Salinity
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<fishfyi-glossary slug="salinity"></fishfyi-glossary>Définition
The concentration of dissolved salts in water, a major factor determining fish distribution. Most fish are adapted to either freshwater or saltwater, with few tolerating both.
Explication détaillée
Seawater salinity averages ~35 parts per thousand (ppt). Stenohaline species tolerate only a narrow range: freshwater fish (goldfish, trout) are damaged by salinities above ~5 ppt; fully marine fish (tuna, cod) cannot survive below ~10 ppt. Euryhaline species tolerate wide salinity ranges: bull sharks enter freshwater rivers; tilapia thrive from 0–40 ppt; killifish survive from fresh to hypersaline (>50 ppt) tidal pools. Osmoregulation in saltwater fish requires continuous gill excretion of salt via chloride cells and production of concentrated urine; freshwater fish continuously excrete water and actively absorb ions — energetically costly processes that define habitat limits.