A rui (rohu) can spend its life in a pond in Bengal without drinking much water. A bangda (Indian mackerel) in the Arabian Sea faces the opposite challenge: surrounded by saltwater, it must prevent too much water from leaving its body.
Put them on ice at a fish market and the difference may seem purely culinary. One might end up in a Bengali jhol; the other could become a crisp fry or a coconut-based curry. But beneath their scales is a biological divide that has shaped how they breathe, drink, eat and survive.
The difference begins with the chemistry of the water around them.
India’s fish live across an extraordinary range of habitats — Himalayan rivers, floodplain wetlands, village ponds, reservoirs, mangroves, estuaries, coastal lagoons and nearly 7,500 km of coastline. For fish living in these environments, survival depends on constantly maintaining the right balance of water and salt inside their bodies.
The body versus the water
The central principle is osmosis — the movement of water across a selectively permeable membrane in response to differences in the concentration of dissolved substances.
A freshwater fish such as rohu, catla, mrigal, mahseer or magur has body fluids that are saltier than the water around it. As a result, water naturally tends to enter its body.
That creates a problem. Cells need a stable concentration of salts and other dissolved substances to function. If too much water enters, that balance can be disturbed.
Marine species such as surmai, bangda and pomfret often have firmer flesh, while oily fish such as mackerel and sardines contain more fat. Photograph: (Jason Holland)
Freshwater fish have evolved an efficient way to deal with this. They generally drink very little water. Instead, they produce large quantities of dilute urine to remove excess water, while specialised cells in their gills actively take up essential salts from the surrounding water.
Marine bony fish face the reverse situation.
Seawater is generally saltier than the body fluids of species such as Indian mackerel, sardines and many pomfrets. Water therefore tends to move out of their bodies. To compensate, many marine fish drink seawater, absorb the water they need and use their gills and kidneys to remove excess salts.
In simple terms, a freshwater fish is trying to prevent itself from becoming too diluted. A marine fish is trying not to lose too much water.
This continuous balancing act is called osmoregulation.
Fish that live between two worlds
The divide becomes more complicated where rivers meet the sea.
India’s estuaries, mangroves, backwaters and coastal lagoons contain brackish water, where freshwater and seawater mix. Salinity can rise and fall with tides, rainfall and river flow.
Chilika Lagoon in Odisha, Pulicat Lake along the Andhra Pradesh-Tamil Nadu coast, Kerala’s backwaters and the Sundarbans are among these transition zones.
Fish living in such habitats must cope with water whose chemistry can change over time. Some species can adjust their salt-and-water regulation as they move between different salinities.
The hilsa, or ilish, is one of India’s best-known examples. An anadromous fish, it spends much of its life in marine or coastal waters before migrating into freshwater rivers to breed. During this journey, its body must adapt to changing salinity.
These ecosystems are also important for fisheries and aquaculture, supporting species such as pearlspot, Asian seabass and milkfish. Their health matters not only for biodiversity but also for communities whose food and livelihoods depend on the shifting boundary between river and sea.
From rui to bangda
The science changes from one habitat to another, but India’s relationship with fish is also reflected in language.
In Bengal and across parts of the Ganga-Brahmaputra basin, rohu is widely known as rui, while catla is commonly called katla. Mrigal, bata, pabda, magur and koi are also familiar names in eastern markets, although local names can vary across regions and languages.
In the sea, small pelagic fish such as sardines and mackerel feed heavily on plankton, while larger predators may eat smaller fish, squid and crustaceans. Photograph: (PTI)
On the west coast, Indian mackerel is known as bangda in Maharashtra. Pomfret is commonly called paplet in parts of the Konkan and Gujarat, while sardines are known as mathi or chaala in parts of Kerala.
Freshwater aquaculture has also made species such as rohu and catla widely available in inland markets. In recent years, efforts to replenish indigenous fish populations in rivers have sought to connect conservation with the livelihoods of fishing communities.
Marine fisheries, meanwhile, remain closely tied to the rhythms of the sea. Seasonal fishing restrictions during the monsoon — imposed for different periods depending on the coast and fishing zone — can affect the availability of wild-caught marine fish while allowing time for breeding and stock replenishment.
What reaches the plate
The biological differences between freshwater and marine fish do not automatically determine how a fish should be cooked. Fat content, texture, bone structure, size and freshness often matter more.
Still, these differences can often be noticed on the plate.
Freshwater species such as rohu, catla and tilapia are generally mild in flavour. Their flesh works well in curries, stews and other preparations where moisture helps keep the fish from drying out.
Marine species such as surmai, bangda and pomfret often have firmer flesh, while oily fish such as mackerel and sardines contain more fat. These qualities make many of them well suited to frying, grilling, roasting and coastal curries.
Their nutritional profiles can also differ by species. Fish are generally good sources of protein and vitamin B12. Oily fish such as mackerel and sardines are particularly known for their omega-3 fatty acids. Some marine fish can also contribute nutrients such as iodine and selenium. The exact nutritional value, however, depends on the species, portion and method of preparation.
Price and availability follow a similar pattern. Farmed freshwater fish are often more consistently available in inland markets, while the price of marine fish can vary with season, catch, species and distance from the coast. A rohu may be an everyday purchase in one city; surmai may be a premium fish in another.
A food web beneath the water
What fish eat also depends on their species and ecological role.
Rohu feeds largely on phytoplankton, algae and plant matter, while other freshwater fish may feed on insects, plankton, crustaceans or smaller fish. In the sea, small pelagic fish such as sardines and mackerel feed heavily on plankton, while larger predators may eat smaller fish, squid and crustaceans.
Together, they form links in a much larger food web.
Small fish transfer energy from microscopic organisms to larger animals. Predatory fish help regulate prey populations. Fish also move nutrients through ecosystems and connect habitats that may appear separate on a map.
That is why the story of a fish does not end at the market.
Freshwater species depend on functioning rivers, wetlands, floodplains and ponds. Marine species need healthy coastal and ocean ecosystems. Fish that move through estuaries and lagoons depend on the connection between freshwater and the sea.
And for a rui in a Bengal pond and a bangda moving through the Arabian Sea, survival begins with the same question, asked in two very different ways: how do I keep the right amount of water inside my body?
The answer is written into their gills, kidneys and cells — and eventually, into the way they reach our plates.
