328 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



Since there are so many variables operating on 

 a longline, determination of the depth at which 

 the hooks were situated (estimated average fishing 

 depth) is based on calculations taken from echo- 

 graph observations of similar gear by Bullis 

 (1955) and Wathne (1959). These calculations 

 showed that, with the gear used, the average hook 

 was placed at a depth of about 173 feet. 



At all fishing stations a bathythermograph cast 

 was made to a minimum depth of 450 feet. "Water 

 temperatures at the surface and at the 173-foot 

 estimated fishing depth were obtained from these 

 casts, and within the limits of the data, tempera- 

 ture ranges and average water temperatures were 

 determined for tuna of each species. 



DISTRIBUTION OF EXPLORATORY EFFORT 



Explorations were carried out as one phase of 

 the overall program of the North Atlantic Fish- 

 eries Exploration and Gear Research Base. Fish- 

 ing effort was distributed through all seasons, but 

 it was necessarily intermittent. In all, eight 

 cruises were devoted to tuna exploration and 111 

 stations were fished with longline gear. Of the 

 111 stations, 9.9 percent were fished in winter 

 (January and February), 37.8 percent were 

 fished in spring (March, April, and May), 14 per- 

 cent in early summer (June), 18.5 percent in 

 middle and late summer (July and August), and 

 19.8 percent in fall (September and October). 

 No stations were fished in November or 

 December. 



The first cruise in the oceanic region was de- 

 signed to cover the entire region. The vessel 

 route ran south from the 1,000- fathom curv^e off 

 New England to the approximate latitude of Cape 

 Hatteras, then east past Bermuda, and north to 

 the point of origin. The cruise took place in 

 the early spring, for the cruise objective was to 

 intercept, and record the distribution of bluefin 

 tuna during a portion of the spring migration. 

 The seven cruises that followed were carried out 

 to cover areas where tuna had been concentrated 

 on previous cruises or where the possibility of 

 tuna concentrations had been indicated by hydro- 

 graphic data or theories on migration I'outes, and 

 to cover systematically the Gulf Stream area with 

 a series of longline sets, 95 to 120 miles apart. 



PHYSICAL CHARACTERISTICS OF THE 

 REGION EXPLORED 



The oceanic region explored by the DeUwa/re 

 extends south and east of the edge of the Conti- 

 nental Shelf of northeastern North America to 

 about latitude 30° N. and longitude 50° W. The 

 region is characterized by the presence of that por- 

 tion of the Gulf Stream system known as the Gulf 

 Stream proper. Since the region is influenced 

 greatly by the Gulf Stream system, an understand- 

 ing of the physical nature of the system is a neces- 

 sary preliminary to an understanding of the 

 distribution of that region's fauna. 



The Gulf Stream system is composed of three 

 principal parts: the Florida Current from the 

 Tortugas to Cape Hatteras; the Gulf Stream 

 proper from Cape Hatteras to the Grand Banks; 

 and the North Atlantic Current from the Grand 

 Banks eastward. As the principal current sys- 

 tem of the western North Atlantic, the Gulf 

 Stream system is analogous to the Kuroshio Cur- 

 rent of the western North Pacific. It has been 

 studied in detail by the Woods Hole Oceano- 

 graphic Institution, and as a result, considerable 

 information is available that relates directly to 

 a study of the environmental conditions that may 

 affect the distribution of the timas. 



The Gulf Stream system is described by Stom- 

 mel (1960) as: 



... a narrow, intense, northeastward-flowing current 

 whicli returns to tlie nortli again the southward-driven 

 Sargasso Sea water that has passed through the Carib- 

 bean and has turned through the Florida Straits. The 

 Gulf Stream flows along the western boundary of the 

 warm Sargasso Sea surface water. As the Stream turns 

 toward the east, off the Grand Banlss, it acts as a kind 

 of dynamic barrier, or dam, which, by virtue of coriolis 

 forces, restrains the warm Sargasso Sea water from 

 overflowing the colder northern water of the North At- 

 lantic. The water in the Stream is not significantly dif- 

 ferent in temperature from the large mass of warm 

 water which lies to the right of its direction of motion. 



Studies have shown that variations occur within 

 the Gulf Stream system in the fonn of wavelike 

 perturbations that can be likened to the meanders 

 of a geologically old river or stream, and other 

 physical complications occur along the edge of 

 the Stream in the form of cyclonic eddies (Iselin, 

 1960). Then too, the water masses of the Gulf 



