Karachi: A strong fish-like odour spread across several parts of Pakistan’s Karachi on Wednesday, leaving residents wondering about its unusual source. People in areas including Clifton, Defence Housing Authority (DHA), Saddar and other parts of the coastal city reported a pungent smell that appeared to have drifted inland from the Arabian Sea.

The unusual odour, however, is not linked to a seafood market, industrial accident or chemical leak. Marine experts have attributed the smell to a natural seasonal phenomenon associated with the death and decomposition of phytoplankton in the sea.

According to marine resource experts from the World Wide Fund for Nature-Pakistan (WWF-P), microscopic marine organisms multiply in large numbers during the monsoon season. As the monsoon period begins to wind down around mid-September, changes in sea temperature and weaker winds can trigger the end of their lifecycle. The dead phytoplankton then decompose close to the surface of the water, releasing sulphur-containing compounds that produce a strong smell resembling rotten fish or decaying vegetation.

Weather conditions around Karachi have also contributed to the odour becoming more noticeable. A low-pressure system over the coastal region, along with light south-easterly winds, has reduced the movement of air and allowed the smell to remain concentrated near the ground instead of dispersing quickly over the sea.

Urban air pollution has further intensified the situation. Meteorological officials said pollutants in the city’s atmosphere can mix with the humid marine air, making the unpleasant odour more prominent across residential and commercial areas.

Despite its intensity, experts say the smell itself does not pose a serious health risk to the public. People with sensitive sinuses may experience discomfort, particularly when the odour combines with dust and stagnant air, but there is no indication of a major health hazard from the phenomenon.

The Pakistan Meteorological Department expects normal sea breezes to return later in the evening. Increased airflow should gradually disperse the trapped marine air and reduce the smell across the city.

For Karachi residents, the strange odour may therefore be unpleasant, but experts say it is largely the result of a recurring natural process linked to seasonal changes in the Arabian Sea.