American Fisheries Society 85 



thirds of a meter (20 to 26 inches) in depth, strewing the 

 manure of various animals on the hottom, spreading a hand- 

 depth of garden earth over this and with a gauze net trans- 

 ferring to this pond the mother Crustacea from any pool or 

 pond. From the manure many protozoa are developed, also 

 unicellular algae, which serve the Crustacea for food, and when 

 the water in the ditches is well warmed up, say up to 20 degTees. 

 and higher (77 Fahrenheit and upward) the development of the 

 crusjtacea may become so rapid that in a few weeks the water of 

 the ditches straightway swarms as with a living soup, and affords 

 a rich booty. 



But, as already noted, the rate of reproduction of these ani- 

 mals depends so much on the temperature that it is not possible 

 to get daphnids in this way in great quantities so early as Feb- 

 ruary or March. It has therefore been recommended instead of 

 ditches in the ground to use petroleum casks in a similar wa}' 

 and set them up in stables where the temperature is even; but 

 here the yield corresponds with the narrow space in the casks 

 and is a very small one; and the whole process is right fussy. 

 Altogether the culture of daphnids in any method yet deviseil 

 has not come into general use; and so it will remain in the 

 future, unless some one succeeds in finding daphnids that with 

 good nutrition reproduce rapidly in confined quarters during the 

 cold months. 



What we have said about daphnids and copepods a])plies also 

 to insect-larvae. These also multii^ly enormously in artificial 

 pools that are drest with manure, but only during the warm 

 months, and in especial abundance only so long as sunny weather 

 prevails. The reason for this is that insects swarm freely and 

 lay their eggs in water only in sunlight, while in cold rainy 

 weather they crawl away. 



Artificially bred insect-larvae, among which we have to count 

 fly-maggots, cannot be had in quantities in the early months of 

 the year, when we are in especial need of fry-food. But on the 

 other hand, iii the rearing of yearlings of which we shall here 

 speak later, the lireeding of maggots plays during the summer 

 months a more important role. 



From the foregoing discussion the conclusion is drawn that 

 we can at anv rate under favorable conditions in some places 



