order of marine animals such as the whale and larger fishes, but 

 these are an extremely small part of the total animal life. 



d. The Deep Sound Scattering Layer 



Volumes have been written on the complex sound production 

 and reception by porpoises and other cetaceans for food hunting, 

 navigation, and communication. Noise making by croakers and snapping 

 shrimp is also covered elsewhere. This section, however, will be 

 concluded with some remarks regarding the acoustic effects of the "deep 

 scattering layer." The deep scattering layer is named from its 

 property of scattering sonar signals and showing up as layers on depth 

 sounding records (fig. 3-4). 



Actually, there appear to be two principal deep scattering 

 layers. The shallower layer has an average midday depth of 240 m, 

 with extremes between 185 and 405 m. Average thickness is about 75 m. 

 The deeper of the two layers has an average midday depth of about 500 m, 

 extreme values being 405 and 590 m. Average thickness of this layer 

 is 130 m. The average migration rate at sunrise and sunset of elements 

 in the 500-m layer is about 7.5 m/min and that in the 240-m layer is 

 about 4.5 m/min. The 500-m layer usually is not found in water depths 

 of less than 1800 m. 



The food chain of the sea is intricately involved in biological 

 communities forming the scattering layers. A range of plant and animal 

 sizes occurs from microscopic to the largest fishes, all feeding on 

 each other. Scattering layers are sensitive to light, approaching the 

 surface at sunset and going deep at sunrise (fig. 3-4). Levels to 

 which the animals descend appear to be related to their negative 

 response to light and sensitivity to sudden temperature decreases. 



Acoustic scattering properties of deep scattering layers are 

 similar to those expected from a collection of simple air bubbles in 

 water and show resonant peaks as a function of size and depth (fig. 3-5). 

 In fact, bubbles scatter sound much more strongly than solid particles 

 of the same size. 



The main scatterers are fish with gas-filled swimbladders . To 

 maintain their constant size, these fishes are able to generate gas in 

 their swimbladders as they descend and vent gas through their gills 

 via the blood stream as they ascend. As verified by photographs and 

 net trawls, most numerous mesopelagic fauna, with swUib ladders , between 

 depths of 150 and 1,000 m are myctophids or lantern fish, bristle- 

 mouths, and hatchetfish (fig. 3-6). Myctophids reach a maximum length 

 of from 4 to 12 cm, with swimbladder diameters of about 4 percent of 

 their length. Resonant frequencies are observed between 3 to 20 kHz, 

 corresponding to fish lengths of from 10 cm to 1 cm. It is interesting 

 that siphonophores have a small carbon monoxide bubble in their float 

 and scatter 12-58 kHz at 100 m depth and 24-111 kHz at 400 m depth. 



