DISTRIBUTION OF TUNAS IN NORTHWESTERN ATLANTIC 



333 



Although bluefin cauglit during the spring were 

 mostly small, several weighed over 300 pounds 

 and one, caught east of Bermuda, was estimated to 

 weigh between 350 and 400 pounds. 



By late spring or early summer (fig. 6A), blue- 

 fin begin to disappear from the oceanic region, 

 and by midsummer (fig. 7A) are uncommon. 

 They apparently remain scarce in oceanic waters 

 through early fall (fig. 8A). 



The range of water temperatures in which blue- 

 fin were taken during the explorations and the 

 average water temperature for all stations at which 

 bluefin were taken are shown in figure 9. The 

 mean temperatures were obtained by using the fol- 

 lowing formula : 



_ (fxt) 



where f=the number of fish in a sample; t= 

 water temperature in degrees F. ; and N = the 

 total number of fish of each species for which data 

 were available. 



Yellowfin Tuna 



Yellowfin tuna apparently do not occur in large 

 numbers anywhere in the oceanic region of the 

 northwestern Atlantic in winter, and none were 

 taken north of the Gulf Stream during that sea- 

 son (fig. 4B). 



In the spring the fish are widely distributed 

 over the southern portion of the region, but ex- 

 plorations indicate that they are not generally 

 pre.sent in large concentrations. However, at one 

 station, east of Cape Hatteras near the Gulf 

 Stream axis, yellowfin were caught at the rate of 

 14.1 fish per 100 hooks (fig. .5B). 



In summer and fall (figs. 6B-8B), the fish were 

 found in greater general concentration, especially 

 in the vicinity of the Gulf Stream. The highest 

 catch rate achieved during explorations in the 

 summer was 5.0 yellowfin per 100 hooks, and the 

 highest catch rate in the fall was 8.5 yellowfin 

 per 100 hooks. A wide variation in weight was 

 noted among yellowfin caught at stations fished 

 in different areas and seasons. For instance, at a 

 station fished in July in the area north of the 

 Gulf Stream the fish averaged only 30 pounds, 

 whereas in April at a station farther south the fish 

 averaged 92 pounds. 



Tiie observed range of water temperature and 

 the average temperature of the water inliabited by 



yellowfin taken during the explorations are shown 

 in figure 9. 



Albacore 



Albacore appear to be widely distributed, but 

 thinly scattered, in the oceanic region in all sea- 

 sons (figs. 4D-8I)), and no specific patterns were 

 discerned, either of migration or distribution. The 

 maximum albacore yield, from a longline set, was 

 at the rate of 2.0 fish per 100 hooks. Many of the 

 albacore taken were large, and the average weight 

 for all albacore taken at all stations was 41 

 pounds. Observed temperature limits and aver- 

 ages for albacore are shown in figure 9. 



Other Tuna Species 



In addition to the dominant species for which 

 the distribution has been described, several other 

 species, of lesser importance, were taken. These 

 included skipjack and bigeye. Individuals of 

 these species, however, were taken so rarely and 

 in such small numbers that little can be said con- 

 cerning their patterns of distribution on the basis 

 of the Delaware explorations. Some of the sta- 

 tions at which bigeye were caught are shown in 

 figure 7C and represent the northernmost records 

 of the species for the western North Atlantic 

 (Mather and Gibbs, 1958). The temperature 

 range of waters in which bigeye were caught is 

 shown in figure 9. 



DISCUSSION 



Although the exploratory coverage was not 

 complete, owing to the intermittent scheduling 

 necessitated to . carrj' out several other program 

 phases during the period of investigation, the out- 

 line of distribution of bluefin and yellowfin tunas 

 that emerges does provide a substantial basis for 

 future work and a more complete understanding 

 than was formerly available. 



"Workers in Japan (Nakamura, 1951; Naka- 

 mura, Yabuta, and Mimura, 1956; Uda, 1953) and 

 the United States (Sette, 1955) have established 

 that concentrations of tuna are generally associ- 

 ated with oceanic, convergent, tropical, and sub- 

 tropical water masses — particularly those of major 

 circulatory systems. Extensive work in the Pacific 

 has resulted in the discovery of definite correla- 

 tions between tuna abundance and the major 

 circulatory systems of the Kuroshio or North 

 Pacific Current, the North and South Equatorial 

 Currents, and the counter currents to these. 



