FISHERY BULLETIN: VOL. 75, NO. 3 



times of fog applicodon 



\ /. / »-x r \ — ~m * \ 



\/\ / " V /\\ 



X \ ,* f~~x „x «•/ 



fiATE OF MOVEMENT 



"I 1 1 T~ 



i — r — i — r- 



TIME OF DAY 



— i 1 1 1 — i r 



02 04 06 



FIGURE 5. — Comparison of mean rate of movement (all sharks) 

 and telemetered swimming speed (sharks with speed sensors) for 

 blue sharks. Note the increase in both parameters at night, the 

 greater values for swimming speeds (as expected), the close simi- 

 larity during times corresponding to shoreward movements (rel- 

 atively straight swimming), and the large disparity in early 

 evening (relatively nonstraight swimming). 



20 22 



TIME OF DAY 



FIGURE 6.— Mean depths of all blue sharks tracked with trans- 

 mitters having depth sensors. The first hour of each tracking is 

 deleted because of the initial plunge in response to tag applica- 

 tion. Note the generally greater depths at night. 



habitat (mesopelagic), but since the blue shark's 

 habitat appears relatively shallow (epipelagic), 

 the large eye would seem best suited to visual 

 hunting at night. 



It is known that cephalopods and small pelagic 

 fishes form a major part of the diet of blue sharks 

 (Strasburg 1958; Stevens 1973; Tricas 1977). The 

 observed seasonal differences in diel movement 

 patterns (Figures 1, 3) may reflect differences in 

 type or location of prey. Fishery landings of mar- 

 ket squid, Loligo opalescens, were high during 

 February to June 1972, but low from July to De- 

 cember (Pinkas 1974), thereby indicating the in- 

 shore presence of spawning congregations (Frey 



TIME AFTER APPLICATION (hr) 



FIGURE 7. — Mean depths of blue sharks for the first 3 h of each 

 tracking. Upper curve, all 12 sharks carrying transmitters with 

 depth sensors. Lower curve, seven sharks judged to have made 

 an "abnormal" plunge in response to the trauma of tag applica- 

 tion. Note that the initial depth response appears to have sub- 

 sided by the recording session 1.5 h after application. 



1971), which are susceptible to commercial 

 fishermen using night-lighting techniques. Cou- 

 steau and Cousteau (1970) described blue sharks 

 gorging themselves on spawning squid that were 

 light-attracted to the surface near their vessel. 



The evening-twilight onshore movements 

 which occurred during March to early June may 

 be due to the nearshore abundance of squid and a 

 possibly reduced availability of prey offshore. 

 Conversely, the offshore pattern from late June to 

 October may be a result of reduced squid popula- 

 tion nearshore, but increased populations of jack 

 mackerel, Trachurus symmetricus, and anchovy 

 offshore. The limited stomach-content data col- 

 lected during this study support this hypothesis. 



In regard to depth/temperature preferences, the 

 results of Strasburg (1958) are somewhat different 

 from those of the present study. His longline 

 catches of blue sharks at equivalent latitudes were 

 from depths of 53 to 93 m (45%), 93 to 143 m (30%), 

 and 123 to 166 m (25%). The blue sharks tracked 

 in the present study appeared to exceed 93 m only 

 about 5.1% of the time (excluding initial plunges). 

 It is conceivable, however, that Strasburg's per- 

 centages may have been influenced by the sharks 

 being attracted deeper than normal by the sloping 

 odor corridors from baits on the gradually sinking 

 longlines. That blue sharks on occasion go even 

 deeper than Strasburg's deepest hooks was noted 

 by Pethon (1970) who reported captures in Norwe- 



526 



