DISTRIBUTION OF TUNAS IN NORTHWESTERN ATLANTIC 



329 



Stream frontal area, which occur between the 

 Gulf Stream axis and the cooler waters to the 

 north, are not stable. They have been described 

 by von Arx, Bumpus, and Richardson (1955) as 

 having "... a structure which, as far as one can 

 tell, is best interpreted as a succession of short, 

 overlapping segments wWch may be described as 

 'shingles'." The "shingle effect" is of left-hand 

 orientation as one looks down the axis of the Gulf 

 Stream system. 



Bathythermograph recordings indicate that rel- 

 atively large variations in temperature occur with- 

 in short distances in the oceanic region lying on 

 either side of the Gulf Stream. On several 

 cruises, bathythermograph casts were made at 

 eacli end of tlie longline set, and even in this rela- 

 tively short distance, marked variations in tem- 

 perature were observed. 



Tlie Gulf Stream, south of Xew England, ef- 

 fectively divides tlie region investigated into a 

 cool-water northern area and a warm-water 

 southern area. 



DISTRIBUTION OF DOMINANT 

 TUNA SPECIES 



EXPLORATORY RESULTS AND OBSERVATIONS 



Catch rates, for the entire investigation and 

 for all species of tuna, ranged from no tuna at 

 several stations to a high figure of 20.8 tuna per 

 100 hooks. Catch rates and catch composition 

 varied widely with season and geographical area 

 fished (figs. 4r-8), as did the weights of individual 

 fish (table 1). 



Bluefin Tuna 



The single exploratoiy cruise conducted in win- 

 ter indicated that bluefin tuna are common in the 

 portion of the oceanic region lying east of the 

 Continental Slielf, from Cape Hatteras to south 

 of Xew England (fig. 4A). The e.xtent of the 

 bluefin population to the north and east toward 

 the Grand Banks during winter and late fall is 

 unknown. The greatest number of bluefin in win- 

 ter were caught along the northern edge of the 

 Gulf Stream (fig. 4A) where, at one station, the 

 catcli rate was 5.7 bluefin per 100 hooks. The fish 

 caught at this station averaged nearly 300 pounds 

 each, although large fish apparently are not com- 

 mon in winter, in the area investigated. 



The distribution pattern assumed by bluefin in 

 spring (fig. 5A) is similar to that in winter, at 

 least in the portion of the region explored in both 

 seasons: and exploratory catches indicate that 

 commercial longline fishing might be feasible in 

 the vicinity of the Gulf Stream in spring, and pos- 

 sibly in winter. Bluefin were taken in spring at 

 the easternmost stations occupied, south of Grand 

 Banks and east of Bermuda. Highest catch rates 

 were achieved farther south and west of these 

 stations, however, in the general area of the Gulf 

 Stream (fig. 6A). At one station, south of Cape 

 Cod on the inshore side of the Gulf Stream, the 

 catch rate was 10.3 bluefin per 100 hooks. Almost 

 directly east of that station, on the northern edge 

 of the Stream, the maximum catch rate for the 

 entire series of cruises was attained — 20.8 bluefin 

 per 100 hooks. 



' Seasons are defincil as follows: Wintcr=January, February; sprine = 

 March, .\pril, May; early summer=June; summer=J<ily, August; fall = 

 September, October. No explorations were conducted in November or 

 December. 



• Average weights of species of minor importance are: Bigeyc, 137 pounds; 

 skipjack, 18 pounds; blackfin, 13 pounds. 

 > Reflects a large catch of fish made at one station. 



