12 FISH HATCHERY MANAGEMENT 



alone (or these plus OH ) and is expressed in terms of equivalent concen- 

 trations of calcium carbonate (CaCO^). 



Hardness represents the concentration of calcium (Ca^^) and magnesium 

 (Mg"^"^) cations, also expressed as the CaCOi-equivalent concentration. 

 The same carbonate rocks that ultimately are responsible for most of the 

 alkalinity in water are the main sources of calcium and magnesium as well, 

 so values of alkalinity and hardness often are quite similar when all are 

 expressed as CaCO^ equivalents. 



Fish grow well over a wide range of alkalinities and hardness, but values 

 of 120-400 parts per million are optimum. At very low alkalinities, water 

 loses its ability to buffer against changes in acidity, and pH may fluctuate 

 quickly and widely to the detriment of fish. Fish also are more sensitive to 

 some toxic pollutants at low alkalinity. 



TOTAL DISSOLVED SOLIDS 



"Dissolved solids" and "salinity" sometimes are used interchangeably, but 

 incorrectly. The total dissolved solids in water are represented by the 

 weight of residue left when a water sample has been evaporated to dryness, 

 the sample having already been filtered to remove suspended solids. This 

 value is not the same as salinity, which is the concentration of only certain 

 cations and anions in water. 



The actual amount of dissolved solids is not particularly important for 

 most fish within the ranges of 10-1,000 parts per million for freshwater 

 species, 1-30 parts per thousand for brackish-water species, and 30-40 

 parts per thousand for marine fish. Several species can live at concentra- 

 tions well beyond those of their usual habitats; rainbow trout can tolerate 

 30, and channel catfish at least II, parts per thousand dissolved solids. 

 However, rapid changes in concentration are stressful to fish. The blood of 

 fish is either more dilute (marine) or more concentrated (fresh water) than 

 the medium in which they live, and fish must do continual physiological 

 work to maintain their body chemistries in the face of these osmotic differ- 

 ences. Hatchery water supplies should be as consistent in their dissolved 

 solid contents as possible. 



TOXIC MATERIALS 



Various substances toxic to fish occur widely in water supplies as a result 

 of industrial and agricultural pollution. Chief among these are heavy met- 

 als and pesticides. 



