954 REPORT OF COMMISSIONER OF FISH AND FISHERIES. 



The filters will of necessity obstruct the water somewhat, and a slight 

 head be created by each one, — perhaps an inch each will be a rough 

 approximation to the truth. Allowance must be made for this by having 

 the filtering-trough several inches higher than the hatching-trough. But 

 do not draw the water away from the lower sides of the filters so as to 

 expose them to the air, for the water will pass through much freer when 

 it is backed up nearly as high on the lower side as on the upper. The 

 number of filters to be used depends upon the amount of foreign matter in 

 suspension in the water, and can only be determined by observation and 

 experiment in each case. 



Another mode of filtering sometimes resorted to, either alone or in 

 connection with the flannel screens, consists in passing the water through a 

 bed of gravel; but the method already described will answer every 

 purpose and is much easier of application. 



As already remarked, there are many places where it is a waste of effort 

 to filter the water, but the advantages of cleanliness are so great that every 

 one who proposes to use water liable to become at any time muddy is 

 advised to put in the necessary troughs, or at any rate to leave space for 

 them. If, however, a hatching-house has been already fitted up without 

 any provision of this sort, a set of filters can be fitted into the upper part 

 of each hatching-trough and be just as effective as if in a trough by 

 themselves. 



7.— HATCHING-TROUGHS AND FITTINGS. 



"We come now to the hatching apparatus proper, the troughs and trays. 

 Whatever may be the advantages derived from the use of very compact 

 apparatus, some forms of which allow us to mature 30,000 eggs to every 

 square foot of trough room, they do not pertain to the hatching out and 

 rearing of the fry. For this work nothing has yet been found better than 

 a long, straight, shallow trough. Ten feet is the length I would recom- 

 mend as most desirable. In no case have them longer than fifteen feet. 

 In passing down a well populated trough fifteen feet any ordinary volume 

 of water will be deprived of so much of its air and oxygen that a new supply 

 is needed, and if necessary to make further use of this water it is best to 

 let it fall in a thin sheet into another trough set a few inches lower. In 

 some cases, where the water as introduced into the house is deficient in 

 aeration, it is best to make troughs as short as five feet, or, what will 

 amount to the same thing, (though a less convenient and less satisfactory 

 mode,) incline the trough from one to two or three inches for every five 

 feet in length, and check the water and keep it up to the proper height 

 in different parts of the trough by a series of transverse partitions or dams. 

 Under ordinary circumstances, with well aerated water at the start, a 

 trough ten or fifteen feet long may be set perfectly level. 



