SECT. 4] SOUND PRODUCTION BY MARINE ANIMALS 563 



May and July measurements made in 1942 in the Chesapeake Bay area showed 

 a drop in peak frequency of the choruses from about 700 c/s to about 225 c/s. 

 It was suggested that this change was caused by fish growth and the resultant 

 decrease in resonant frequency of the then larger swim-bladder (the sound 

 source in this species). Such a frequency shift, however, would necessitate a 

 doubling in the fish's size, and, since this does not occur, changes in the com- 

 position of the croaker population through migration is likely the correct 

 explanation (Marshall, 1954). In this connection the observations of the fishery 

 biologist Wallace (1941) are of great interest. He finds that male croakers 

 become sexually mature at a smaller size than the females do and, on maturing, 

 leave the Bay. Thus he observed a 50 : 50 sex ratio in June but a proportion of 

 males to females of 35 : 65 in August. It is obvious that if the significance of 

 this sound and the physical factors determining its spectrum were understood, 

 much could be learned of the population dynamics of this important fish. 



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