Warming seas drive jellyfish surge in Bay of Bengal, hurting deep-sea fishing

Jellyfish surge disrupts deep-sea fishing

A prolonged dry spell combined with climate change-driven ocean warming is disrupting marine ecosystems in the Bay of Bengal, triggering a spike in jellyfish and forcing fish away from Bangladesh’s traditional deep-sea fishing grounds.

Industry insiders say the shift intensified from late January and became more pronounced in February after minimal rainfall between November and January. With no significant rain to cool surface waters, sea temperatures have remained elevated, salinity has increased, and acidification has accelerated, creating conditions that experts say are unsettling marine habitats across the Bay.

Many fish species struggle to survive in their usual habitats when temperature, acidity and salinity rise, and they begin moving toward cooler, more stable waters. Jellyfish, by contrast, adapt more easily.

Fishermen report falling catches

Operators say the effects are now visible in authorised deep-sea zones, particularly beyond 40 metres in depth, where dense jellyfish concentrations have been reported. Trawlers returning from those waters are bringing back sharply reduced catches.

“The higher the temperature and acidity, the more favourable it becomes for jellyfish,” said Tanvir Shahriar Rimon, chief executive officer of Rancon Sea Fishing. “Once jellyfish dominate an area, fish cannot sustain there. They move elsewhere.”

Rimon said lower fish hauls are leaving many trawler owners struggling to recover fuel costs, let alone make a profit. Enam Chowdhury, president of the Bangladesh Marine Fisheries Association, said the situation is unlikely to improve without rain. If rainfall returns, he said, the sea may cool and the jellyfish could disperse, but without it the outbreak may not subside quickly.

The pattern is not new. Similar surges have occurred in recent years, and in 2024 a severe outbreak left many vessels unable to cover even fuel expenses.

Warming, salinity and overfishing worsen the imbalance

Scientists have long cautioned that climate variability, including phenomena such as El Niño, can intensify ocean warming and acidification in the Bay of Bengal, reshaping fish distribution and threatening marine livelihoods. A study published in *Marine Environmental Research* in March found that warming of 1.5 to 3 degrees Celsius significantly accelerates jellyfish proliferation.

Marine researcher Dr Mazharul Islam said rising sea surface temperature and salinity, partly due to oceans absorbing large amounts of heat-trapping carbon dioxide, directly contribute to higher jellyfish density. He added that overfishing worsens the problem by reducing natural predators and competitors, creating favourable conditions for jellyfish blooms.

“When large fish populations decline due to overfishing, jellyfish increase and the food chain gets disrupted,” he said.

In a balanced ecosystem, large fish feed on medium-sized fish, which in turn consume smaller species. Larger fish also prey on jellyfish. When top predators decline, jellyfish populations can expand unchecked, reducing smaller fish stocks and deepening the imbalance.

Industry faces mounting strain

Bangladesh’s deep-sea fishing industry operates mainly in the Bay of Bengal, targeting tuna, mackerel, sardine and various demersal species beyond coastal waters. The sector gained momentum after maritime boundary verdicts in 2012 and 2014 expanded the country’s exclusive economic zone and opened wider offshore areas for commercial fishing.

Over the past decade, private operators and state-backed initiatives have increased the number of longliners and trawlers in hopes of tapping underexploited offshore resources. But high fuel costs, limited cold storage, ageing vessels and dependence on a handful of landing centres already leave operators vulnerable even in favourable seasons.

As ocean temperatures begin rising earlier in the year, operators fear disruptions may last longer and become more frequent. Until sustained rainfall cools and stabilises sea conditions, the sector remains exposed to a warming Bay and a marine ecosystem under mounting strain.