The city has been hit by the worst six months of red tides in 26 years, with the highest number of potentially harmful algae species recorded, analysis by the Post has revealed.

Scientific experts said the algal blooms might indicate that pollution is increasing and climate patterns shifting. They would not rule out the possibility that a lethal red tide - that can kill off marine life en masse - could strike soon, although these are almost impossible to forecast.

“The more algal blooms we have, the higher the probability that a toxic red tide will strike,” said Professor Rudolf Wu Shiu-sun, director of the University of Hong Kong’s school of biological sciences. “Even if they are not toxic, the blooms can still deprive the sea of oxygen and harm fish.”

An algal bloom is an area of seawater discoloured by large concentrations of microorganisms, and is often a murky brown rather than red. Peak season is between March and June.

From January up to July, the city had 35 days with red tides - the highest in the six-month period since records peaked at 45 days in 1988. The findings were made in a South China Morning Post review of the Agriculture, Fisheries and Conservation Department’s red-tide database.

The natural phenomenon was triggered by 16 algal species, five of which were known to carry potentially harmful effects, the highest number in the same six-month period over the last 26 years. Three of those can release toxins that poison shellfish, while the other two directly harm fish.

The lethal algae Karenia digitata, which almost wiped out stocks at local fish farms in 1998, was not among them.

Dr Johnny Chan Chun-yin, clinical assistant professor of the medicine department at the University of Hong Kong’s Li Ka Shing Faculty of Medicine, warned that close contact with toxic algae could lead to pain in the limbs and reversed temperature sensations. Consuming marine life that had eaten the toxins could cause serious headaches, nausea and loss of balance.

Professor Ho Kin-chung, a red-tide expert and dean of science and technology at the Open University, said the city should remain alert for a possible repeat of the 1998 disaster.

However, there was no crystal ball to show when the next outbreak would happen and whether it would be as lethal, he said.

“Definitely, this is something we don’t know but we can’t rule out,” Ho said. “This is just like infectious diseases. The virus is always there; it just explodes … in the right conditions.”

Both Ho and Wu agreed that the flow of waste water from the Pearl River was the major factor boosting red tides in Hong Kong.

“In the past, red tides might stem from a local fish farm’s discharge, but now the whole sea has been almost completely eutrophicated,” said Ho.

Eutrophication refers to excess nutrients in the water, and their composition - dominated by nitrogen and phosphorous - was key in defining the nature of a red tide. Warmer weather and slow currents also favoured the formation of red tides, according to the experts. Ho hypothesised that the El Nino phenomenon - weather events resulting from the warming of the Pacific - tended to create slower currents.