SWIMMERS, YOU MAY think, would be most at risk from water pollution. Not so: it is a bigger hazard for regular consumers of seafood. The reason, says Professor Mike Dickman, Chair of Ecotoxicology at Hong Kong University's Department of Ecology and Biodiversity, is that fish and shellfish can absorb up to a million times the amount of pollution in the water around them. So even pollutants found in trace quantities (or at such low levels they are undetectable in water to scientists) can build up to toxic levels in fish and shellfish.
This is why shellfish are often used in studies of pollution levels. The possible effects on the health of the fish (and humans) are as varied as the pollutants in the water: they may be killed, deformed, rendered incapable of reproducing or otherwise made sick. Some pollutants have no effect on the fish but are harmful to anything or anyone that eats them (see examples below).
Pollutants that can accumulate in fish include pathogens harmful to humans such as hepatitis A; a third of all oysters harvested in Hong Kong farms are contaminated with hepatitis A, according to Friends of the Earth data. Oysters, says Professor John Hodgkiss of the Department of Ecology and Biodiversity, are a big danger to health in Hong Kong. Shellfish expose their tissue to filter water for nutrients, which means they also pick up and retain a 'phenomenal level of pathogens', says Hodgkiss. These include viruses like hepatitis, bacterial illnesses like cholera, red-tide toxins and heavy metals. Most local oysters are used to make oyster sauce.
In his research on the possible links between mercury in fish and male infertility, Professor Dickman has discovered that levels of pollution in fish caught locally are not significantly different from levels in fish imported from the Philippines. Note, however, that because of over-fishing in Hong Kong, Philippine fish grow older and larger than local specimens, meaning they accumulate contaminants over a longer period and spread them over bigger bodies. Because the sea here is more polluted, fish absorb the same levels of toxins more quickly.
heavy metals Heavy metals like mercury, cadmium, zinc and lead are among pollutants fish and shellfish retain. This is how they find their way into water: In household paint washed down sinks, or from pipes - lead.
Manufacturing industries used to send much of their heavy-metal-laden waste water towards the sea via rivers and storm drains. Now the heavy-metal deposits found in marine sediments are those left when industries shifted wholesale across the border to take advantage of lower labour and land prices. Metals are used in agricultural products that enter streams as run-off after rain. After a couple of days heavy metals settle into the sediment because for the most part they are not water-soluble.
Neon-light manufacture - mercury Battery manufacture (cadmium and lead) Manufacture of agricultural products, medicines and bronze (copper) Electroplating (chromium and copper) Textile manufacture (chromium and copper) Circuit-board printing (chromium and copper) Particulates from vehicle exhausts on the ground and washed into the sea (lead) Some metals are essential in the right quantities. Copper and zinc, for example, are important physiological components of some marine organisms like shrimps and crabs, but the levels of metals in some of Hong Kong's waterways, such as the harbour, are far higher than the background levels needed to support marine life.