Govoni et al : Distribution of larval Xiphias g/adius off the soutfieastern United States 



69 



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Figure 4 



Advanced very high resolution infrared radiometer images of sea-surface 

 temperatures off the southeastern Atlantic coast of the United States on 28 

 May 1997 from NOAA polar orbiting satellite (darker to lighter shades denote 

 cooler to warmer water; circles denote locations of hydrographic stations occu- 

 pied on 3 June 1997). 



yielded more than one larva, i.e. two larvae in each. Ten 

 collections with larvae present were within the frontal 

 zone, whereas two collections were in the body of the 

 Gulf Stream (Table 1). The probability of observing the 

 presence or absence of larvae with a frequency at least 

 this extreme was 0.176% (two-tailed test). 



In September 1988 (CH8807), the Gulf Stream fron- 

 tal zone was defined by 27° to 29°C surface isotherms. 

 With the defining criterion of horizontally compressed 

 surface isotherms within this temperature range, the 

 width of this frontal zone ranged from 18 to 93 km. 

 This area, however, encompassed a westward intru- 

 sion of the Gulf Stream that developed over the tran- 

 sect grid (Fig. 3) in the wake of the Charleston Bump, 

 a topographic rise at the continental shelf break that 

 typically forces an eastward deflection of the Gulf 

 Stream (Pietrafesa et al., 1985). Isolated surface pools 

 of 27° to 29°C water inshore of the Gulf Stream front 

 proper (Fig. 6, B and F) indicated this intrusion. This 

 convolution of the Gulf Stream front proper mani- 

 fested three fronts across the shelf but these were 

 considered a composite single front for analysis. Shear 

 zones (determined by rapid increases in the ship's 

 set), drift lines of Sargassum, and discontinuities in 

 sea-surface texture, were embedded within the fron- 



tal zone and evidenced probable convergence of sur- 

 face water (Stommel, 1966; Olson et al., 1994). The 

 156 collections of CH8807 yielded 12 swordfish larvae. 

 One collection yielded three larvae; another two. Lar- 

 vae were collected exclusively within the frontal zone 

 (Table 1), one at the tip of the intrusion (station 58; 

 Fig. 6F). The probability of observing the presence or 

 absence of larvae at least this extreme was 0.020%. 



In May 1997 (CH9703), the Gulf Stream frontal 

 zone was defined by 27° to 29°C surface isotherms. 

 The frontal zone was moving rapidly eastward. On 

 23 May, 8 d before the collection of swordfish larvae 

 within the frontal zone, an elongate filament of a Gulf 

 Stream intrusion lay inshore of the collection area 

 (Fig. 41. The isolated pool of 25°C water at the sur- 

 face and eastward of the frontal zone (Fig. 7) indicated 

 that this intrusion was extant on 31 May 1997 when 

 collections of CH9703 were taken. The six neuston col- 

 lections of CH9703 yielded nine larvae. One collection 

 taken along the frontal axis and one collection taken 

 on the Gulf Stream side of the axis yielded two larvae 

 each; the other collection taken along the axis yielded 

 three. Larvae were present in two collections along the 

 frontal axis, two collections 1 km to the Gulf Stream 

 side of the axis, and one of two collections taken 0.5 km 



