well publicized, have produced no 

 transatlantic returns at all. 



In comparing the numbers of lo- 

 cal and transatlantic returns, the 1 ,396 

 local recaptures which occurred in 

 the release season have been elimi- 

 nated. This leaves 1,517 local recap- 

 tures after eight months or more at 

 large, in companson with the 40 in 

 the Bay of Biscay. Transatlantic re- 

 turns have exceeded 1 percent of the 

 releases for only two years, 1954 and 

 1965, These figures indicate a much 

 stronger tendency for young bluefin 

 in the western Atlantic to return an- 

 nually to their usual nursery ground, 

 rather than to migrate across the At- 

 lantic. Also, the data indicate that 

 when such migrations do occur, they 

 take the form of pulses, rather than a 

 constant small flow Finally, the fact 

 that the fish are in a given area in one 

 summer and m such a distant one in 

 the next does not support the concept 

 of an annually repeated cyclical mi- 

 gration. 



Deviations from established mi- 

 gratory patterns may be caused by 

 changes in environmental conditions 

 Rodewald (1967) showed that the 

 transatlantic migrations of small blue- 

 fin recorded prior to 1967, like the 

 corresponding ones of giant tuna, 

 might have been positively related to 

 anomalies of the atmospheric circu- 

 lation pattern over the North Atlantic 

 which caused unusually strong west- 

 erly winds in the midcUe latitudes 



Two east to west transatlantic 

 migrations of small bluefin tuna have 

 been recorded (Aloncle 1973). These 

 fish were tagged in or near the Bay of 

 Biscay in September 1968 and Sep- 

 tember 1969 The former was recap- 

 tured in August 1970 south of Cape 

 Cod, and the latter in July 1 970 in the 

 New York Bight (Figure 75) These 

 migrations, in regard to dates and 

 localities of release and recapture, 

 constituted an almost exact reversal 

 of the west to east transatlantic mi- 

 grations just described These results 

 show that small bluefin sometimes 

 cross the Atlantic from east to west 

 as well as from west to east. The 

 transatlantic returns constitute a much 

 higher proportion of the total num- 

 ber (40%) for the eastern Atlantic 

 taggings, than for those in the west- 

 ern Atlantic (0.4%). The numbers of 



releases in the eastern Atlantic (34 

 fish from 1967 through 1972) and of 

 returns from these (5 fish) (Aloncle 

 1973), however, arc so small that this 

 comparison may not be statistically 

 significant. Likewise, they give no 

 basis for estimating the aimual fre- 

 quency with which east-west transat- 

 lantic migrations occur. 



We lack the data required to esti- 

 mate the approximate total numbers 

 of fish which have made transatlantic 

 migrations from these tag returns. 

 There are indications, however, that 

 the volume of these migrations has 

 sometimes been sufficient to affect 

 the fisheries in the respective aieas 



As noted above, the most impor- 

 tant west-east transatlantic migrations 

 of young bluefin tuna were from the 

 release groups of 1954 and 1965. 

 Catch records for young bluefin in 

 the Bay of Biscay in this penod are 

 somewhat confusing, but some data 

 indicate considerable increases in the 

 landings there between 1 954 and 1955 

 and between 1965 and 1966 There 

 was also a great decrease in the west- 

 ern Atlantic purse seine catch in 1 966 

 Fourteen tag returns furnish fairly 

 strong evidence for a significant west- 

 east fransatlantic migration between 

 the 1965 and 1966 seasons. The case 

 is less clear for the 1954 releases. 

 Since these fish had been at large for 

 five years, the actual date of their 

 transatlantic migration is uncertain. 

 These fish were of age 2 (year class 

 of 1 952) when released, however, and 

 nearly all subsequent small bluefin 

 which have made similar migrations 

 were recaptured in European waters 

 before they had reached age 4 There- 

 fore, it is most probable that these 

 two migrants had actually crossed the 

 Atlantic between the 1954 and 1955 

 seasons. On the basis of these returns, 

 Hamre et al (1966) suggested that 

 the great contnbution of the 1 952 year 

 class to western European catches 

 might have been due to a major move- 

 ment of fish of this year class from 

 the western to the eastern Atlantic 



The only certain period of east- 

 west transatlantic migration was 1 969- 

 1970. New England purse seine fish- 

 ermen informed us that tagged fish 

 were scarce among the early catches 

 in 1970 in comparison with the nu- 

 merous returns obtained in previous 



years from earlier local taggings. The 

 senior author attributed this situa- 

 tion to a probable influx of fish from 

 European waters, where very few 

 small bluefin had been marked. The 

 subsequent recovery of two French 

 tags verified this conjecture This im- 

 migration may have been at least 

 partially responsible for the increase 

 in the northwestern Atlantic purse 

 seine catch of small bluefin from 

 1,565 tons in 1969 to 4,200 tons in 

 1970. 



c Trans-Equatorial 

 Migrations 



The two South Atlantic recap- 

 tures of giant bluefin which had been 

 tagged in the Straits of Florida (Fig- 

 ure 75) have been discussed in the 

 context of the rmgratory pattern pro- 

 posed for the large bluefin which 

 occur m the western North Atlantic 

 during the warm season (part 2b of 

 this section). As noted there, how- 

 ever, bluefin are widely distributed 

 in the tropical Atlantic during this 

 same penod Among the possible ex- 

 planations of this situation are the 

 following: 



1 Some large bluefin may 

 switch their summer feeding grounds 

 from the northwestern Atlantic 

 coastal waters to the frontal zones of 

 the Equatonal Currents. Concentra- 

 tions of tunas of various species have 

 been observed in the frontal zones of 

 the Pacific Equatorial Current sys- 

 tem (Blackburn 1965, Laurs and 

 Lynn 1977, Murphy and Shomura 

 1972). 



2 A separate stock of large blue- 

 fin may exist in the tropical Atlantic. 



Neither of these explanations 

 would be ruled out by the 

 transequatonal tag returns, if fish of 

 the hypothetical tropical stock re- 

 produced in the western North At- 

 lantic spawmng area. These possi- 

 bilities will be discussed fiirther in 

 the conclusion of this section (Sec- 

 tion D). 



d. Stock Identification 



i. Biometric Studies 



Preliminary' biometric compan- 

 sons (Ginsburg 1953, Schuck and 

 Mather 1950, Mather 1959) of mor- 

 phometric and meristic data suggest 

 that there are slight morphological 

 differences between bluefin tuna of 



131 



