OPERATING THE HATCHERY 



be treated. This can be done by de- 

 termining the volume of water con- 

 tained in a trough or tank at an 

 arbitrarily predetermined depth, 

 say 2 inches (5 cm.)- By multiply- 

 ing the inside length of the trough 

 by the inside width and the product 

 of these two numbers by the prede- 

 termined depth, all expressed in 

 inches or centimeters, one can cal- 

 culate how much salt to use. A 

 3-percent concentration can be ob- 

 tained by using 1 ounce of salt to 

 each 60 cubic inches of water, 30 

 grams to the liter, 4 ounces to the 

 gallon, or 30 ounces to the cubic foot 

 of water. To give fish a salting, 

 shut oflf the inflowing water, drain 

 the trought to the predetermined 

 depth, remove some of the water in 

 a bucket and add the required quan- 

 tity of salt, dissolve the salt, and 

 mix this strong solution with the 

 water in the trough. 



Fingerling trout will withstand a 

 3-percent concentration for 6 to 10 

 minutes. Most minnows will prob- 

 ably stand this treatment for only 2 

 or 3 minutes. When several of the 

 weaker fish have turned over, the 

 inflow is resumed at the maximum 

 rate which the fish will withstand 

 and the drain is partially opened to 

 permit a rapid replacement of the 

 salt water. This method may be 

 applied to fish as often as desired 

 without apparent injury and, in- 

 deed, with a definitely tonic eflfect. 

 When repeated three times at 24- 

 hour intervals, salting is quite ef- 

 fective in curbing epidemics caused 

 by external protozoans, and it is 

 the only treatment which should 



ever be applied in the absence of 

 definite knowledge regarding the 

 cause of any mortality. Salting, 

 however, becomes progressively 

 more expensive, less effective, and 

 more difficult to apply as the size 

 of the body of water to be treated 

 increases. 



Flushing 



Control of fish pathogens or para- 

 sites by flushing does not have wide 

 use, but in certain instances it is 

 quite effective. Flushing consists 

 in routinely adding a stock solution 

 of a disinfectant or drug of known 

 strength to the upper end of a 

 trough and allowing it to flow down 

 the trough and out. Most fre- 

 quently it is used with malachite 

 green to control fungus infection 

 of fish eggs during hatching. Pre- 

 liminary experiments also indicate 

 that such treatment with malachite 

 green may be of use in control of 

 the external infection of fish by 

 fungi belonging to the genus Sapro- 

 legnia. The most-frequently used 

 procedure is to make a stock solu- 

 tion containing 0.5 gram of pure 

 malachite green per liter of water 

 (2 grams to the gallon, 1 ounce to 

 14 gallons) and after the dye is 

 completely dissolved, 100 milliliters 

 (3.5 fluid ounces) of the solution 

 should be poured in at the head of 

 the trough and half of that amount 

 in the middle of the trough. Bur- 

 rows (1949) described a treatment 

 of eggs by a constant flow of mala- 

 chite-green solution. 



Malachite green also can be 

 added in powdered form to a fish 



52 



