FISHERY BULLETIN: VOL. 75, NO. 3 



FIGURE 1. — Positions of seven blue sharks tracked from late 

 March through early June 1972. Note that all day positions are 

 offshore from the island, while the majority of night positions are 

 nearshore, often in relatively shallow water. 



wards the island shoreline. Examples of trackings 

 of this type are shown in Figures 1 and 2. 



These sharks remained offshore in the general 

 vicinity of the tagging during the daylight hours. 

 Approximately at dusk, the sharks initiated a rel- 

 atively straight-line course towards the island. It 

 is difficult to place precise times on when the 

 sharks began this move, but it appeared to be from 

 about 1.6 h before to 1.3 h after sunset, with a 

 mean slightly after sunset. During the shoreward 

 movement, the sharks swam at depths varying 

 from near the surface to over 90 m. Once near the 

 island, the sharks usually moved in an easterly 

 direction parallel to the shoreline. Several hours 

 before sunrise, there was a directed movement 

 away from the island back to the offshore envi- 

 ronment. The closest estimated nighttime ap- 

 proaches to the island for these individuals aver- 

 aged 1,100 m (range, 200-4,000), corresponding to 

 water depth averaging 115 m (range, 80-380). 



Although three preliminary trackings in early 

 and mid-March ended prior to nightfall, the last of 

 these appeared to show the beginnings of a shore- 

 ward movement prior to transmitter release. One 

 tracking in mid-June ended prematurely prior to 

 dusk. From late June until early October, the 

 three sharks successfully tracked remained off- 

 shore throughout the day and night over bottom 

 depths of 500 m or more (Figures 3, 4). 



Rate of Horizontal Movement 



Rate of movement was calculated for each shark 

 from its half-hourly estimated positions such as 



522 



shown in Figures 2 and 4. The mean values for all 

 sharks tracked (Figure 5) showed an increase in 

 rate of movement at sunset which continued 

 through most of the night. The mean rate of 

 movement for the daytime was 1.2 km/h (range, 

 0.3-7.0); for the nighttime, 1.8 km/h (range, 0.4- 

 4.0). 



Swimming Speed 



There was a definite increase in telemetered 

 instantaneous swimming speed at night (Figure 

 5). However, no abrupt increase in speed occurred 

 at the dusk transition, as might be expected in 

 view of the rate of movement increase at that time. 

 Swimming speed peaked a few hours after sunset 

 and remained comparatively high until a few 

 hours before sunrise. The artifactual burst of 

 speed immediately after tag application was short 

 lived, even in those sharks that did not promptly 

 return to the bait cannister. 



Although the maximum speed capability of the 

 sensor was 5 km/h, this speed was not often 

 reached during the half-hourly data recording 

 periods, which suggests speeds in excess of 5 km/h 

 seldom occurred. The mean swimming speed for 

 the daytime was 1.3 km/h, for the nighttime 2.8 

 km/h, while the range for both covered the entire 

 sensor range. 



Increases in swimming speed were often as- 

 sociated with brief dives during the same record- 

 ing session (Figures 2, 4). In seven of the eight 

 trackings in which both speed and depth were 

 telemetered, and where tracking extended at least 

 into dusk, the highest mean speeds occurred at 

 relatively great depths (means: 4.8 km/h, 69 m) 

 while the lowest speeds occurred at much shal- 

 lower depths (means: 0.5 km/h, 20 m). This 

 suggests that some factor in deeper water stimu- 

 lated this speed increase, possibly presence of food. 



Swimming Direction 



Figure 5 shows clearly the relationship between 

 swimming speed and rate of movement through- 

 out the diel cycle. As expected, swimming speeds 

 had the higher values, as the two measures would 

 have been equal only in cases where the shark 

 swam in a straight line for the entire 30-min in- 

 terval between position determinations. During 

 daylight hours both rates were moderately close, 

 suggesting that the sharks made gradual changes 

 in swimming direction rather than abrupt 



