Today is World Oceans Day and what better occasion is there for celebrating Hong Kong's own underwater natural treasure, even though it is on the edge of extinction?

Despite the impact of increased urban development, reclamation, overfishing, marine pollution, climate change and physical damage from boat anchors, divers and discarded fishing nets, our precious coral communities still boast a greater number of species than the entire Caribbean Sea.

Those holidaymakers who jet off to the pristine tropical waters of the so-called "coral triangle" joining Indonesia, Malaysia and the Philippines to enjoy snorkelling and diving may not be aware that they can see abundant coral habitats simply by taking the No7 minibus from Sai Kung town centre.

According to the Agriculture, Fisheries and Conservation Department (AFCD), which is tasked with the welfare of these fragile assets, there are 33 officially recognised coral communities on the eastern coast of Hong Kong and these stunning underwater gardens include up to 84 species of hard coral, 29 species of soft coral, 38 species of gorgonians (sea fans) and six species of black coral. They provide a vital habitat for a vast diversity of fish and invertebrates, from butterfly fish and wrasse to lobster and sea cucumbers. Hong Kong's most popular commercial fish such as wrasse and grouper cannot survive without coral habitats and about two-thirds of all our fish spend some of their life in and around coral.

This month, more than 500 local scuba divers, supported by scientists and academics, will begin the annual reef-check programme organised by the AFCD. Last year's programme reported that "corals at all 33 sites were in general healthy condition" with only "minor bleaching observed at two sites". It also reported increased coral coverage in areas where marker buoys were installed by the AFCD in 2002 to deter boat anchoring and other types of damage.

The AFCD also organises an annual underwater photography competition to encourage a wider awareness of the local seabed and some of the images submitted by scuba divers are nothing short of amazing. It is incredible that these natural wonders flourish just a few nautical miles from commercial container ports, high-rise housing estates and major construction sites.

Everything, it seems, is rosy in the underwater gardens of Hong Kong.

Even scientists are surprised that these corals can thrive in such conditions. A team of researchers at the Swire Institute of Marine Science (Swims), located at Hong Kong's only marine reserve, at Cape D'Aguilar, and led by American coral scientist David Baker, is investigating the reasons why the city's corals are apparently so robust.

"When I came to Hong Kong, I was dreading the diving. I thought everything would just be trashed," says Baker, whose expertise in all things coral started with a simple childhood interest in keeping tropical aquarium fish.

"Everything in science suggests corals should just not be here but some of these are truly astounding," he says, as he tends the Swims coral farm, a cluster of large, land-based circular tanks constructed in 2008 with support from the Ocean Park Conservation Foundation and containing hundreds of native corals.

"There are big variations in water temperature and salinity in these waters that would normally prohibit coral growth. Corals like clear water with lots of light and low nutrients," he explains. "Development increases sedimentation and habitat loss - sediment reduces light and smothers the coral. This is true for all types of development - housing, reclamation, bridge-building, dredging, deforestation, all of it."

If that is not enough of an assault on Hong Kong's corals, Baker also explains the dangers of "nutrient loading" and the negative impact of nitrogen, which is contained in human waste.

"Our urine is nitrogen-rich and that nitrogen is a fertiliser for the stuff which is bad for coral. It favours the growth and proliferation of algae. You can connect nutrient loading to red tide, loss of ecosystems and loss of biodiversity," he says.

One of his team's projects, funded by the Research Grants Council, is to fingerprint sources of nitrogen in the coral and establish whether human sewage is a likely pollutant. Early indications are that even corals in the protected waters of the marine reserve are being polluted by human waste emanating from nearby Shek O.

"Almost every dive we go on we find coral but only at a few sites are they thriving and have high biodiversity," he says. His research indicates that Hong Kong's corals are "living on the edge" in terms of survival.

With great enthusiasm he talks of the "cutting edge of coral science", which involves the creation of "designer reefs" that can withstand pollution and climate change - and Hong Kong's super-robust corals could have a part to play in this research.

Baker is not the only scientist worried about our corals. Others with decades of experience researching Hong Kong waters suggest privately that things are not nearly as perfect in the underwater garden of Eden as the government might have us believe. A dive and snorkel inspection with Baker's team, on the pristine coral communities at Hoi Ha Wan Marine Park, on the northern coast of Sai Kung Country Park, reveals the reasons for their concerns.

Hoi Ha Wan was designated as a marine park - which differs from a reserve by having less strict controls over who can do what within them - in July 1996. A beautiful horseshoe bay covering an area of 260 hectares, it is one of only four marine parks in Hong Kong. It is widely regarded as one of the two most important sites for coral communities (along with Tung Ping Chau Marine Park, on the eastern side of Mirs Bay) and it is under severe threat.

The water at Hoi Ha Wan is usually crystal clear but after weeks of heavy rain, underwater visibility is reduced to a few metres - although that does not diminish the beauty of this coral community during an investigative dive along the shallow southeastern shore.

Just a few metres from the boulder-strewn shoreline there are beautiful pink and powder blue boulder, honeycomb and brain corals, some of which Baker identifies as the Favia and Platygyra species, which have the ability to remove sediment from themselves.

"The sediment-tolerant species can 'inflate' their tissues like a balloon by taking up seawater and using those hydraulics to lift the sediment on the tissue surface high enough so it slides off the colony," says Baker. These corals are smart as well as beautiful.

Slightly lower down the shallow boulder reef, where it meets the sandy seabed, there are colourful daisy corals, or Goniopora, with long fleshy tentacles. Baker is most enthused by a species called Porites, which he refers to as the "redwood of corals". Members of his team estimate that this one, near the end of Hoi Ha's pier, is at least a century old.

Despite the impressive diversity (some 64 of the 84 hard corals in Hong Kong waters can be found in this one bay), Baker points out some worrying indicators. Here, there are many mussels and barnacles, organisms not usually seen near corals, and, attached to a boulder coral, a "sea squirt", which thrive on nutrients and pollution that are harmful to corals and are often spotted near sewage outflows. He points out a Drupella, a sea snail that eats coral and is prevalent when its main predator, the lobster, has been overfished.

There are a few butterfly fish and damselfish, and the distinctive bronze stripes of a rock grouper can be seen darting into the distance, but there are fewer fish than you might expect in such a vibrant coral habitat.

"Of all the places where I have worked, Hong Kong is probably one with the least amount of fish," says Baker. "The only fish you see are about this big [indicating with his thumb and index finger a size of about 3cm to 4cm]."

Sadly, there is no need to seek the advice of an eminent scientist at Swims to work out why this might be. Right in the middle of the marine park, fishing nets are being flung into the water from a small blue sampan. On the shore, just above the dive site, an elderly woman casts her fishing line into the water and throws in some dubious-looking groundbait for good measure.

Back in the village of Hoi Ha, Baker introduces David Newbery and his wife, Nicola, who have lived here for 20 years and run an organisation called Friends of Hoi Ha.

"Over 400 fishing licences have been issued for Hoi Ha Wan Marine Park," says Newbery, "and we often see fishing with tangle nets about 1km long."

Discarded and snagged fishing nets are a huge prob-lem for coral and marine life and are known locally as "ghost nets".

"Almost every time we dive in Hong Kong we see a ghost net," says Baker. "They'll snag a diver just like they snag a fish - anything that swims in gets trapped. The nets saw through the coral as they are moved by the wave action."

It is not the fishing, though, that is Newbery's primary concern. Friends of Hoi Ha is campaigning vigorously against a proposal currently under consideration by the Town Planning Board to build another 40 to 60 village houses, sandwiched between the marine park and the country park in a designated site of special scientific interest (SSSI).

Newbery guides Baker through the sodden forest and overgrown paddy fields adjacent to the beach, an area dissected by countless small streams and river systems. He explains the elaborate hydrology that creates an essential filtering system, cleaning the water entering the bay. He fears the new houses will mean 40 to 60 simple septic tanks, some only 10 metres from the beach, which will leach human waste directly into one of Hong Kong's most impressive coral communities.

"We will have a lagoon that will be turned into an open sewer," says Nicola Newbery.

So how serious is Hong Kong about conserving its corals when housing development and commercial fishing are considered perfectly acceptable in and around the parks set up to preserve these natural assets?

The government's main coral man is Chow Wing-kuen, senior marine conservation officer at the AFCD. Chow is not the stuffy government bureaucrat you might expect but an enthusiastic graduate of marine biology who looks as though he would be far more comfortable in a wetsuit than a shirt and tie.

"I dive for fun. I also dive for my job," he says. "Not many people know about our corals but they are amazing for a subtropical area. "The marine parks and marine reserve protect the corals by law. This year there will be a record number of teams involved in reef check."

Launched in 1997, Reef Check is widely recognised as a successful programme for engaging with the sports-diving fraternity and involving them in conservation.

"We face big challenges," says Chow. "We are surrounded by sea but few people know about these marine treasures - so we must try and educate as best we can," he adds, pointing to a plethora of posters, leaflets, guide books and maps on the desk in front of him at his Cheung Sha Wan office.

"We have banned destructive fishing in the marine parks," says Chow, "but bona fide fishermen and local villagers can apply for a permit."

To prove his department's efficacy, he points to the widely applauded success of the trawling ban that was implemented in January last year and made possible by the healthy working relationship between the AFCD and the fishing industry.

Surely though, banning fishing in a small conservation area would be much easier to implement than a wholesale trawling ban?

"We did have a plan last year to suspend all the current permits but, as you can imagine, there were lots of objections from local fishermen, so we have to have proper consultation," he says.

"We cannot do this without support from the public," he adds, admitting that there are only three full-time AFCD staff dedicated to coral conservation (excluding the marine park management staff).

Chow says there are plans to introduce more marine parks and no-anchoring zones have recently been set up to stop boat anchors destroying coral heads. He is also working on a code of conduct for divers. Chow acknowledges there are many threats to the coral but points out that any development in Hoi Ha Wan would have to comply with government ordinances and there's a need to conduct proper environmental impact assessments.

Environmentalist Andy Cornish is a global programme leader for WWF International. He spent three years undertaking more than 500 dives to study the reef fish of Hong Kong and has little sympathy for the AFCD line.

"In 2008, the government said it would ban commercial fishing in marine parks and they still haven't done it, which is outrageous," says Cornish. "These corals are already on the edge of their limits."

"The solution is not complex. Just ban fishing [in marine parks] and keep the water clean. It's simple," he says.

He is equally forthright in his views about Hoi Ha Wan, which was one of his main study sites.

"The village house development at Hoi Ha Wan should not go ahead under any circumstances in its current format," he says.

Nevertheless, Cornish believes there is a bigger threat to our coral, and it is one no one is addressing: climate change. He says now is the time to make the corals as healthy as we can, not the time to assault them with yet more pollution.

And Hoi Ha Wan is not the only marine park and coral community threatened by development. PetroChina is proposing to reclaim land for a Shenzhen liquefied natural gas terminal just 5km from the coral communities of Tung Ping Chau. The other two marine parks - Yan Chau Tong and that encompassing Sha Chau and Lung Kwu Chau - are considered inferior in terms of coral life.

Despite these concerns, Baker, at least, is philosophical about the future of this enchanting underwater world.

"As a society, Hong Kong has to decide how it values these coral communities," he says. "I remain optimistic because at least the government has the resources to deal with the problem and [can] spend money on improving the water quality if it wants to."

The Newberys have been battling with government departments for years to preserve Hoi Ha and are understandably less sanguine about the future.

"If we can't save the corals of Hoi Ha, within a marine park and an SSSI area, then there isn't much hope for the rest of Hong Kong," says David Newbery.

There is something truly astounding, even heroic, about Hong Kong's corals. They are assaulted with human sewage, sawed by ghost nets, poisoned by nutrients, covered in sediment, dumped on with plastic lapsap, heated by climate change and damaged by tourists, yet somehow they survive.

We have all probably played some small part in pushing these coral communities towards extinction - and it can only be hoped that somewhere there is the authority, the will and the resources to take the bold actions that will preserve this uniquely Hong Kong natural treasure for future generations.