276 FISH HATCHERY MANAGEMENT 



toxicity of formalin to fish. Above 70°F, formalin becomes more toxic; the 

 concentration used for channel catfish should not exceed 167 parts per mil- 

 lion for 1 hour (5.9 milliliters per 10 gallons; 43.8 milliliters per 10 cubic 

 feet). At such high temperatures, concentrations higher than 167 parts per 

 million should be used for bluegills or largemouth bass only with caution. 

 In water temperatures above 50°F, salmonids become more sensitive to 

 higher concentrations of formalin, and treatment levels should not exceed 

 167 parts per million for 1 hour. At higher temperatures and lower concen- 

 trations of formalin, it may be necessary to repeat the treatment on two or 

 more successive days to effectively control ectoparasites without damage to 

 the fish. Aeration should always be provided during bath treatments to 

 prevent low oxygen conditions from developing. At the first sign of stress, 

 fresh water should be added to flush out the treatment. 



Formalin also can be used effectively as an indefinite treatment of most 

 fish species in ponds, tanks, and aquaria at 15-25 parts per million if cer- 

 tain precautions are used. Do not exceed 10 parts per million as an indef- 

 inite treatment for striped bass fingerlings because the 96- hour LC50 (the 

 concentration that kills 50'/u of the fish in 96 hours) is only 12 parts per 

 million. Formalin removes 1 part per million oxygen for each 5 parts per 

 million formalin within 30-36 hours, and it should be used with extreme 

 caution, particularly during summer months, to minimize the chance of an 

 oxygen depletion in the unit being treated. Formalin also is a very effective 

 algicide so it should not be used in ponds with moderate to heavy phyto- 

 plankton blooms. If it is necessary to use formalin in a pond that has a 

 phytoplankton bloom, drain out one-third to one-half of the water prior to 

 treatment. Within 12 to 16 hours after treating, start adding fresh water to 

 bring the pond level back to normal. 



Fish treated with excessive concentrations of formalin may suffer delayed 

 mortality. Rainbow trout yearlings, channel catfish fry and fingerlings, and 

 bluegill fingerlings often are vulnerable in this way. Onset of deaths can 

 occur anytime within 1 to 24 hours after treatment but may not occur until 

 48 to 72 hours later, depending on species of fish, size and condition of 

 fish, and water temperatures. Clinical signs associated with delayed mortal- 

 ities include piping at the water surface, gaping mouths, excess mucus, and 

 pale color. Formalin also is toxic to humans but the strong odor and eye ir- 

 ritation usually warn of its presence. A few people develop allergic 

 responses to formalin. 



COPPER SULFATE 



Copper sulfate (registered by the Food and Drug Administration only as an 

 algicide) is one of the oldest and most commonly used chemicals in fish 

 culture and is considered to be 100"o active. It has been applied widely in 



