764 BULLETIN OF THE BUREAU OF FISHERIES. 



REQUIREMENTS SATISFIED. 



After the first step was taken and the excellent result of rearing bottom- 

 living animals in native water was recognized, it seemed most desirable to 

 follow up the advantage gained in the rearing of other forms by extending and 

 developing the procedure so that it would be applicable to pelagic fry. For- 

 tunately we were able to hit upon a method which solved at once all the main 

 difficulties arising from the peculiarities of pelagic existence of larvae and other 

 free swimming animals. This method consists essentially of creating and 

 maintaining within an inclosure of "native" water a gentle upward swirling 

 current. It obviates the several difficulties which we have enumerated as 

 peculiar to pelagic fry in the following ways: 



It effectually prevents the crowding of the fry to one wall of the car, for 

 the force of the current carries them round and round continuously, nor can 

 they work their way to the bottom, for the current has an upward as well as a 

 rotary direction. Even the cannibalistic propensities, which are so pronounced 

 in the larval stages of lobsters and some other animals, are rendered innocuous 

 to a great extent by the forced separation of the fry and are mitigated by the 

 availability of other food. 



The current being wholly internal, and its main component circular in its 

 course, it does not force the fry strongly to one side nor allow them to remain 

 in one place as does the tidal current passing through the inclosure. The 

 pressure of the current against the sides varies, of course, with the rapidity 

 with which the outside water is drawn into the car, with the extent of the area 

 through which the water can pass out, and with the rapidity of the current. 

 Since any or all of these factors can readily be controlled there is no difficulty 

 in obtaining a proper adjustment of current for the requirements of particular 

 cases. 



Stagnation is prevented even when no new water is admitted from the 

 outside, for the water in the car is constantly being turned over and the lower 

 strata brought to the top and aerated. When, therefore, the water of a car of 

 considerable size is kept cool by being sunk into the ocean and shaded from 

 the sun and is continuously forced to the surface so as to be relieved of waste 

 gases as well as recuperated with oxygen, there is comparatively little need of 

 continuous or frequent renewal. It is at least reasonable to suppose that, in 

 what we may call (after Birge) the "respiration" of a small inclosed body of 

 water containing a considerable quantity of animal life, the elimination of the 

 waste or toxic gases is necessary, and that aeration which is accomplished by 

 forcing more air into the water only partially fulfills the requirements of respi- 

 ration. The analogy with the physiological process of respiration would seem 

 to be real. In case of small, verv thin, flat animals, where the ratio of surface 



