Archive · 27 Aug 2026
How India’s Fish Survive in Freshwater and the Sea
Fish must keep the right amount of water and salt inside their bodies.
Freshwater fish live in water that is less salty than their bodies, so water moves into them.
They avoid drinking much and release extra water as dilute urine.
Their gills take useful salts from the water.
Sea fish live in saltier water, so water tends to leave their bodies.
They drink seawater and remove the extra salt.
Some fish, such as hilsa, travel between the sea and rivers and must adjust to both environments.
This balancing process is called osmoregulation.
Healthy rivers, wetlands, estuaries and oceans help fish survive and support people who depend on them.
Freshwater fish such as rohu have saltier body fluids than their surroundings, so water naturally enters their bodies.
They drink little water, produce large amounts of dilute urine and absorb essential salts through specialised gill cells.
Marine bony fish face the opposite challenge: they lose water to seawater, drink it and remove excess salts through their gills and kidneys.
Fish such as hilsa can adapt to changing salinity while moving between marine waters and freshwater rivers to breed.
Water chemistry, habitat health, species biology and seasonal conditions influence fish ecology, nutrition, availability and cooking.
- Who
- Freshwater and marine fish found in India, including rohu, catla, Indian mackerel, sardines, pomfrets and hilsa.
- What
- The article explains how fish maintain water-and-salt balance through osmoregulation and how this affects their habitats, food webs and use as food.
- When
- The balancing process occurs continuously; seasonal monsoon fishing restrictions also affect marine fish availability.
- Where
- Across India’s rivers, ponds, wetlands, estuaries, mangroves, lagoons, backwaters and surrounding coastal waters, including the Arabian Sea.
- Why
- Fish must maintain stable internal concentrations of water and salts so their cells can function.
- Core biological process
- Osmoregulation, the regulation of water and dissolved salts inside the body.
- Freshwater strategy
- Fish such as rohu generally drink little, produce dilute urine and absorb salts through their gills.
- Marine strategy
- Many marine bony fish drink seawater and remove excess salts through their gills and kidneys.
- Migratory example
- Hilsa spends much of its life in marine or coastal waters before entering freshwater rivers to breed.
- Transition habitats
- Estuaries, mangroves, backwaters and coastal lagoons contain changing mixtures of freshwater and seawater.
- Nutrition
- Fish generally provide protein and vitamin B12; oily fish such as mackerel and sardines are known for omega-3 fatty acids.
- Coastline
- India has nearly 7,500 kilometres of coastline.