338 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



MV DELAWARE 



Cruise 60-6 



April l8-Moy6, I960 



EslimotBd 



Fishing D«pib 

 Isoiherm— 'f 



Stohon No Call 







Figure 10. — Effect of a temperature gradient in allowing 

 the presence of two ecologically dissimilar tuna species, 

 bluefln (BF) and yellowfin (YF), in close proximity. 

 Lines represent isotherms ; numbered circles on lines 

 represent the position of fishing stations with respect 

 to the temperature. Station 100 (upper figure) was 

 fished directly in the axis of the Gulf Stream. 



perature was indicated throughout the investiga- 

 tion. 



A comparison of the weighted average temper- 

 ature of the water at stations where bluefin and 

 yellowfin were caught (fig. 9) provides additional 

 evidence that bluefin prefer areas of cooler water 

 and are most abundant in cool-water areas despite 

 the wide spread of temperatures (36° F.) included 

 within the range observed for this species. 



In early summer, when the bluefin begin to mi- 

 grate from the oceanic areas, the occurrence of 

 bluefin is reported, each year, on the Continental 

 Shelf. This and evidence obtained from tagging 

 tend to indicate tliat at least a part, of the bluefin 

 tuna of the New England and Nova Scotia sum- 

 mer fishery migrate to the Continental Shelf from 

 tlie Gulf Stream area. One tuna, captured and 

 tagged by the Delaware on May 24, 1959, 325 

 miles east of Ocean City, Md., was recaptured in 



a commercial purse seine near Cape Cod about 3 

 months later. But earlier evidence from tagging 

 work accomplished by the Woods Hole Oceano- 

 graphic Institution on the Continental Shelf in- 

 dicates that tuna make transoceanic migrations 

 (Mather, 1960). It is not unreasonable to sup- 

 pose, therefore, that part, of the tuna that winter 

 near the Gulf Stream migrate to more distant 

 waters than the waters of the nearby Continental 

 Shelf. Nor is it unreasonable to suppose that at 

 least part of the tuna found on the Continental 

 Shelf of New England and Canada in summer 

 months have migrated from points farther distant 

 than the Gulf Stream area, as would be necessary 

 if the popular theory that the large fish winter in 

 the Caribbean were confirmed. 



The available temperature data show that mean 

 surface temperatures at the Boston light vessel 

 ranged from 53.7° to 64.2° F. during the summer 

 of 1957 (Day, 1959). The weighted average tem- 

 perature of 59.9° F., that was calculated for 

 oceanic stations yielding catches of bluefin tuna 

 near the Gulf Stream (fig. 9), is within this range. 

 Migration of bluefin into the shelf area from 

 oceanic stations in late spring or early summer 

 would, therefore, be accompanied by only minor 

 temperature changes. 



In contrast, Rivas (1955), in his discussion of 

 the possible migration of bluefin from the Straits 

 of Florida to the Gulf of Maine, stated "... 

 they go in two to three weeks from temperatures 

 of 28 to 29 degrees centigrade into waters which 

 are 16 to 18 degrees centigrade." ( From 82° to 84° 

 F. to 61° to 64° F.) This spread of 18° to 23° rep- 

 resents a much greater change than woidd be 

 faced by fish moving into the Gulf of Maine from 

 the Gulf Stream area to the north, but seems to be 

 well within the realm of possibility. 



The explorations indicate that the early sum- 

 mer period, marking the migration of the blue- 

 fin from the oceanic area north of the Gulf Stream, 

 also represents a period of transition in species 

 dominance in that region. As the bluefin, which 

 have been the dominant tuna through the winter 

 and spring, migrate from tlie area, yellowfin ap- 

 pear and assume dominance through the summer 

 (figs.4C-8C). 



The migratory patterns assumed by the yellow- 

 fin during this spring-summer shift and the routes 

 taken have not been observed; but, on the basis 



