[23] FISH CULTURE IN CENTRAL EUROPE. 561 



monoids to have more ready access to small crustaceans and inollusks, 

 which form an important part of their food. 



Mr. Haack considers it also necessary to place in ponds those trout 

 which have been deprived of their eggs by an artificial process, be- 

 cause here they will find more favorable conditions for gaining flesh and 

 for recovering from the sudden exertion incidental to forced spawning. 



Although on general principles it is preferable that the water in ponds 

 destined for salmonoids should during summer be kept cool, and that 

 its temperature should iii no case exceed 31° C. [SS° F.] (according to my 

 observations the temperature did not exceed this limit in any of the 

 establishments which I visited), it will be well to note that aeration, 

 an abundance of water, frequent agitation of the same, and a just pro- 

 portion between the capacity of the water and the number of fish to be 

 kept in it, will allow the fish to do well even if the summer temperature 

 of the ponds should not altogether come up to the conditions as men- 

 tioned above. 



It would be useless to describe the arrangements for obtaining the 

 best hydraulic movement in the ponds and to provide for their draining 

 without losing the fish. This is done by patting partitions in suitable 

 places ; perforated zinc plates being at present preferred for that pur- 

 pose. More or less ingenious apparatus is employed in this connection, 

 as well as sewers and pipes; but as they are known from models, it will 

 not be necessary for me to describe them. 



In constructing the bottom of the conduits, cement or stone is at pres- 

 ent preferred to wt>od, which is only seemingly more economical. 



Planting young salmonoids. — When should the young salmonoids be 

 planted ? The answers to this question differ somewhat, and reflect the 

 individual opinions of various fish-culturists. 



Mr. Haack prefers to plant them when the umbilical bag has begun 

 to disappear, stating that as soon as they are placed in the water they 

 will hide under stones as long as they do not feel the desire to seek 

 food, and during this time they become acclimatized in the new ele- 

 ment. 



In the Netherlands the young salmon are not planted until they have 

 lost the umbilical bag, and after they have been kept and fed artificially 

 for an entire year. 



At Wiltheu the young trout are not placed in brooks until they have 

 been kept for a certain time in the apparatus where they were born, 

 where they have lost the umbilical bag, and where they have been fed. 

 After they have passed the fine season of the year in the brook they are 

 in autumn placed in x)onds, where they remain about a year; and after 

 that they are sold, if they have reached the weight of at least half a 

 pound. 



It is not customary to place in open waters embryonated eggs of sal- 

 monoids which are near being hatched, for fear of some voracious fish, 

 especially the Chondrostoma nasus aud the Barbus fluviaiilis. Although 

 S. Mis. 90 36 



