Coastal Aquaculture and Mariculture (2+1)

Unit 4 - Water and soil quality management

4.1.1.b. Salinity

• Salinity is a measure of the concentration of dissolved ions in water expressed as parts per thousand (ppt). The major dissolved ions are sodium, chloride, Magnesium, calcium, potassium, sulphate and bicarbonates.
• Seawater varies in salinity from about 33 to 37 ppt with an average of about 34. Estuarine and brackish waters have varying salinities from full strength seawater to about 3 ppt.
• The composition and concentration of dissolved salts in the body fluids of fish and invertebrates must be maintained within fairly narrow limits to buffer against changes that can cause physiological disruptions.
• Since fish and shellfish live in water, salinity of external water can disrupt concentrations of dissolved salts in the body fluids.
• Fish maintain homeostasis through osmoregulation which is influenced by salinity of water. Some species can osmoregulation at wide range of salinity; they are called ‘Euryhaline’. Others have limited tolerance to salinity changes, such species are called ‘stenohaline’.
• Every species has an optimum salinity range. When forced outside of this range, metabolic energy is spent on osmoregulation at the expense of growth. If salinity deviates too far fish die.
• Euryhaline species such as the Asian sea bass can be cultured in a broad range of salinity from fresh to seawater, but stenohaline fish such as cobia can be cultured only in full strength seawater.