THE HILSA (Tenualosa ilisha) is more than a fish. It anchors festival meals across the Ganga-Brahmaputra-Meghna delta and sustains millions of small-scale fishers and traders. Of late, warming seas, a stressed delta and national borders that cut across its range are putting the fishery under growing pressure. A regional fisheries review finds that rivers accounted for 94 per cent of Bangladesh’s hilsa catch between 1956 and 1962, while the sea supplied just 6 per cent. Today, the balance has reversed. Data from Bangladesh’s Department of Fisheries shows about 72 per cent of the catch comes from marine waters, compared with 28 per cent from rivers and estuaries.
A scientific review of the species adds that riverine catches peaked in the 1960s, while marine catches the fish rose sharply and became more common from the 1990s onwards, partly because of barriers to upstream migration. This geographical shift is reflected in the changing catch patterns. Data from the Central Marine Fisheries Research Institute, under the Indian Council of Agricultural Research, show a marked decline in hilsa landings along the West Bengal coast. Meanwhile, figures from Gujarat’s fisheries department, as reported by media, show that the state landed 14,587.72 tonnes of hilsa in 2025-26, its highest yield in four years. Gujarat’s inland catch also rose sharply, from 123.62 tonnes to 4,645.99 tonnes, with almost all of it sourced from the Narmada that flows through Bharuch district.
Hilsa is an anadromous fish that lives primarily in marine environments and migrates upstream into freshwater rivers to breed. This spawning journey relies on strict ecological thresholds. A review in Aquaculture, Fish and Fisheries places the ideal breeding temperature between 29.3°C and 30.2°C. Furthermore, estuarine research demonstrates that their spawning grounds require near-freshwater conditions, with a salinity level below 0.1 parts per thousand (ppt). Successful upstream migration also depends on deep channels, 18 to 20 metres deep, and strong river currents to guide the fish. Regional warming of climate severely disrupts these variables.
Elevated sea surface temperatures in the northern Bay of Bengal make the marine environment less favourable for breeding cues, while delayed and erratic monsoons weaken the freshwater river surges that historically kept seawater at bay. This drop in freshwater pressure creates a salt wedge, where dense, heavy seawater creeps inland along the riverbed beneath the fresh surface water. Consequently, environmental monitoring at the Meghna River Estuary shows dry-season salinity averages peaking around 10 parts per thousand, nearly double the historical baseline. While the population persists, large, mature hilsa are increasingly deterred by these barriers, choosing to stay offshore in marine depths rather than migrating into vulnerable river mouths.
Climate change is compounding the effects of development infrastructure and long-standing environmental pressures. Fisheries researchers have noted that the Narmada’s hilsa catch initially collapsed after the Sardar Sarovar dam restricted downstream freshwater flows. Heavy seasonal rainfall later helped restore freshwater discharge and reduce salinity in the estuary, potentially encouraging the anadromous fish to migrate upstream. The episode illustrates how strongly hilsa migration depends on hydrological conditions.
In West Bengal, around the Farakka Barrage, overfishing and agricultural and industrial pollution have all been implicated in the long-term decline of hilsa catches. Siltation adds another strain: expanding sandbanks can reduce the depth of river channels used by migrating fish. Pollution compounds these pressures elsewhere, too. In Gujarat, chemical waste can intensify the ecological damage associated with reduced river flows.
Small-scale fishers bear much of the immediate economic cost of declining catches, often falling into cycles of debt after successive poor seasons. Under such pressure, some turn to catching jatka, or juvenile hilsa, using illegal monofilament gillnets with very small mesh sizes that can trap fish indiscriminately. Though individually expedient, such practices can further deplete shared fish stocks—a classic example of the “tragedy of the commons”. Catching juveniles before they reproduce also reduces the population’s capacity to replenish itself. Field research reported by various media suggests that heavy exploitation has affected hilsa size in the Padma river. Whereas hilsa have historically reached sexual maturity at lengths of around 22-30 cm, fishers are increasingly catching immature fish measuring just 17-18 cm, before their first spawning cycle.
Hilsa ignores geopolitical borders, yet marine management remains fragmented. Bangladesh actively protects juveniles (jatka) and spawning mothers through a strict 22-day breeding ban and a 65-day marine ban, while the Indian states of West Bengal and Odisha enforce separate marine calendars that lack regional synchronisation. Because these closed periods are uncoordinated, they fail to form a continuous trans-boundary shield for the migrating fish. This misalignment leaves a critical regulatory loophole: when Bangladesh halts fishing, the transboundary shoals travel across maritime boundaries into India’s Exclusive Economic Zone (EEZ), where domestic trawlers can legally harvest them. Consequently, net fishing pressure shifts rather than declines. Bangladesh carries the socio-economic burden of conservation subsidies while its neighbours reap the harvest. Policy analysts at the Observer Research Foundation, a global think tank based in Delhi, and various environmental scientists confirm that hilsa stocks are heavily overexploited across the entire northern Bay of Bengal due to this lack of joint bilateral governance.
Shared fish need shared management. Instead of single-species rules written country by country, the region must adopt an Ecosystem Approach to Fisheries Management (EAFM). This framework expands protection to the entire shared habitat upon which trans-boundary stocks depend.
Three synchronised policy implementations can make this operational. India, Bangladesh and Myanmar should align their seasonal fishing closures to reduce transboundary fishing pressure during breeding and migration periods. These nations must also execute joint stock assessments, using shared monitoring systems and fish-tagging data. Collaborative hilsa fishery advisories, informed by real-time sea-surface temperature and salinity data, could help fishers and regulators respond to changing conditions. Such cooperation is critical, given that India, Bangladesh and Myanmar account for over 95 per cent of global hilsa landings. Restrictions on destructive, fine-mesh monofilament nets must be consistently enforced across borders to eliminate regional supply loopholes. Connected protected corridors and a comprehensive network of marine protected areas should be created to cover spawning regions and migration paths. While ecological degradation and barriers such as the Farakka Barrage have disrupted hilsa migration along traditional routes up the Hooghly river, favourable environmental conditions have been associated with a surge in hilsa landings on Gujarat’s coast. These contrasting trends illustrate the limits of managing a migratory species through fixed national boundaries alone.
(Sarvesh JP is project associate with the Bay Of Bengal Programme Inter-Governmental Organisation or BOBP-IGO, Chennai; Deepanjana Saha is doctoral scholar with Ashoka Trust for Research in Ecology and the Environment or ATREE)
This article is part of the cover story, “Great Migration”, which examines how climate change is redrawing the map of life on Earth. It was first published in the November 1-15, 2026, print edition of Down To Earth.