Salinity refers to the amount of salt in water (expressed as total dissolved solids, TDS). Most common salts in Victorian inland waters are derived from common salt, gypsum, and lime (calcium carbonate or sometimes magnesium carbonate). Salinity is expressed in electro-conductivity units abbreviated as EC. (1.6EC =1mg/L, (milligrams per litre); mg/L=parts per million (ppm). Salinity is also expressed as parts per thousand. 1ppt = 1,600EC.
Categories of TDS given in Water Victoria (1989) are:
- Freshwater less than 800EC
- Marginal 800-2,400EC
- Brackish 2,400 - 8,000EC
- Saline more than 8,000EC
Adverse biological effects begin to occur in freshwater rivers when salinity exceeds 1,500EC. Almost all adult native freshwater fish species can tolerate quite high salinities (16,000EC) with some species able to withstand sea water (56,000EC). In order to maintain this salt/water balance, active and passive transport of salts and water, both into and out of tissues takes place.
Fish in freshwater have an internal salt concentration greater than that of the external medium and large volumes of fresh water enter the body via osmosis through the gills and oral membranes. In order to maintain their salt concentration, they actively absorb salts through their gills and also reabsorb salts from their kidneys. Their urine is therefore very dilute.
Marine fish have an internal salt concentration less than that of the external medium. Water moves out of their body by osmosis and they dehydrate. To compensate for this, they drink large amounts of water, which in conjunction with salts is absorbed by the gut. Salts (Mg, SO4 & CL) are filtered out of the blood by the kidneys while Na, K & CL are excreted by the gills. Their urine is very concentrated. The survival of freshwater fish in saline conditions therefore requires them to be able to maintain their correct level of internal salt concentration or osmolality. All teleost (bony) fish, both marine and freshwater, maintain their internal osmolality at about the same level (7-13g/l TDS).
If a freshwater fish exposed to saline or brackish water does not have the necessary biological mechanisms to control internal water and salts, then its osmotic balance will be upset and the fish will die.