OPERATING THE HATCHERY 



solution and must be resterilized daily 

 until the epidemic is over. 



Addition of some disinfectants and 

 drugs to water in tanks holding fish is 

 recommended by numerous investigators 

 for the control of various fish infections. 

 Since a number of these methods are of 

 recent origin they have not been tried 

 vmder various conditions and with dif- 

 ferent species of fish. Therefore, it is 

 po.ssible that drugs listed in the section 

 on diseases (p. 57) may be found toxic 

 to .some species of minnows or aquarium 

 fishes and entirely harmless to other. 

 All of these drugs are used in highly 

 diluted forms and the fish should be able 

 to remain in such medicated water for 

 2 to 3 days, or even longer, without any 

 apparent injury. If treatment should 

 last more than a week, water in the tanks 

 should be changed frequently, prefer- 

 ably every 3 to 6 days. After each change 

 of water, the required quantity of fresh 

 drug should be added. Treatments 

 should not be carried on for more than 2 

 weeks at a time to reduce the possibility 

 of devlopment of drug-resistant strains 

 (varieties) of the disease-producing 

 organisms. 



Since some of these chemicals may be 

 found to be toxic to some fishes under 

 all conditions or under special condi- 

 tions like high temperature of water or 

 oxygen deficiency, all treatments should 

 be made with great caution and tried 

 first with a small number of fish. The 

 medicated water should be replaced with 

 fresh water whenever fish show any signs 

 of distress. 



Infected minnows should not be pur- 

 chased or seined, as the danger from 

 loss and spread of disease to healthy fish 

 is too great. During times of scarcity 

 when only infected minnows are avail- 

 able, the fish should be dippe<l for 10 

 seconds in a 1 : ir),(K)0 solution of mala- 

 chite green before being placed in the 

 holding or tempering tanks. The mala- 

 chite green should be discarded after 100 

 pounds of minnows have been treated or 

 at the end of the day. 



14. Minnows that have not spawned 

 .should be placed in warm-water ponds 

 until they spawn. Then they can be put 

 in cold-water tanks or iK)nd.s and held 

 for long periods without heavy losses. 



When men of the Wisconsin Fish Man- 

 agement Division were conducting feed- 

 ing experiments with brassy minnows, 

 they noticed an increased mortality dur- 

 ing May and June. Post-mortem exami- 

 nations revealed that the majority were 

 gravid, egg-bound females. That they 

 were ready to lay eggs but could not be- 

 cause of the cold water was interpreted 

 as the cause of their deaths. This may 

 account for part of the heavy loss of min- 

 nows in cold-water holding tanks during 

 the warm summer months. 



Holding minnows in ponds 



In some States, lioldin^ ponds are 

 a mucli debated subject. The bait 

 dealers insist that they must have 

 holding ponds to sta}^ in business, 

 and the conservationists maintain 

 that holding ponds are one of the 

 chief causes of minnow losses. The 

 opponents to holding ponds point 

 out that most ponds are used to hold 

 lake shiners from early spring 

 when they are abundant until sum- 

 mer when tliey are needed for bait, 

 tliat the shiners do not thrive in 

 holding ponds because most of the 

 ponds are too cold, and the fish be- 

 come egg bound, and furthermore, 

 that fish held in ponds are not able 

 to spawn that year and both the 

 adults and tlieii- reproduction are 

 lost from the lake. The problem is 

 usually solved on a compromise 

 basis that limits the number and 

 size of the ponds that each dealer 

 can use. 



On the other hand, all parties 

 agree that holding ponds are in- 



44 



