A submerged shoal is an area of the seabed that is shallower than the surrounding area, also known as pinnacles these areas can be very popular with holiday divers. Usually these shoals are well offshore, although this is not always the case. Diving on such shoals is a great experience because you have the sensation of exploring an underwater island.
One of the main attractions of shoal diving is that such places act as oases for marine life. The best shoals are those composed of hard rock. Which protrude some distance from a sandy seabed. These provide a substrate on which marine life will settle. Often there will be large shoals of pelagic fish near such shoals, and frequently the tops of the shoals have large solitary fish. At shoals there is the increased chance of seeing some of the large oceanic wanderers. Examples of these include: whale sharks, reef sharks and giant barracuda.
Although not really a diving matter, the location of shoals can be difficult. In some locales dive sites at shoals are permanently marked by a buoyine. For inshore shoals, transits from the shore may be necessary, although it may be possible to detect shallow shoals by looking for sea-weed or breaking waves. However, for unmarked offshore shoals electronic means will have to be used and these can be very successful for finding large shoals offshore. Small shoals on the other hand can be very difficult to locate and repeated runs with an echo sounder may be required.
Once a shoal is located it should be marked with a shotline, This should have a heavy weight (perhaps 25 kg) connected by a heavy line (at least 10mm diameter) to a substantial buoy (say 50-100 kg buoyancy). This shotline will act as a reference for both the divers as they descend and ascend, and also for the surface party.
It is most essential that surface marker buoys be used on all shoal dives. Care must be taken to ensure that the buoyline does not entangle the shotline - if there is any tidal stream then the buoyline can be trailed slightly downtide. The delayed surface marker buoy technique can be very effective for stage decompression if there is little or no tidal stream and if the surface visibility is good.
One of the most novel aspects of diving on a reasonably deep shoal is that the descent has to be made though midwater, with no visual reference other than, perhaps, the rope of a shotline. Once out of site of the surface and the seabed, the diver appears to be surrounded by a sphere of blue water. This is referred to as being 'in the blue' and can be somewhat disconcerting. It is caused by light being reflected from a light-coloured bottom so that all directions appear equally bright. It mainly occurs in tropical water, in Britain the greener water leads to the expression 'in the murk'. In this case the water colour usually becomes darker with depth, thus hinting at the direction of greater depth. However, deep blue water is encountered in the clear, offshore waters of Northwest and Southwest Britain.
Many shoals have substantial depth nearby, so great caution must be exercised not to venture too deep, especially when tidal streams are present. Consider an extra margin of bottom time for some shoals, as they may lie some distance from the shoreline and potential rescuers may take extra time in arriving.
An increasing water swell can rapidly put shoal diving out of sensible reach. It also becomes very difficult to stay near the top of a shallow shoal in a big swell. It is also very uncomfortable and somewhat difficult to attempt to carry out stage decompression stops on a shotline, although a lazy shot or a delayed surface marker buoy can get around this problem.
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