High Island Reservoir, Hong Kong

by Keith Wilson

High Island used to be an island but was joined to the mainland by the construction of two cofferdams to form the High Island Reservoir. Thousands of dollos, which are curiously shaped mass concrete structures, were placed in front of the eastern cofferdam in 1974, to protect it from rough seas. Incidentally the origin of the word dollos comes from South African witch doctors, who used the name for similarly shaped bones of mysterious origin. The reservoir was completed in 1978 providing a then much needed capacity of 281 million cubic metres of potable water supply. Five men were killed during the construction, and 400 village people were displaced.

In forming the reservoir, Hong Kong's marine environment lost many protected, shallow, coral-filled bays, which would have supported rich fisheries. Unwittingly the dollos have to some extent compensated for the loss by supporting rich communities of hard corals and associated reef fish. Some of the best examples of table corals in Hong Kong can be found growing on the dollos structures. The dollos stretch in a line, some 500 meters long, reaching down to 12 metres depth. The irregular pile of interlocking units, form a complex wall with many caves, nooks and crannies, suitable for supporting a great variety of invertebrates and fish.

On the Saturday preceding our scheduled diving it appeared quite likely that expected bad weather would mean our trip would have to be called off. After a week of flooding, and several amber and black rainstorm warnings, following the passage of a severe tropical depression, sea and diving conditions were not exactly perfect. We needn't have worried as Sunday turned out to be very calm and sunny for most of the day with less than 0.5 m swell. The chosen dive spot was the southern end of the High Island Dam dollos. This site is well protected from southwesterly winds, which had dominated during the passage of the severe tropical depression. Water conditions were better than expected with up to 8 metres visibility above 12 m depth and sea surface temperature of 25ºC. A strong thermacline was found at 12-13 metres below which temperature dropped to 21ºC and visibility fell to less than 2 metres, with the presence of very "milky" coloured water.

As Dive Marshal I struggled to count up the divers before we left Tso Wo Hang. In total eighteen divers and one snorkeller made the trip.

The dollos were shrowded with thousands of fish, especially between 2 and 8 metres. the highlights of the day were thousands of small Barracuda (Syphraena sp.), large Mangrove jack (Lutjanus argentimaculatus) up to 70 cm, 11 species of butterflyfish, hundreds of damselfish (Chromis notata), Western Gregory (Stegastes obreptus) lots of different wrasses including many Cockeral wrasse (Pteragogus ennaecanthus), a pair of Sixbar wrasse (Thalassoma hardwicke), a Peacock wrasse (Halichoeres poecilopterus), Azure tuskfish (Choerodon azurio), and a Bird wrasse (Gomphosus varius), parrotfish, Moorish idol, Turkey lionfish (Pterois volitans), Zebra lionfish (Dendrochirus zebra), a large pufferfish (Takifugu alboplumbeus) and Yellow boxfish (Ostracion cubicus), dartfish (Ptereleotris sp., cf. evides, new Hong Kong record), Silver mooney (Monodactylus argenteus), lobster, and several jellyfish with hundreds of associated small jacks seeking protection. Also a new fish record for Hong Kong was a pair of Pearlscale angelfish (Centropyge vrolicki).

To the western side of the dollos is a natural reef area and a large cave, which is know to harbour the occasional large grouper but according to Andy Niven the cave was empty. The reef area contained many bright red-coloured anemonies with the usual associated anemone fish and cleaner shrimps. A Giant Reef Ray (Taeniura meyeni) and small lobster were found here.

During the day Steve Pearson took the opportunity to take his "rebreather" washing machine for a spin and rustled up a small group to explain where you put the soap powder and fabric softener, or was that sofnalime?

I took the trouble to demonstrate that a fin sinks if you drop one in the water. Nila's boyfriend told me that he'd already shown that snorkels sink too, with the snorkel that I'd lent him earlier. Since he was at least 6' 4" and a fit 230 lbs or so I didn't enquire as to whether he bothered to retrieve it.

To sum up the High Island dollos have better than average viz, and although man-made make a great dive site for corals and fish. We should go there more often.

South China Dive Club

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