HATCHERY RE(^LTREMENTS 13 



Heavy Metals 



There is a wide range of reported values for the toxicity of heavy metals to 

 fish. Concentrations that will kill 50"o of various species of fish in 96 hours 

 range from 90 to 40,900 parts per billion (ppb) for zinc, 46 to 10,000 ppb 

 for copper, and 470 to 9,000 ppb for cadmium. Generally, trout and salmon 

 are more susceptible to heavy metals than most other fishes; minute 

 amounts of zinc leached from galvanized hatchery pipes can cause heavy 

 losses among trout fry, for example. Heavy metals such as copper, lead, zinc, 

 cadmium and mercury should be avoided in fish hatchery water supplies, as should 

 galvanized steel, copper, and brass fittings in water pipe, especially in 

 hatcheries served by poorly buffered water. 



Salinity 



All salts in a solution change the physical and chemical nature of water 

 and exert osmotic pressure. Some have physiological or toxic effects as 

 well. In both marine and freshwater fishes, adaptations to salinity are 

 necessary. Marine fishes tend to lose water to the environment by diffusion 

 out of their bodies. Consequently, they actively drink water and get rid of 

 the excess salt by way of special salt-excreting cells. Freshwater fishes take 

 in water and very actively excrete large amounts of water in the form of 

 urine from the kidneys. 



Salinity and dissolved solids are made up mainly of carbonates, bicar- 

 bonates, chlorides, sulphates, phosphates, and possibly nitrates of calcium, 

 magnesium, sodium, and potassium, with traces of iron, manganese and 

 other substances. 



Saline seepage lakes and many impounded waters situated in arid 

 regions with low precipitation and high rates of evaporation have dissolved 

 solids in the range of 5,000-12,000 parts per million. Fish production in 

 saline waters is limited to a considerable extent by the threshold of toler- 

 ance to the naturally occurring salt. Rainbow trout, as an example, gen- 

 erally tolerate up to 7,000 parts per million total dissolved solids. Survival, 

 growth and food efficiency were excellent for rainbow trout reared in 

 brackish water at an average temperature of 56°F. The trout were con- 

 verted from fresh water to 30 parts per thousand over a 9-day period and 

 were reared to market size at this salinity. 



Mineral deficiencies in the water may cause excessive mortality, particu- 

 larly among newly hatched fry. Chemical enrichment of water with calcium 

 chloride has been used to inhibit white spot disease in fry. Brook trout can 

 absorb calcium, cobalt, and phosphorous ions directly from the water. 



