550 SCHEVILL, BACKUS, AND HERSEY [CHAP. 14 



to these small sharks {Squalus acanthias). We have over several years listened 

 in the presence of sharks, including Squalus acanthias, without hearing any- 

 thing except occasional crunching of food. As nearly as one can tell from 

 Azhazha's description, the sounds that he correlates with poor herring catches 

 are very like those that we, from repeated experience, associate with the smaller 

 odontocetes, and we therefore suppose that he was reporting porpoise calls. 



5. Purposeful and Adventitious Sounds 



Two classes of sounds generated by marine animals have been noted and 

 described. These have often been called "biological" and "mechanical", or 

 "purposeful" and "adventitious" sounds, or more simply "sounds" and 

 "noises". However, as will be seen, it is not always easy for us to make this 

 distinction. In the first case the sounds are generated by the body of the 

 animal alone — generally by structures which have been modified for sound 

 production. The sounds thus produced may be assumed to be of survival value 

 to the individual making them, and this has been demonstrated in a number of 

 cases. In the second case the sounds are usually generated by the contact of 

 the animal with its surroundings. Sounds made by the motions of the animal 

 through the water or over the bottom and those incidental to feeding, foraging, 

 and other activities are placed in the second category. In certain fishes with 

 swim-bladders, furthermore, sounds may be produced accidentally through 

 sudden motions of the animal, particularly bending, which may cause the swim- 

 bladder to vibrate. Such sounds should be distinguished from purposeful 

 resonation of the swim-bladder by drumming muscles or other means. 



Adventitious sounds, since they may inadvertently attract attention, 

 probably most generally affect their maker adversely by causing competition 

 if not predation. However, adventitious feeding sounds in the swell-fish 

 Spheroides maculatus attract others of this kind (Breder and Clark, 1947) and 

 it might be supposed that these sounds are of value to the species in enabling 

 them more effectively to exploit a food source. Such sounds have been described 

 by Tokarev (1958) in the silverside, Atherina hepsetus, the jack, Trachurus 

 trachurus, and the maenid, Smaris smaris. in the Black Sea. He states that each 

 species makes characteristic feeding sounds, and, furthermore, that these 

 sounds vary according to the nature of the food. Tokarev argues that these 

 sounds are important in forming aggregations of fish. That these adventitious 

 feeding sounds are of value to the species is perhaps supported by the fact 

 that in certain marine animals purposefully produced sounds of a characteristic 

 nature are made in the presence of food and at that time only. This has been 

 observed in certain pomacentrid fishes (Fish, 1948) and in the porpoise, Tursiops 

 truncatus (Wood, 1954). 



Although naturalists have generally supposed that the submerged swimming 

 of marine animals is essentially silent, fishermen who locate schools of fish by 

 underwater listening with the unaided ear have long attributed certain sounds 

 to such a source (Westenberg, 1953). These sounds are usually likened to 



