TRICAS: BLUE SHARK AND ITS PREY SPECIES 



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2 H 



2 I 



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0700 0900 1200 1500 1800 



TIME OF SHARK CAPTURE 



FIGURE 4. — Frequency of digestive states of anchovies in rela- 

 tion to time recovered from wild blue shark stomachs. Freshly 

 ingested i light bars): anchovy body, scales, skin, and fins intact; 

 represent anchovies about 0-8 h after ingestion. Moderate diges- 

 tion (hatched bars); anchovy with head detached, open body 

 cavity, and exposed myotome; represents anchovies about 9-20 h 

 after ingestion. Advanced digestion (identification of anchovy 

 possible only by vertebrae or otolithsl not included in distribu- 

 tion because of broad time span represented by this state (about 

 20 h or longer). Numbers indicate sharks sampled that contained 

 anchovies in freshly ingested or moderately digested states. 

 Water temperatures in the field ranged from 13^ to 19°C, 



Free-swimming sharks responded to moving 

 prey (bait on slowly retrieved light fishing line) 

 with a consistent posterior orientation, as illus- 

 trated in my field notes: "As I retrieved the bait 

 towards the boat, a 1,5-m male shark sighted the 

 anchovy and then swam in a wide arc so as to 

 approach the bait from behind. He then made a 

 rapid posterior-oriented dash up to the anchovy, 

 bit the bait once at mid body, and swallowed it 

 whole," Replicate tests using Pacific mackerel 

 elicited similar posterior attacks; in these cases, 

 the shark rolled partially on its side to take the 

 larger fish prey. Tooth marks on bait in these test 

 situations were similar to those on anchovies re- 

 covered from stomachs of wild sharks. 



Sharks also showed several distinct patterns of 

 predatory behavior while feeding on schools of 

 spawning squid. Each feeding patterq appeared to 

 be correlated with the size and level of activity of 

 each shark as well as the physical configuration 

 and alertness of the squid within the school. Sur- 

 face and underwater observations of sharks feed- 

 ing on night-light attracted squid revealed four 

 feeding responses: 



nounced lateral head movements and correspond- 

 ing broad tail sweeps. Squid were generally cap- 

 tured in the corners of the mouth and swallowed 

 whole. In this behavior, sharks did not show rapid 

 head shaking (as often occurs when sharks bite on 

 relatively large prey) although lateral head jerks 

 to position prey for swallowing were common. 

 Sharks moved in a relatively straight path, and 

 created minimal disturbance to the school. 



2) TURNING: Turning behavior was most fre- 

 quent among sharks feeding at the surface when 

 squid were in an alert state or not in tight schools. 

 As the shark approached the school, the squid 

 (which swam backwards and could view the pred- 

 ator's approach) began to turn in tight arcs away 

 from the shark's path. The shark would respond by 

 turning in an accelerated pursuit, but was most 

 often eluded by the squid. Sharks that were suc- 

 cessful quickly whipped their heads to one side 

 and captured squid in the corner of their mouths. 



3) CHARGING: This behavior can best be de- 

 scribed as a straight accelerated rush through a 

 dense school of squid. Charging was most preva- 

 lent among the more active sharks that had just 

 arrived at a squid congi'egation. Typically, the 

 shark showed no orientation to specific individu- 

 als, and indiscriminately engulfed large numbers 

 of prey. 



4) TAIL STANDING; Sharks also fed on the 

 lower portions of squid schools. As previously de- 

 scribed, squid would often be concentrated directly 

 beneath the light source so as to form a dense 

 school. In this feeding behavior, the shark first 

 circled the lower portion of the school and then 

 moved up to the squid and assumed a near vertical 

 attitude, using broad tail sweeps to maintain posi- 

 tion. Then the shark lunged its head into the bot- 

 tom of the school and engulfed many individual 

 squid. The longest duration of a tail-standing 

 posture was 20 s in which approximately 30 squid 

 were consumed by one individual. This behavior 

 was observed only when squid schools were most 

 dense and was not as common as other feeding 

 modes. 



DISCUSSION 



1) SLOW HEAD SWAYING: This feeding be- 

 havior was most common among larger sharks 

 moving either through the center of moderately 

 dense squid schools, or at the periphery of large, 

 m.ore diffuse aggregations. Sharks swam among 

 the .squid at a relatively slow speed, with pro- 



Blue sharks fed on a variety of small fishes and 

 cephalopods associated with both pelagic and in- 

 shore habitats. Northern anchovies were the 

 major prey for sharks in this investigation, and off 

 Newport Beach, Calif (Bane 1968), while small 

 schooling fishes composed a major portion of blue 



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