SCMP: Mainland sewage fuelling Hong Kong’s ‘red tides’

According to the SCMP, sewage from the mainland is fuelling an increase of harmful algal blooms – known as red tides – in Hong Kong waters.

Professor Ho Kin-chung, dean of the Open University’s school of science and technology and an expert on algae, told the SCMP the red tides were “fed” by nutrients flowing in from mainland waters to the east and west of Hong Kong.

“The economic boom across the border leads to more sewage discharge into the sea and rivers, and in the right seasons [the nutrients] come down to us. So this is no longer a local phenomenon but a regional one,” Ho said.

Within the last fortnight brown algae been spotted across the territory – off Lamma Island, Cheung Chau, Lantau and within Victoria Harbour.

Agriculture, Fisheries and Conservation Department staff said they had spotted red tides at fish culture zones within Tolo Harbour, Sha Tau Kok and Tai Mei Tuk, while a member of the public reported another at Pak Sha Wan in Sai Kung.

Most algae absorb nutrients such as phosphates or nitrates that are commonly found in cities’ wastewater. At the right temperature, well-fed algae will proliferate in a short period of time.

Ho said Hong Kong was sandwiched by the Pearl River in the west and Mirs Bay in the east and these were the two key origins of red tides in local waters.

As a result, Tuen Mun, Lantau, Tolo Harbour and Sai Kung were becoming increasingly prone to the phenomenon.

Recent reports of large amounts of seaweed being washed onto beaches in South Lantau should not be confused with red tides. Red tides are composed of free-floating (planktonic) algae, seaweed however are larger marine plants normally growing attached to the seabed, reefs or rocks.

A spokeswoman for the Agriculture, Fisheries and Conservation Department said it would announce today whether tests had found the algae off Lamma Island to be toxic.

Four red tides were also spotted in Hong Kong waters last week.
Three red tides at fish culture zones within Tolo Harbour, Sha Tau Kok fish culture zone and Tai Mei Tuk were observed by staff members of the Agriculture, Fisheries and Conservation Department (AFCD) during April 8 to 10. Another red tide was spotted on April 9 by a member of the public at Pak Sha Wan, Sai Kung.

None of the red tides were associated with the death of fish in these occurrences.  “The red tides at fish culture zones within Tolo Harbour and Tai Mei Tuk were formed by Gonyaulax polygramma, Prorocentrum minimum and Heterosigma akashiwo. The one at Pak Sha Wan, Sai Kung, was formed by Gonyaulax polygramma whereas the one at Sha Tau Kok fish culture zone was formed by Heterosigma akashiwo and Prorocentrum minimum. All the above algal blooms are commonly found in Hong Kong waters. Gonyaulax polygramma and Heterosigma akashiwo are non-toxic. Studies have shown that Prorocentrum minimum may produce toxin, but no such reports or associated fish kills have been recorded in Hong Kong,” a spokesman for the working group said.

Red tides are a natural phenomenon. The AFCD’s phytoplankton monitoring programme will continue monitoring red tide occurrences to minimise the impact on the mariculture industry and the public.

In 1998, a red tide killed 80 per cent of the stock at Hong Kong fish farms.

 Source: SCMP 17/04/2014 and AFCD Press Release 11/04/2014

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