FISHERY BULLETIN: VOL. 75. NO. 3 



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FIGURE 3.— Positions of three blue sharks tracked from late 

 June to early October 1972. Note that both day and night posi- 

 tions are well offshore over relatively great depths. 



tween rate of movement and swimming speed was 

 during the early evening, evidence that much of 

 the swimming then was variable in direction — a 

 possible indication of searching for and/or pursu- 

 ing prey. Beginning in the early morning and con- 

 tinuing through dawn, the differences between 

 the two rates lessened. 



A compass sensor for direct measurement of in- 

 stantaneous swimming direction (azimuth) was 

 incorporated during only one successful tracking. 

 The compass data from this tracking (Figure 4) 

 show that the greatest number of multiple- 

 direction recordings (i.e., during single-recording 

 periods) occurred at night, suggesting that vari- 

 ability of swimming direction is generally greater 

 at night. During one nighttime recording, a 

 change of at least 360° coupled with a speed 

 change of 1 to 5 km/h was noted during one 15-s 

 period. 



variability in depth. During four trackings, the 

 sharks may have been close to the bottom when in 

 the relatively shallow water near the island. 



The first hour of depth data were excluded from 

 Figure 6 because of what appears to be an initial 

 plunge induced by tagging trauma. As shown in 

 Figure 7, the data also suggest that this initial 

 effect decreased or disappeared within 1.5 h after 

 tagging. About half of the sharks tagged exhib- 

 ited this "abnormal" plunge (to a mean depth of at 

 least 95 m) within 0.5 h of being tagged. The others 

 apparently did not — possibly a result of the tag 

 dart penetrating in a less sensitive spot. Of the 

 first nine sharks tagged, six were seen to return to 

 the bait cannister within seconds after transmit- 

 ter application — suggesting little, if any, tagging 

 trauma. Two of these six sharks, however, still 

 made a deep dive by the next recording session. 



Temperature 



Blue sharks in the study area appeared to prefer 

 a relatively narrow range of water temperatures. 

 Overall, the telemetered sharks were found in a 

 temperature range of 8.5° to 17.5°C, but occurred 

 in the much narrower range of 14.0° to 16.0°C for 

 73% of the time. Seasonality of diel depth/tem- 

 perature selectivity was not apparent from either 

 the temperature or depth data. As expected, the 

 telemetered depth and temperature data usually 

 corresponded quite well, i.e., an increase in depth 

 accompanied by a decrease in temperature (Fig- 

 ures 2, 4). Individuals were most often seen 

 swimming at the surface during the cooler 

 months, but rarely during either the coldest or 

 warmest months, a behavior that may have been 

 influenced by surface temperatures. 



Vertical Movement 



DISCUSSION 



Figure 6 illustrates the mean depths teleme- 

 tered from all sharks with transmitters equipped 

 with depth sensors. The sharks were within a 

 depth range of 18 to 42 m for 92% of the time; they 

 appeared to equal or exceed 100 m only during 

 3.9% of the readings (excluding initial plunges). 

 The apparent tendency was a slight increase in 

 mean depth at night. The mean daytime depth was 

 30 m; at night 40 m. Individual tracking graphs 

 show that the sharks covered the entire depth 

 range of the sensors (0-110 m) during both day 

 and night, but that at night there were more verti- 

 cal excursions from shallow to deep, i.e., greater 



It is not surprising that the blue shark appears 

 more active at night than during the day. Car- 

 charhinids in general are considered by Randall 

 (1967) to be nocturnal. In addition, most sharks 

 studied quantitatively in this regard have proven 

 to be basically nocturnal, the bonnethead shark 

 studied by Myrberg and Gruber (1974) being a 

 possible exception. Like other nocturnal sharks, 

 however, blue sharks certainly feed diurnally at 

 times, and it is common knowledge that they read- 

 ily respond to opportunistic feeding stimuli (e.g., 

 bait) during the day. There have also been obser- 

 vations of blue sharks feeding naturally during 



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