4 EFFECTS OF EXPLOSIVE SOUNDS UPON FISHES. 



our shores. Are such devices favorable, inert, or prejudicial to our 

 fisheries, and, if prejudicial, in what ways can they be modified to 

 make them least harmful? 



Motor boats driven by exploding gasoline are equipped, as a rule, 

 with an escape pipe which is situated close to the level of the water 

 and through which the exploded gas is discharged in violent jets. 

 This pipe is sometimes so arranged that its end may be dropped 

 below the water level or kept in the air. "When the gas is delivered 

 into the air each discharge is usually accompanied by a familiar 

 explosive noise of much penetration. Wlien the delivery is into the 

 water the sound is greatly muffled and freed for the most part from 

 its objectionable penetrating character. This method of reducing 

 the noise is so easily applied that in certain communities efforts have 

 been made to require all motor boats to be thus muffled, at least be- 

 tween certain hours. The objection from the standpoint of the motor 

 boats to this form of muffling comes from the fact that when the 

 escape pipe is under water the obstruction to the free outward pas- 

 sage of the gases is so much increased that the efficiency of the 

 motor is considerably reduced, and hence the running of the boat is 

 impaired. 



To the human ear under ordinary circumstances most motor boats 

 either with or without mufflers are noisy appliances, generating 

 sounds that are carried a long distance through the air. But in 

 the water these sounds are very much less penetrating. To test 

 this, a 7-horsepower motor boat with an exceptionally loud sound 

 was run in open water and an observer plunged under the surface 

 as the boat passed. When within 10 or 12 feet of the boat, whose 

 escape pipe was in the air, the explosions of the gas could be faintly 

 heard, though they were disagreeably loud to the observer when in 

 the air. With the escape pipe under water and at the same distance 

 as before the noise of the explosions could scarcely be detected at 

 all under water. Thus both methods of running the boat delivered 

 into the water surprisingly little sound as compared with what 

 escaped into the air, and of the two conditions the muffled boat 

 yielded to the water much less sound than the unmuffled boat. 



In testing the effect of the motor-boat noises on fishes, a number 

 of kinds of fish known to be sensitive to sounds, such as killifish 

 {Fundulus heteroclitus) ^ young scup {Stenotomus chrysops)^ and 

 young kingfish {Menticirrhus saxatilis) were placed in a large 

 wooden cage, 4 feet square by about 2 feet deep, whose walls were 

 of strong netting. This cage was fastened in quiet water at the 

 end of a float and a motor boat of 3^ horsepower and with a pene- 

 trating noise was started at a distance of some 400 feet from the 

 cage and run at full speed past it. 



