412 Donald R. Griffin 



when the recording was played through a 500 c.p.s. tuned filter both the call and the 

 apparent echo could be seen clearly in oscillographic records. When the intervals 

 between peaks of original call and apparent echo were measured in 1 1 oscillographic 

 records showing the highest signal-to-noise ratio, the average interval was 1-58 

 seconds, the extremes being 1-47 and 1-77 seconds (cr = 0-08 second). The ratio of 

 amplitudes of the apparent echo and original call averaged 0-405, the extreme values 

 of this ratio being 0-27 and 0-56. 



If these recordings do indeed consist of calls and echoes, the time interval between 

 them, together with the amplitude ratio and the known depth of 5100 metres, estab- 

 lishes certain geometrical requirements for the location of the " echo fish ". The 

 difference in length of the direct path from source to hydrophone and the path 

 followed by sound waves reflected from the bottom was approximately 2400 metres 

 (the distance travelled by underwater sound in 1-58 seconds). In the simplest case 

 we might assume that the source was directly below the hydrophone, so that its 

 distance above the bottom would be 1200 metres or 3900 metres below the surface. 

 If we make the further assumption that in these cases, when the bottom reflection was 

 maximal, the angles of incidence and reflection were equal, the possible locations 

 of the source are limited to a locus having the form of a shallow dish-like surface 

 with its deepest point 3900 metres below the hydrophone and gradually rising towards 

 the surface as the source is assumed to be displaced laterally. Unless one assumes the 

 source to have been some miles away it must have been at a considerable depth, 

 probably well below the levels to which light can penetrate, and below the depths 

 reached by whales or other marine mammals. These geometrical relationships are 

 thus consistent with the hypothesis that this recording reveals a deep sea fish emitting 

 calls loud enough for them and their echoes from the bottom to be audible at the 

 surface three to four thousand metres away. 



This explanation of the " echo fish " recordings can be considered only as a specu- 

 lative possibility, however, until means are devised to explore the depths of the 

 ocean in greater detail, both acoustically and biologically. Alternate explanations for 

 the sounds recorded in this instance are: (1) that a single fish emitted a double call, 

 (2) that the second call came from a different fish, or (3) that the sounds had a non- 

 biological origin. The third alternative seems quite unlikely because this type of sound 

 was heard on several occasions during the day the recording was made, because it 

 became louder and fainter from minute to minute, and because it was recorded when 

 the ship was many miles from land or other ships under conditions when all possible 

 precautions had been taken to avoid sound production by the ship herself. Reflections 

 from other surfaces than the bottom are rendered most unUkely by the relatively high 

 amphtude ratio and the difference of 2400 metres in path length for the direct and 

 reflected sound. Finally it must be pointed out that even though this recording does 

 reveal a fish call plus its echo from the bottom, we have no direct evidence that the 

 unknown fish could hear such an echo, and still less that it would pay any attention 

 if it did. Yet the " echo fish " could easily have heard these bottom echoes if it had 

 an auditory sensitivity equal to that of any fish adequately studied to date, and in the 

 unhghted depths of the ocean echolocation could be as advantageous to a fish as it is 

 to a bat flying in darkness through the air. We are deahng here with one of many 

 phenomena pertaining to the deeper layers of the ocean concerning which we can 

 only speculate, and dream of future investigations by methods yet to be devised. 



