DISTRIBUTION OF TUNAS IN NORTHWESTERN ATLANTIC 



337 



q: 

 d: 



UJ 

 Q. 



UJ 



95' - 



85°- 



75° - 



65° - 



55° - 



45° - 



35° 



FiouKE 9. — Temperature ranges and mean temperatures for four species of tunas. Data based on temperatures 

 recorded at stations where tunas were captured. The thin line represents the total recorded temperature 

 range; the thick line represents 1 standard deviation on either side of the mean; and the cross bar repre- 

 sents the mean. The number of tunas used in the calculations are recorded below the species names. 



Further, the areas yielding the greatest abundance 

 of tuna have generally been found near the bound- 

 aries of tliese systems. In many cases the abrupt 

 temperature gradients present at tlie edges of 

 frontal zones of the current systems serve to 

 separate the environments on either side of the 

 gradients to such an extent tliat tuna of two or 

 more species (for instance bluefin and yellowfin) 

 may exist witliin a short distance of one another 

 and yet be distinctly separated. 



Delaware explorations indicate that tempera- 

 ture is an important environmental key to species 

 distribution in the nortli frontal area of the Gulf 

 Stream. To illustrate the close geographical prox- 

 imity of tuna of one species to those of another 

 in the presence of a temperature gradient, two 



series of isotherms were plotted from tempera- 

 ture recordings made at tuna stations in and near 

 the Gulf Stream frontal area (fig. 10). The re- 

 sulting plots repi-e.sent conditions over a period 

 of several days. Despite the rapidity with which 

 individual points in the frontal area may change 

 temperature, tlie general temperature structure 

 represented should remain essentially the same, 

 and the way in which two species — with differing 

 ecological requirements — can exist as dominants 

 in close proximity is indicated. Fishing results 

 at the same points show clearly the definite change 

 in species composition, from bluelin in the cooler 

 waters on the edge of the Gulf Stream to yellow- 

 fin in the warmer waters in the Gulf Stream. This 

 change in species composition with change in tern- 



