By Andrew Gray
Something was different on the recent PIFSC Main Hawaiian Islands Reef Fish Survey cruise led by the Coral Reef Ecosystem Division (CRED). In addition to the usual scuba surveys conducted by scientists, the small boats also deployed scientific divers that resembled a species of bionic frogmen. Instead of scuba, these divers used closed circuit rebreathers (CCR), and while it may look like they were a bit overdressed for the occasion, there was a good reason for it.
When conventional open circuit scuba divers take a breath of air from the tanks on their backs, they exhale it into the surrounding water and create a burst of bubbles containing CO2, residual oxygen not metabolized by the body, and nitrogen, an inert gas. Closed circuit rebreathers (or just rebreathers) are a specialized type of dive equipment that removes the CO2 and re-uses the exhaled oxygen. The diver breathes from a “loop” of gas where CO2 is removed from the exhaled gas by a chemical filter called a “scrubber.” Three oxygen sensors analyze the gas and a computer-controlled solenoid adds additional oxygen to replenish oxygen metabolized by the diver or add more oxygen when specified by the diver. Rebreathers include two small tanks of gas, one of pure oxygen and a second of a gas to dilute the oxygen called the diluent, usually air for depths less than 130 feet.
This equipment has a number of benefits over conventional scuba. The efficient use (and re-use) of gas allows divers to stay underwater much longer without having to bring an excessive number of tanks. Since the rebreather is mixing an ideal nitrox gas mix for whatever depth the diver is at, rebreather divers can have much longer no-decompression dive times when compared with conventional scuba divers at the same depth. Also, because no air is released into surrounding water, there are no bubbles or noisy exhalations—and that is exactly why we are using them.
It’s understood that visual fish surveys may be prone to certain biases. One of which is that fish behavior may be altered by the presence of a diver in the water. Some fish species may be attracted to the diver, others afraid. In areas where spearfishing is common, targeted fishes likely react to divers very differently than in areas where fishing does not occur. The CRED fish team specializes in conducting Stationary Point Count (SPC) visual fish surveys, which have always been conducted on open circuit scuba. But what effect do the noisy, conspicuous bubbles produced by scientific divers using scuba have on our fish counts and biomass estimates? By using closed circuit rebreathers we can replace the blaring bubble machines with silent divers.
So that’s where the “bionic frogmen” come in. On the recently completed research cruise, CRED and partners conducted stationary point count visual fish surveys throughout the main Hawaiian Islands using both open circuit scuba and closed circuit rebreathers to compare results. Would we record more fish when using the less obtrusive rebreathers? Would we see more of the fish species targeted by fisheries when using rebreathers and the same amount of other smaller non-fished species like butterflyfish and damselfish? Surely we’d see the same number of hawkfish, those small wary fish perched on top of coral heads. Right? Well, we are only about halfway through our comparison study so it’s too early to say just yet, but preliminary results suggest that we are seeing some differences when using rebreathers and not always what we expected. We hope to work up the results in a scientific paper later this year.
If open circuit diving is so obtrusive, why don’t all scientific divers switch to using rebreathers? Because there are some disadvantages to diving with rebreathers which can make life difficult when conducting hundreds or thousands of surveys each year. For one, rebreathers are heavy and bulky, even for me, a 190-pound 6’2” guy. After coercing your body into the 65-pound rebreather and clipping on another 15-pound bailout tank, regular scuba gear feels like floating on a fistful of helium balloons. You also have to deal with much longer setup and breakdown times, more pre-dive preparation and buddy checks, and significantly more expensive gear and consumables. Rather than have everyone switch to rebreather, we are more interested in understanding any differences in fish biomass between open circuit scuba surveys and rebreather surveys. For now, rebreathers are another interesting tool we can use to help monitor and assess reef fish populations around the Hawaiian Islands and U.S. territories in the Pacific Ocean.