SKOT. 4] SOUND PRODITCTION BY MARINE ANIMALS 549 



array. Local circumstances of the searcli, as well as the character and spectrum 

 of the sound, must determine the choice of directional array. The most versatile 

 equipment now available includes the several echo-rangers used in European 

 fisheries. Some of these are trainable in vertical angle as well as in azimuth. 

 They are especially attractive because they are designed for use in small fishing 

 boats (examples are Simrad, Atlas, Basdic and others). 



4. Identification of Source 



Once the instrumental preliminaries are over, there is little trouble in hearing 

 marine sounds, unless one has had the bad luck to strike an acoustic desert. 

 In many regions, however, the listener will be rewarded by sounds. By what are 

 they made? Many non-biological sounds are readily attributed (waves, ship 

 noises). The biological sounds are not always as easy, and are often weaker 

 than the listener's own ship noises. In some cases the sound sources have long 

 been known ; these are usually animals from shallow depths, and include 

 commercial fish (e.g. grunts, croakers) and such familiar creatures as snapping 

 shrimp or pistol prawns, which make sounds that may be heard by the unaided 

 ear, with the animal nearby or in the hand. These are a very small minority of 

 the known marine sounds, and many others will continue to be quite difficult 

 to identify. 



Most marine animals remain immersed and are, therefore, not easily noticed 

 by human observers. ^ Among the minority that are readily seen are the air- 

 breathing marine vertebrates : reptiles, birds and mammals, the latter including 

 the sirenians, pinnipeds, and cetaceans. Of these, only the cetaceans have been 

 demonstrated to make underwater sounds, although the above-surface sounds 

 of pinnipeds have been heard under water. The identification of cetacean sounds 

 with a particular cetacean rests upon the visual identification of the whale at 

 the time the sounds are heard, the likelihood of error diminishing with the 

 number of observations (sometimes a specimen must be collected for identifica- 

 tion). There is, none the less, the lurking likelihood that one may be listening 

 to unseen animals while watching others. 



With fish the difficulties are appreciably greater. Many of the smaller ones 

 may be kept in aquaria (likewise the smaller cetaceans) ; the tendency of many 

 animals to behave and phonate differently in captivity presents difficulties, 

 since it is often difficult to elicit natural responses. 



An example of the dangers of identifying sounds without careful investiga- 

 tions or good luck is the recent report by Azhazha (1958), just cited, on trawling 

 in the Norwegian Sea. It appears from this report that all his experiences 

 occurred during the hours of darkness. Without any supporting evidence that 

 squalids can in fact make sounds, he attributes the sounds, to him unfamiliar, 



1 Some, of course, may be seen by divers, but unless the diver is equipped with under- 

 water listening gear, he is likely to notice only the loudest of underwater sounds. This is 

 because of man's well-known lowered hearing acuity by bone-conduction (as when im- 

 mersed), and also because most underwater breathing apparatus is very noisy. 



