Dive Atlas of the World. Jack Jackson

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       Typical view of a northern Red Sea reef. Colourful anthias forage over a mixture of soft and stony corals on Jackson Reef in the Strait of Tiran.

      However, things must be placed into perspective. Sharks have attacked in areas where no feeding occurs and without obvious reason. When wearing light-coloured fins, I have had them bitten by large groupers and sharks at dive sites where no feeding had ever taken place. Possibly, the larger fish considered the fins to be smaller, prey-sized fish. A large barracuda has also attacked me in water with poor visibility. I was wielding a camera at the time so a glint of sun on the lens may have looked like the flash of a small silver fish. I know two divers who have been bitten by sharks while swimming too close to bait-balls that the sharks were feeding on. Several well-known operators have been badly bitten by groupers or moray eels that they fed regularly, but at the time of the respective incidents they were feeding another fish. Several people have suffered small grazes at organized shark feeds in the Bahamas.

      Even where hundreds of non-cage shark feeds are performed yearly with hand-feeding and/or large amounts of bait, there have been few injuries and those that did occur were mostly to those doing the hand-feeding. When sharks attack spear-fisherman, they are usually carrying dead, or worse still, struggling-while-they-die fish. Eventually, by the law of averages a tourist will suffer a serious injury or die during a feeding operation. However, the incident-rate is well within the range of adventure sports in general and much safer than mountaineering, skiing or snowboarding. Many more people are killed by bee-stings.

      There are many locations where fish feeding is restricted or prohibited. Recently the anti-feeding lobby in Florida, backed by spear-fishermen and commercial fishermen, managed to have fish-feeding banned. Media frenzy claimed that more shark attacks than usual had occurred locally, but this was not true. The rule-makers ignored the fact that currents had driven fish-shoals inshore; that people were filmed swimming among shoals of fish on which sharks were preying; and that a myriad of commercial fishermen were chumming the water, catching, killing and cleaning fish right off the tourist beaches. Florida now has a situation where dive operators are not allowed to use chumsickles (large blocks of frozen fish parts) to attract sharks, yet spear-fishermen and commercial fishermen are still permitted to use this baiting technique to attract sharks and other marine animals.

      With reference to feeding sharks, some species are more belligerent than others, and Grey Reef Sharks can be more so in some areas than in others. Having regularly organized shark feeds in the Red Sea since the early 1980s, my feeling is that many operators use too much bait. A couple of 25cm (10in) fish hidden in the coral are enough to keep the sharks interested for 20 minutes. It is also better not to hand-feed, even with chain-mail gloves, as this gives the sharks the impression that man supplies the food and could result in sharks harassing divers who are not involved in feeding.

      The case of researcher Erich Ritter being bitten by an adult Bull Shark at Walkers Cay, Bahamas, while being filmed for the Discovery Channel’s Shark Week series, will not help the pro-feeding lobby. However, those who regularly dive with sharks believe that if done in a responsible manner, shark dives are reasonably safe. We are privileged to have close encounters with wildlife underwater, often within arm’s length. Not everyone wants to get this close to a shark, but there have been many instances where other animals such as large barracuda, large groupers, Moray Eels and even Titan (Moustache) or Yellowmargin Triggerfish have either bitten or butted divers in situations not connected to feeding. Feeding fish is an emotive issue – you will have to make up your own mind.

      DIVING WITH GASES OTHER THAN NORMAL AIR

      ENRICHED AIR NITROX

      The term Nitrox is commonly applied to oxygen-enriched air. By increasing the percentage of oxygen, and thus decreasing the percentage of nitrogen, divers will absorb less nitrogen during a dive and have less to eliminate during the ascent.

      Diving on Nitrox can be treated in several ways. By calculating dive-plans from Nitrox tables, divers will have longer no-decompression stop times at their maximum depth. If calculating dive-plans from air tables, they will have an extra safety factor. If divers go into decompression, it will be shorter if calculated from Nitrox tables, but have a greater safety factor if calculated from air tables. Many divers feel less fatigued after diving on Enriched Air Nitrox, though there is no scientific proof of this, and many experience a lower rate of gas consumption.

      Another way in which Nitrox can be used to divers’ advantage is that divers, who have been deep while breathing air or other gas mixtures, can shorten their decompression times at shallow depths by changing to a mixture containing 50–80 per cent oxygen. This mixture enables faster elimination of excess nitrogen (if using air) or helium (if using mixed gases). However, due to oxygen toxicity, the depths to which divers can descend depend on the percentage of oxygen in the Nitrox mix used. The higher the percentage of oxygen, the shallower will be the maximum depth to which they can go. Divers should not descend to depths where the partial pressure of oxygen exceeds 1.4ata. Atmospheres absolute (ata) is the sum of atmospheric pressure and the hydrostatic pressure – the total weight of water and air above us.

      There may be circumstances when a diver breathing Enriched Air Nitrox has to go deeper than oxygen toxicity allows on that particular Nitrox mixture. In this situation, if the diver has a separate small cylinder of normal air fitted with its own regulator, it is possible to switch to breathing from this cylinder for a brief foray deeper than the depth allowed on the Nitrox mixture. The diver can then switch back to breathing Enriched Air Nitrox after returning to a depth where oxygen toxicity is no longer a problem.

      Another problem with oxygen breathed at higher than normal partial pressures, is that when used over long periods it affects the central nervous system. Divers must be careful not to exceed the recommended oxygen tolerance units (OTUs), particularly on repetitive dives.

      High concentrations of oxygen cause combustion on contact with oils and greases. Scuba cylinders and their valves come in contact with pure oxygen during filling, so they must be scrupulously clean. Standard regulators should be suitable for Nitrox mixtures of less than 40 per cent oxygen, but for higher concentrations, their O-rings must be replaced with ones that do not require lubricants.

      HELIOX AND TRIMIX

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