BIOLOGICAL OCEANOGRAPHY 



crustaceans, have been collected in net tows to reinforce this conten- 

 tion. At least one observer has concluded that concentrations of 

 tiny jelly fish equipped with gas-filled floats are the source of the 

 scattering layer. 



The complexity of the problem is indicated by the apparent 

 likelihood that several agents are involved, for a single layer often 

 "splinters" during vertical migration into as many as four separate 

 elements. These elements never cross one another, each seeking a 

 precise depth as though adjusted to a particular level of twilight. 

 Because of this stratification, exact identification of each element is 

 dependent upon the selectivity of sampling devices which, at present, 

 leaves much to be desired. 



The inevitable development of improved sampling techniques 

 and echo sounding apparatus will, undoubtedly, resolve the problems 

 of the deep scattering layers. But apart from the acoustical problems 

 they present, biologists view the phenomenon as a valuable ecological 

 tool for understanding the mass distribution of life within the sea. 



NATURAL SEA NOISES 



Marine animals contribute another source of acoustic interference. 

 Contrary to the belief that the sea was a silent environment, many 

 aquatic animals are now known to produce noises of widely varying 

 nature. At times, concentrations of animal sound-emitters can set 

 up a veritable clangor beneath the sea, and because of the diverse 

 properties of air and water, little of it ever reaches the casual listener 

 above the surface. Over hydrophones, however, these "fishy" noises 

 can be plainly heard and recorded. While the sea's inhabitants are 

 apparently well adjusted to the condition (having no other choice) 

 such noises can have serious effects on naval operations. 



The military implications of natural sea noises were not fully 

 realized until World War II, when their intensity and worldwide 

 distribution were recognized as limiting factors in anti-submarine 

 operations. Early researchers found that hydrophone reception was 

 decidedly hampered by the ambient noise level produced by animal 

 sound-emitters. Some of the sounds detected and identified as bio- 

 logical were remarkably similar to sounds characteristic of surface 

 and underwater craft. At least one organism, or group, produces a 

 sound hke an unrythmic hammering on steel to the understandable 

 dismay of a hydrophone operator. 



Mindful of the obvious operational and psychological stress the 

 noises placed on ship's crews, the Navy initiated a program to record 

 and identify them. The object of the program was to train operators 

 of acoustic gear to discriminate between biological sounds and mechan- 

 ical sounds and to possibly redesign equipment to filter out undesir- 

 able sounds. 



The problem of identification is complicated by the fact that, 

 over hydrophones, the sounds made by an individual animal may 

 differ from the effect produced by a group. Thus, the croaker often 

 makes a drumming sound, but the noise produced by a dense shoal 

 of croakers has been described as resembling that of a pneumatic drill 

 tearing up a pavement. In some instances their chorus becomes so 

 voluminous that it drowns out completely any ship or propeller 

 noise. The snapping shrimp, an inedible variety smaller than a man's 

 little finger, produces a sharp "snap" with its lobster-like claw, but 

 the over-all noise created by a large number of such animals is a con- 

 tinuous crackling noise similar to radio static. 



In the effort to catalog marine animal noises, workers found it 

 necessary to correlate the sounds recorded with more familiar terres- 

 trial ones. Today, it is known that porpoises and whales whistle, 

 click, bark, and moan; barnacles slurp; black mussels crackle; toadfish 

 croak, growl, and whistle; weakfish, silver perch, and spot perch 

 produce a rapid, raspy croak; the Atlantic croaker, true to its name, 

 makes a similar, almost drumming sound at a slower rate; the northern 

 puffer or swellf ish squeaks and coughs among other noises ; the striped 

 barnfish coughs; and the striped sea robin makes a sound like finger- 

 nails being scraped over a drum. On the other hand, many fish have 

 been found to be practically mute, such as the flounder and mullet. 



The manner in which the organisms produce their sounds is fairly 

 well known. Crustacea sounds are normally of the percussion variety, 

 such as the claw-snapping of shrimp. The noises made by free-swim- 

 ming fish are usually the product of their swim-bladder. This organ 

 varies in form with the species of fish, and the type of sound emitted 

 is a particular function of its size, shape, and the manner in which it 

 is vibrated. Fish also produce rasping, scraping, grinding, or whining 

 noises by either grinding their teeth or by rubbing their fins together. 



The exact function of marine animal noises is still uncertain. 

 They appear in some instances to be associated with breeding and 

 spawning, in others to be defensive and protective. It seems likely 

 that sounds produced by at least some fish are used by them for 

 spatial orientation in a manner similar to sonar. It has also been 

 determined that some of the sounds made serve no useful purpose at 

 all, and in some cases even reveal the originator's presence to a lurking 

 predator. 



Marine biologists are particularly interested in determining the 

 exact biological significance of sounds and look upon the non-military 

 use of hydrophones as a valuable oceanographic instrument. It is 

 evident that if certain sounds are peculiar to certain animal groups, 

 underwater listening could be used to plot the distribution of such 

 animals. Moreover, by classifying the creature's various sounds, 

 such as those made while spawning and feeding, and those believed 

 to be "emotional", biologists hope to broaden their understanding of 

 the behavioral traits of marine animals. 



CORAL REEF BUILDERS 



Probably no branch of oceanography involves as many of the 

 earth's sciences as the study of the origin and development of coral 

 reefs. Primarily an active biological process, the building of a reef 

 embraces geological, chemical, and physical processes, the extent of 

 which varies with the location and type of formation under construc- 

 tion. While considerable work has been done in this field over the 

 years, there remain wide gaps in knowledge which impede progress, 

 particularly in the physiology and ecology of reef organisms. 



A bed of both calcareous and stony coral 



Common sea coral 



21 



