SUNRISE 



SUNRISE 



SUNSET 



FISHERY BULLETIN: VOL. 79, NO. 2 



SUNRISE SUNSET 



Figure 8. — Depth record for swordfish no. 7 off Cape Hatteras in the Atlantic superimposed on a 1° C isotherm plot drawn from 

 expendable bathythermograph casts. On 9 November the fish passed under a cold streamer of shelf water (see Figure 7) and rose 

 toward the surface in the middle of the day, probably in response to the decreased light. On 10 November the deepening isotherms 

 indicate that the fish was following an isolume, descending 1 h before dawn, reaching its greatest depth at midday and coming to the 

 surface 1 h after sunset. Continuous recording of depth on 11 November shows that the rise toward the surface at sunset was made as a 

 series of steps with several minute pauses at each level. The flattening isotherms toward the end of the record indicate that the fish had 

 entered the Sargasso Sea. 



opposite in phase to that of the skipjack tuna and 

 swordfish, with the sharks coming inshore at 

 night. The authors suggested that the movements 

 were associated with the nighttime availability of 

 squid near the beach. It is likely that all of these 

 diel cycles of movement are linked to changes in 

 the location and availability of food. 



Swordfish no. 4, 5, and 6, which were offshore 

 in water 400-800 m deep, moved slowly west after 

 they had been harpooned (Figure 4). We aban- 

 doned no. 4, but no. 5 and 6 continued until they 

 were over San Jose Canyon, the most prominent 

 submarine canyon in the area. Near nightfall, 

 both of these swordfish changed course and moved 

 south along the length of the canyon. The cor- 

 respondence with the axis of the canyon is not 

 exact, but the course change and the movement of 

 both fish along the canyon suggests they were 

 responding to the bottom topography. Commercial 

 longline fishermen feel that the submarine can- 

 yons and hummocky areas along the edge of the 

 continental shelf are good places to find swordfish. 



Currents flowing over rough bottom produce 

 eddies and flow separation features which may 

 extend to the surface. The patterns of such turbu- 

 lent flow affect the distribution of biological sound 

 scattering particles and produce concentrations of 

 organisms which can be seen in 200 kHz echo- 

 grams made over Hudson Canyon in the Atlantic 



(Orr ). The concentration of organisms as a result 

 of turbulence generated by the rough bottom may 

 be the feature that attracts swordfish to the 

 waters over submarine canyons. 



Fishermen had suggested that swordfish in the 

 area where no. 2 and 3 were followed were part of a 

 resident population, while those in the area of no. 

 4,5, and 6 were transient. Our results are consis- 

 tent with this notion. 



In the Atlantic, swordfish no. 7 swam in a 

 southeasterly direction at a speed of 1.5-2.0 km/h 

 during the first day. On the second day it entered 

 the Gulf Stream as indicated by the deepening of 

 the isotherms in Figure 8. While in the Gulf 

 Stream it moved on a course a little north of east at 

 a speed of 5.5 km/h, with most of this velocity 

 contributed by the current which was flowing 

 northeast. The course of the fish was to the south 

 of the direction of the current, indicating that 

 there was a southerly component to its swdmming. 

 It probably continued to swim on its original 

 course and speed after entering the Gulf Stream. 

 By the time we abandoned it on the third day, it 

 had crossed the Gulf Stream, which is about 75 km 

 wide at this point, and entered the Sargasso Sea. 

 This can be seen from the flattening of the deep 



'Marshall Orr, Woods Hole Oceanographic Institution, Woods 

 Hole, MA 02543, pers. commun. 1980, manuscr. in prep. 



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