84 REPORT OF COMMISSIONERS OF INLAND FISHERIES. 



sessile animals, this is all that is necessary for rapid and healthy 

 growth. For pelagic animals, however, like the young of most fishes 

 and the larval forms of Crustacea and other marine invertebrates, it 

 is not sufficient. The very peculiarities of structure and instinct 

 which adapt these creatures to their pelagic life make it difficult to 

 confine them for a long time even in relatively large inclosures of the 

 water in which they normally live. 



One is baffled now by one peculiarity and now by another. The larvae 

 or fry are often strongly heliotropic, and in going toward or away from 

 the light soon strike the boundary wall of their confine, and when they 

 are numerous, as they must be in practical culture, die from the effects 

 of crowding, if, indeed, they are spared to this fate by their can- 

 nibalistic comrades. Often in the blind struggle to go toward the 

 light regardless of the boundary wall, they gradually work their way 

 to the bottom and become entangled in debris or covered with silt. 



If, for the sake of good circulation of water, the tidal current is 

 allowed to pass through the car, as in the case of sessile or bottom- 

 living forms, the pelagic fry are apt to be swept against one side, or to 

 collect in eddies, with disastrous results. If, on the other hand, the 

 current through the inclosure is not supplied, the water becomes stag- 

 nant and not well aerated, and since the time required to rear most 

 animals to a considerable size is long, the stagnation under these 

 circumstances is almost inevitable. 



The minuteness of many larval animals constitutes a fourth diffi- 

 culty, for perforations or meshes large enough to permit sufficient cir- 

 culation frequently permit also the escape of the fry, while meshes too 

 small for the fry to go through become clogged with silt and do not 

 allow free circulation. 



The fifth difficulty in the rearing of pelagic fry in inclosures of this 

 kind, depends upon the fact that normally they capture their prey 

 "on the fly." A dilemma presents itself: If the fry are fed upon 

 smaller animals or plants, these too must be pelagic, involving all the 

 difficulties over again, while, if artificial food is used, there is no pro- 



