SCIARROTTA and NELSON: DIEL BEHAVIOR OF BLUE SHARK 



Application, Tracking, and Recovery 



The sharks to be tagged were attracted with bait 

 to the 7-m tracking boat. Cut Pacific mackerel, 

 Scomber japonicus, in two bait cannisters, was 

 suspended at depths of about 5 and 15 m. Since 

 drifting of the boat established the odor corridor 

 necessary for shark attraction, the time needed for 

 attraction decreased as the wind (and drift rate) 

 increased. The time necessary to attract the first 

 blue shark ranged from 10 min to 4 h and the mean 

 was 1.5 h. 



Whenever a choice was possible, a larger indi- 

 vidual shark was selected for tagging in order to 

 lessen the possible effect of the transmitter on its 

 behavior. The shark to be tagged was enticed to 

 the surface next to the boat using a short baited 

 line, then harpoon tagged in the middorsal region 

 anterior to the first dorsal fin. The sex of the shark 

 was noted and its total length estimated (Table 1). 

 An attempt was made to prevent the shark from 

 actually taking the bait, as this might have 

 influenced subsequent feeding motivation. 



The transmitter was attached to the shark by a 

 stainless steel dart (Floy FH 69) thrust beneath 

 the skin with a hand-held applicator pole. The 

 transmitter package included a syntactic foam 

 float and a magnesium breakaway link which cor- 

 roded through in a roughly predictable time, al- 

 lowing the unit to float to the surface for recovery. 



The tracking procedure involved continuous 

 monitoring of the signal from the drifting boat 

 using the omnidirectional hydrophone. As the 

 signal became weak, its direction was determined 

 with the directional hydrophone, and the boat was 

 then moved closer to the shark. Distance to the 

 shark was estimated primarily from approximate 

 signal strength and by triangulation from suc- 

 cessive positions of the moving boat. To minimize 

 the effect of the boat on the shark's behavior, an 

 effort was made to maintain a distance of at least 

 200 m between the boat and the shark. 



Ultrasonic tracking in the study area at times 

 presented certain problems. Noise from crusta- 

 ceans, echo-locating cetaceans, ship traffic, wave 

 action, hydrophone turbulence, and bottom echoes 

 could be picked up by the receivers, and if of high 

 enough level, would mask the data pulses. Signal 

 reception was also affected when the shark went 

 below the thermocline (reflection) or was swim- 

 ming very near the surface (wave shielding, 

 bubble attenuation, downward ray refraction). 

 These factors at times caused signal losses that 



could be counteracted only by lowering the hydro- 

 phone to a depth of about 10 or 15 m. 



Data Recording and Reduction 



Approximately once per half-hour, a 30-s data 

 sequence was recorded on magnetic tape and the 

 estimated position of the shark plotted. The 

 omnidirectional hydrophone was preferred for 

 recording purposes whenever the signal was 

 sufficiently strong. It was less convenient to use 

 the directional hydrophone for recording long data 

 sequences because of the difficulty of maintaining 

 continuous accurate aim, thus resulting in greater 

 signal-strength variability. 



Decoding of the single-channel depth data re- 

 quired only a stopwatch and calibration graph. 

 Ten pulse intervals were timed and converted to a 

 depth value. For the multichannel data, the tape 

 recordings were converted into paper oscillograms 

 on which the pulse intervals were measured man- 

 ually. For analysis, the mean value for three clear 

 8-channel sequences were graphed for each half- 

 hour recording period. 



RESULTS 



The telemetered blue sharks were generally 

 most active at night, with highest activity in the 

 early evening and lowest activity in the early day- 

 light morning. While some activity occurred 

 throughout the diel cycle, the mean recorded val- 

 ues for all trackings were greater at night for 1) 

 rate of horizontal movement, 2) swimming speed, 

 3) variability in depth, and 4) variability in 

 swimming direction. Experienced tracking per- 

 sonnel were also able to detect by ear subtle 

 changes in the multiplexed pulse intervals. Al- 

 though not quantified, the trackers received the 

 distinct impression that these changes occurred 

 more often at night — thus further supporting a 

 nocturnal activity maximum. 



Horizontal Movement — 

 Island-Oriented Migration 



The most striking behavior demonstrated by the 

 present study was a seasonal, evening-twilight 

 migration from the epipelagic offshore habitat to 

 the shallower waters bordering the island. Be- 

 tween late March and early June, each of the 

 seven sharks tracked made this movement to- 



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