FISHERY BULLETIN: VOL. 69, NO. 4 



Figure 10. — The caudal fin of a swordfish caught in the 

 Gulf of Mexico, showing a crescentic wound and a com- 

 pleted wound cut through the trailing edge of the fin. 



found squid remaiiLs in most of the stomachs 

 of preserved specimens he inspected, and calcu- 

 lated that the squids which were eaten were as 

 large or larger than the sharks. He wondered 

 how small sharks that apparently swim slowly 

 could catch and capture such large, swift prey. 

 This question is also pertinent in considering 

 how IsLstius succeeds in contacting fast-swim- 

 ming animals such as tunas, marlins, or por- 

 poises. It would appear to be no problem for 

 Isistius to approach and make contact with 

 basking or drifting whales or fishes. In the case 

 of tunas, however, there is no evidence that they 

 ever drift or stop swimming (Magnuson. 1970). 



A possible sequence is that the potential prey, 

 seeing Isistius as an object apparently suitable 

 for food, makes the initial approach, identifies it 

 at a short distance, rejects it as food, and veers 

 off. At that instant, the shark may be able to 

 achieve contact by means of a short dash. 



It is also possible that the shark, to some de- 

 gree, simulates other organisms such as squids 

 in the pattern of its luminous ventral surface. 

 A more remote possibility is that Isistius is mis- 

 taken by large teleosts for a cleaner, and is in- 

 vited to make contact. 



Large squids appear to be killed by Isistius 

 more often than merely deprived of plugs of 

 flesh. It may be that squids also make an ini- 

 tial approach but, unlike teleosts, do not veer 

 off from their attack and are subsequently bested 

 in the encounter. 



Isouchi (1970) provided the only record of 

 an Isistius eaten by a large teleost when he found 

 a living shark in the stomach of Scomheromorus 

 sp. This record indicates that Isistius is a po- 

 tential food item; on the other hand, records 

 of teleosts having ingested any species of small 

 or young sharks are limited to five or six (S. 

 Springer and M. R. Bartlett, personal commu- 

 nications) . This certainly supports a hypothesis 

 of usual rejection. Rejection of the young as 

 food by teleosts, in fact, may account for the 

 survival of most elasmobranch species, consider- 

 ing their extremely low reproduction rates and 

 relatively low swimming speeds. 



It may not be necessary to assume any compli- 

 cated behavior patterns of Isistius or its prey; 

 perhaps contacts by means of short dashes can 

 be made during chance proximities. Thomas 

 Dohl, The Oceanic Institute, Hawaii, has in- 

 formed me that young porpoises of sizes that are 

 assumed to be still nursing do not bear wounds 

 or scars, but those which are larger do. Similar 

 restriction of wounds and scars to older por- 

 poises is suggested by the data of van Utrecht 

 (1959). This may be simply a matter of an 

 increased jirobability of encounter with time; 

 but it may, on the other hand, indicate that 

 porpoises are not attacked by Isistiiis until the 

 pori)oises become predatory on fish. 



Several crescentic wounds which I have exam- 

 ined on tunas were made from a frontal attack 



796 



