and increases corre- 

 spondingly during spawning and 

 their post-spawning migrations. 

 As with the smaller groups, their 

 feeding, general activity and 



growth are presumed to be re- 

 duced during their wintering pe- 

 riod in deep waters. Movements 

 within some of these seasonal 

 habitats have been observed, as 

 well as the migrations between 

 them. 



We shall discuss the migratory 

 patterns of the different size groups 

 of bluefin tuna in terms of this sim- 

 plified model. 



C. STUDIES OF MIGRATIONS 

 AND STOCK IDENTITY 



1. Mediterranean and Eastern 

 Atlantic 



a. Introduction 



The earliest speculations and hy- 

 potheses about the migrations of blue- 

 fin tuna, and in fact nearh all of 

 those before 1920. were concerned 

 with the Mediterranean and eastern 

 North Atlantic areas. In fact, the study 

 of these migrations undoubtedly de- 

 veloped from observations made dur- 

 ing fishing operations in the Medi- 

 terranean and its approaches during 

 the pre-Christian era. Traps similar 

 to those still in use apparently ex- 

 isted then, but movable nets, whose 

 successful operation required the 

 assistance of watchers (thynnoscopi) 

 situated at coastal vantage points, 

 were probably more numerous 

 (Parona 1919, Thomazi 1947). Both 

 methods depended on the movements 

 of schools of fish along the coasts, 

 which occurred mainly during the 

 runs of maturing (now known as "ar- 

 rival") tuna in May and June, and 

 post-spawning (now known as "re- 

 turn") fish in July and August. There- 

 fore, before recapitulating the devel- 

 opment of hypotheses on the migra- 

 tions and populations of tuna in this 

 region, we will summarize some of 

 their local movements which have 

 been indicated over the ages by the 

 operations of the traps which have 

 harvested these periodic passages. 

 Basic data which will be considered 

 include the location of the traps, the 

 direction from which fish must ap- 

 proach them, and the periods when 

 they catch tuna. The relative impor- 

 tance of their catches will be dis- 

 cussed in special cases. 



Sara ( 1 964, his Figure 5) showed 

 the approximate locations of the 

 significant traps still in use in the 

 early 1960s, indicating, for each, the 

 fishing period and the direction from 

 which the fish entered the trap. The 

 data for those in the Ibero-Moroccan 

 Bay and the extreme western Medi- 

 terranean (Figure 45) strongly sug- 

 gested eastward movements from the 

 Atlantic into the Mediterranean ("ar- 

 rival") in May and June, and move- 

 ments in the opposite direction ("re- 

 turn") in July and August. The arrays 

 of "arrival" traps on the sides of the 

 Bay terminated at Tarifa (north coast) 

 and Cape Spartel (east coast), show- 

 ing that the "arrival" run extended at 

 least up to the very threshold of the 

 Strait of Gibraltar. 



Data for the "return" traps sug- 

 gested that some west-bound tuna fol- 

 lowed botli coasts of the Mediterra- 

 nean as the\ approached the Strait. 

 In the Ibero-Moroccan Bay, however, 

 they occurred in much greater 

 strength along its northern (Iberian) 

 coast, in contrast to the complete ab- 

 sence of a "return" trap fishery along 

 its eastern (Moroccan) coast. 



The other important group of 

 traps shown by Sara (1964) was in 

 the central Mediterranean. Most of 

 these fished the "arrival" run only. 

 The indications from the dates and 

 directions in which the fish entered 

 the traps were as follows. 



Off Tunisia, the "arrival" tuna 

 apparently travelled eastward from 

 Bizerte to the Cape Bon peninsula, 

 which they rounded, then moved 

 southward to Ras Kapudia. "Arrival" 

 bluefin also evidently moved east- 

 ward along the Libyan coast, from 

 Zuara near the Tunisian border to 

 Cape Misurata (longitude 15°E). 



The situation around Sardinia, 

 Sicily, and Italy differs because of 

 the disjointed configuration of the 

 coasts. The "arrival" fish tended to 

 move in southerly directions along 

 the western shores of Sardinia, and 

 also along the Calabrian coast in the 

 Gulf of Sant' Eufemia. With local 

 exceptions at its two extremities, "ar- 

 rival" fish travelled westward along 

 the north coast of Sicily, following 

 the western sides of bays and the 

 eastern sides of promontories along 

 the way. This westward movement is 



98 



