Why the fish died

Two main causes for the recent fish deaths in seawater are algae clogging up their gills and suffocating them, and increased ammonia in the water that irritates fish gills, says an environmental expert.

The high level of ammonia in the water comes from the decomposing algae and other organic matter.

The ammonia causes swelling and irritation of the fish gills, and the algae clogs the gills, killing the fish, said Dr Hans Eikaas, 43.

Dr Eikaas is the head of Environmental Technology and Chemistry at DHI, a not-for-profit group offering environmental consultancy and water-modelling services.

For freshwater fish, ammonia is more harmful as it also changes the acidity of the water to be alkaline, making it hazardous for the fish.

He said that algae blooms are a natural phenomenon and can occur in clean, unpolluted waters, but it happens more often in nutrient-rich environments.

There are hundreds of species of algae that bloom, collectively called Harmful Algae Blooms, but he believes the species of seawater algae causing the recent fish deaths is Karlodinium.

It has no known detrimental effects on human health.

When asked why fish deaths have spread inland, he said it was likely they had common causes as the fish deaths at sea.

A prolonged dry period followed by short intense rainfall stirs up sediments that are loaded with nutrients, causing algae and bacteria to thrive.

He feels that fish farms should leverage on technology like chlorophyll and acidity sensors to get a heads up on rising algae levels.

Ideally, farms should isolate the fish from the surrounding water with canvas bags or tanks, or use UV filtration to minimise algae and bacteria numbers.