of the Mediterranean) as well as the 

 Ibero- Moroccan Bay, and also failed to 

 collect any early stages of bluefin there. 

 It seems inconceivable that bluefin do 

 not spawn in the eastern Atlantic. On 

 the other hand, the collection of bluefin 

 larvae in so many parts of the Mediter- 

 ranean and the collection of "thousands" 

 (Sella 1929a) of juveniles less than 10 

 cm long in Sicilian waters offers a strik- 

 ing contrast. It is difficult to escape the 

 conclusion that the Mediterranean is 

 most important to the reproduction of 

 the bluefin tuna of the eastern Atlantic. 



4. Comparison 



The distribution and migratory pat- 

 terns of the bluefin on the two sides of 

 the Atlantic are quite analogous (Table 

 35). The extension of its range to much 

 higher latitudes off Europe than off 

 North America may be explained by 

 the warmer temperatures in northern 

 European waters. 



The similarity in the patterns is 

 especially ev ident during the warm sea- 

 son when the distributions are best 

 known. The Norwegian coast and the 

 North Sea correspond to the Gulf of 

 Maine and Canadian waters, being oc- 

 cupied principally or entirely by large 

 and medium bluefin. The surface 

 temperatures and salinities of these wa- 

 ters, excepting the salinity in the North 

 Sea, are relatively low. South of these 

 areas, we find the principal summer 

 habitats of the small bluefin, the Bay of 

 Biscay and the Cape Hatteras-Cape Cod 

 area. The surface temperatures and sa- 

 linities in these areas are somewhat 

 higher than those farther north. The 

 northerly distribution of small bluefin 

 in the eastern Atlantic is limited by the 

 English Channel and the British Isles, ■ 

 and the North Sea. Some large and 

 medium fish are taken in the Bay of 

 Biscay, however. The summer habitat 

 of the small bluefin in the western At- 

 lantic, on the other hand, is contiguous 

 with that of the larger fish at Cape Cod. 

 In some years, great numbers of .small 

 fish have entered the southwestern Gulf 

 of Maine. Large and medium tuna also 

 occur ft-equently in the Cape Cod-Cape 

 Hatteras area, but usually in moderate 

 numbers and for short periods. 



Small bluefin gather off the south- 

 west coast of Portugal from October 

 through June, and very small or small 

 ones are found off the coast of Mo- 



rocco tliroughout the year. There are 

 no counterparts of these occurrences in 

 the western Atlantic, since the species 

 is rarely encountered in United States 

 coastal waters south of Cape Hatteras. 



The cold season occurrences of 

 the bluefin are not as well known, but 

 some differences between the patterns 

 on the two sides of the Atlantic are 

 evident. In the western Atlantic the 

 small and medium fish winter sepa- 

 rately from the large ones, north of 

 latitude 35"N, whereas in the eastern 

 Atlantic, the medium and small fish 

 share the northeastern part of the win- 

 ter habitat of the large ones. The me- 

 dium fish occur farther offshore than 

 the small ones in both areas and the 

 large ones are distributed all the way 

 across the ocean from about 35°N lati- 

 tude to the equatorial region. There- 

 fore, there is no clear cut separation 

 between tlie westem and eastern groups, 

 but the distributions of relative abun- 

 dance suggest that the former group is 

 more extensively distributed than the 

 latter. 



Tlie spawning seasons and areas 

 of the Atlantic bluefin, especially for 

 the medium fish and the mature indi- 

 viduals of the small group, are far ft-om 

 completely known. 



The most important spawning 

 grounds which have been described are 

 in the Gulf of Mexico (part of the 

 American Mediterranean) and in the 

 south central part of the European Medi- 

 terranean Sea. These locations are 

 analogous geographically, but the sur- 

 face temperatures and salinities of the 

 waters differ considerably. They are 

 about 23°C - 27°C and 36.0 o/oo, re- 

 specfively, during the late April-early 

 July spawning season in the Gulf of 

 Mexico, and 1 8.0"C - 22"C and 37. 5 o/ 

 00 in the mid June-August spawning 

 season in the south central Mediterra- 

 nean. The stock structure is much sim- 

 pler in the Gulf than in the Mediterra- 

 nean. The former area is occupied only 

 by large bluefin during the winter and 

 the spawning season, and by newborn 

 fish from the time of their birth through 

 the fall and perhaps into the winter. 

 The spawning fish presumably come 

 trom various parts of the westem At- 

 lantic. Bluefin of all sizes occur in the 

 Mediterranean throughout the year, but 

 large fish are relatively scarce there 

 except between the "arrival" run of 



maturing fish in May-June and the "re- 

 turn" run of spent fish in July-August. 

 These runs include many giant fish and 

 lesser numbers of medium ones from 

 the eastern Atlantic. Maximum spawn- 

 ing of large fish takes place from mid- 

 June to mid-July, but the reproduction 

 by medium fish is later and may extend 

 into August. 



Thus spawning occurs earlier in 

 the westem Atlantic than in the east- 

 ern-Atlantic-Mediterranean system. 

 Also, the stock structure appears to be 

 less complicated in the former area than 

 the latter. 



The occurrences of larvae and ju- 

 veniles less than 10 cm long are much 

 better known in the westem Atlantic, 

 where they are widely distributed, than 

 in the eastern, where their supposed 

 range is relatively small. The larger age 

 fish form a stable and regular con- 

 centration offAfrica at least six months 

 earlier than any known occurrence of 

 this nature off the American coasts. 

 Tlie routes which these fish follow from 

 their birthplaces to their earliest con- 

 centrations in the respective areas are 

 not known. It seems probable, how- 

 ever, that analogous post-spawning 

 migrations of large and new-bom blue- 

 fin occur in the Sfraits of Florida and 

 the Strait of Gibraltar. After spawning 

 has taken place in the Gulf of Mexico 

 on the one hand, and the Mediterra- 

 nean on the other, the large spent fish 

 pass through the respective straits into 

 the Atlantic. These migrations may be 

 followed, after a few weeks, by similar 

 migrations of their offspring. 



The migratory patterns joining 

 these seasonal occurrences are likewise 

 quite analogous, insofar as they have 

 been determined. The longest migra- 

 tions are carried out by the largest fish. 



After feeding in the northem part 

 of their range, they make long south- 

 ward and generally offshore migrations 

 to their vast winter habitats which ex- 

 tend, in the western Atlantic at least, 

 into the South Atlantic. The fish in the 

 North Sea and in Scandinavian waters 

 pass north and east of the British Isles, 

 instead of going directly south through 

 the English Channel. 



Shorter northward and more in- 

 shore migrations in the spring bring 

 them to concentrations in spawning ar- 

 eas which are in the northem parts of 

 their wintering areas, or between their 



139 



