It's the window cleaner!

Cleaning algae off the windows of a massive new tank at the Bermuda aquarium is a bit like painting the Forth bridge. Volunteer divers, like the one pictured here, are on the job every day. Nigel Merrick, who does his bit when asked, reports on the North Rock exhibit, the most ambitious project in the aquarium's history

Blessed with a sub-tropical climate from the Gulf Stream, Bermuda, an island group in the north-west Atlantic, possesses the world's northernmost coral reef, and is home to multitudes of sea creatures more commonly seen in the Caribbean, or around Florida. Parrotfish, wrasse, grouper, angelfish, butterflyfish and barracuda are permanent residents.
Poking from the ocean seven miles north of the mainland on the outer reef, and visible on a clear day, is the last remnant of Ice-Age Bermuda. North Rock, a 5m spike of limestone, is a lonely spot where, it is said, "not a seabird can find a resting place". The next nearest landfall is in North Carolina, about 600 miles west.
Lashed by winter gales and the occasional summer hurricane, life at the Rock is tricky at best. Ageing brain corals, brittle firecorals and delicate seafans are easily dislodged from their anchorages by the fierce breakers. Only on the calmest days can a few of Bermuda's many tourists visit this unique place.
So the Bermuda Aquarium and Zoological Society (BAZS) decided to bring North Rock and some of its residents to the tourists, with the most ambitious marine project in its 70-year history.
Its North Rock exhibit was painstakingly designed and constructed over five years at a cost of US$1.5 million, and opened in April last year. The 636,000-litre container is affectionately referred to as the shark tank, although sharks are not due to arrive there until this year.
The tank, 20m long, 8m wide and 4m deep, is divided by an acrylic panel, maintaining the appearance of a single unit but protecting the smaller reef fish from the jaws of the predators - groupers, jacks and barracuda.
A powerful surge piston creates an artificial swell in the coral tank to keep the water moving over the delicate corals. This helps to distribute nutrients and remove harmful waste products.
All the seawater is exchanged every two hours. It is supplied directly from the nearby lagoon of Harrington Sound, and is therefore subject to seasonal fluctuations in water temperature, from about 65*F in winter to around 84*F in summer.
Fortunately the minimum winter temperature is just above that required for the continual growth of coral.
Unfortunately, these conditions are also perfect for algae. Allowing green slimy things to grow unchecked would be disastrous, as well as obscuring the visitors' view of the fish. The tank is therefore cleaned daily by a stream of divers, mostly volunteers, under the watchful eye of Norvell Wright, the North Rock aquarist.
Cleaning is a bit like painting the Forth Bridge. Windows have to be wiped every day, the walls scrubbed about once a week. It is hard work, but never a chore.
Entering the shark tank is an experience unlike any other in sport diving. This is one dive where both the fish and visibility are guaranteed. Gone are the cumbersome dustbins we normally wear on our backs, replaced by a yellow air-hose connected to an alarmingly small (but efficient) electric compressor.
Wetsuits must be balanced by extra weight to compensate for the lack of a cylinder, and weightbelts are covered with a neoprene shield to
prevent accidental scratching of the acrylic windows.
Unlike their timid counterparts in the wild, the inhabitants of the tank are
unusually friendly. Curious groupers follow the divers around as they work, peeking over their shoulders, while juvenile wrasse consider it their duty to clean the insides of the divers' ears or remove extraneous facial hair.
The exhibit is currently home to more than 70 species of fish, invertebrates and corals. At full capacity there will be almost twice this number.
The specimens are collected from local waters, and are subjected to at least 30 days' quarantine before being introduced into the exhibit.
Visitors are entertained and informed by the divers, who give talks and answer questions using a two-way communication system. This gives people an understanding of the exhibit that would be difficult to match, short of allowing them to swim inside the tank.

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Appeared in DIVER - January 1998

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