Dive Atlas of the World. Jack JacksonЧитать онлайн книгу.
MAJOR CURRENTS
Although local currents vary during the day due to winds, upwellings, downwellings and the heat of the sun, there are more consistent current patterns in the world’s oceans that affect the climate, conditions for diving and which migratory species can be found at a given time in a normal year.
Ocean currents are horizontal and vertical circulation systems of ocean waters that are produced by the earth’s rotation, gravity, wind friction, and the variations in water density that result from differences in temperature and salinity.
For instance, the currents that form the Gulf Stream bring warm waters northward, affecting the climates and waters of the Bahamas, Bermuda, eastern North America, the British Isles and the Atlantic coast of Norway. This leads to tropical species off Bermuda and some surprising species such as Ocean Sunfish and Leatherback Turtles visiting the west coast of the UK. Similarly, part of the South Equatorial Current that flows towards East Africa joins the Agulhas Current and relatively warm water flows southward at high speed along the east coast of South Africa. However, when this current is reversed, cold water flows north, bringing with it huge quantities of sardines, which in turn attract large predators.
The Lionfish (Pterois miles) is the Indian Ocean relative of the Pacific’s Pterois volitans. They are often treated as a single species.
Tube worms burrow into sediment or live coral. They retract instantly into their tubes if a shadow passes over them.
The Red Sea Bannerfish (Heniochus intermedius) is endemic to the Red Sea. Juveniles form shoals, but adults are solitary or in pairs.
TIDES
Tides are primarily caused by the combined effects of the centrifugal force of the spinning earth and gravitational attraction between the moon and the earth. The sun, despite its huge size, is so far away that its effect on the tides is only about half that of the moon. The cycle of one tide, to go from high water to low water and back to high water again, usually takes roughly 12 hours (semidiurnal). In some parts of the world it may take roughly 24 hours (diurnal), depending on whether the sun or the moon is dominant. Some areas experience a mixture of both diurnal and semidiurnal tides. The normal tidal day is 24 hours and 50 minutes. Around some islands and reefs you may, effectively, get four tides per day if the flow along one side of the obstacle is longer than along the other.
Spring tides, those of maximum range and flow, occur twice a month at, or near, a new or full moon. Equinoctial spring tides, those that are of greater than average range and flow, occur near the equinoxes in March and September, at new and full moon. Neap tides, those of minimum range and least flow, occur twice a month at or near the first and last quarters of the moon. These are best for wreck diving and photography. The word spring (an outflow of water) and the word neap (Anglo-Saxon for scanty) are both from Old English. Local tide tables enable divers to calculate incoming flood tides, (sea level rising), outflowing ebb tides (sea level falling) and slack water, the time of least flow when tides are changing from flood to ebb or vice versa.
The shape of the shoreline has an effect on the height of the tide. Where stretches of water are enclosed by a shoreline with a funnel shape, tides are amplified as the funnel narrows. The upper parts of Nova Scotia’s Bay of Fundy have the world’s highest tidal range – 16m (52ft).
THE MARINE ENVIRONMENT
Viewed from space the oceans dominate the earth, covering 70 per cent of our planet. They provide us with food, a large area for recreation and they regulate the climate. Mankind has treated the oceans as a rubbish pit for centuries. However, modern agricultural and industrial pollution is much more damaging and is accompanied by prodigious overfishing, often by detrimental methods. The combined effect has been threefold: huge plankton blooms (that suffocate organisms below), damaged reefs and depleted fish stocks. Ships taking on seawater as ballast in one region and discharging it in another, and aquariums emptying exotic fauna and flora into seas where they have no natural predators, have severely upset the ecology, often with disastrous results. We are slowly learning that there is a limit to the way in which we can treat the oceans.
Storm-driven wave action will occasionally damage coral reefs. However, some human activities, such as blast and cyanide fishing, coral mining, landfill, dredging, siltation caused by dredging or logging, and the indiscriminate collection of corals to sell as curios, are just as destructive. Similarly, overfishing depletes fish life, upsets the food chain and, in the case of herbivorous fish, leads to the corals becoming overgrown with algae. Corals found deep down in temperate waters are also being damaged by destructive fishing methods.
As diving becomes more popular, environmentalists are becoming increasingly concerned by the damage done by careless divers to live corals. Some diving operators in warm waters have banned the use of gloves, except on wrecks, in an effort to stop divers from holding on to live coral. If divers have to settle on the seabed to practise diving exercises or adjust equipment, they should do so only on dead sand to avoid killing live coral.
The growing awareness of environmental issues has given rise to ecotourism – tourism with an ecological conscience. Ecotourism is often summed up as ‘take nothing but photographs, leave nothing but footprints,’ but even footprints, as indeed any form of touching, is a problem for corals. It may be better to manage tourism, and the tourists themselves, in such a way as to be ecologically sustainable. The capital investment necessary to develop ecotourism is minimal, much-needed employment becomes available to the local population and in the long term the profits exceed those of logging or overfishing.
Although many divers, dive operators and diving resorts lead the field in protecting marine ecosystems, we all require somewhere to eat and sleep. If a small resort is built without a waste-treatment system, the nearby reefs may not be damaged irreparably. However, if those same reefs attract increasing numbers of tourists and more resorts, then controls on the resorts, visiting divers from nearby areas, and visiting live-aboard boats, become necessary.
Coral reefs are not the only places affected by divers, but that is where concentrations of divers are found. There is also concern over some divers’ behaviour in places where annual congregations of larger animals occur, but this can be controlled by educating divers and operators.
It has been suggested that in a few cases environmentalists have gone too far. If rules in one area are too strict, divers and snorkellers will lose interest in that area and either give up entirely or go elsewhere. Either way, if divers and snorkellers are not around regularly to keep an eye on the animals or coral reef, and the local people do not gain employment from tourism, there is more chance of unscrupulous fishermen wiping out the animals or using damaging fishing methods on reefs.
ECO-FRIENDLY DIVING
Ecological sustainability of the marine environment depends as much on individual divers as on dive operators and resorts.
■ Do not touch living marine animals or organisms with either your body or your diving equipment.
■ Control your fins. Their size and the force produced by the fin-stroke can damage large areas of coral. Do not use deep fin-strokes next to the reef, the surge of water can disturb delicate organisms.
■ Master good buoyancy control. Much damage is caused by divers descending too rapidly or crashing into corals while trying to adjust their buoyancy. Be properly weighted and if you have not dived for a while,