Synchronized underwater audio-video recording
Phillip S. Lobel
One advanced technique used in undersea research to study fish acoustic behavior is simply the interdisciplinary application of established acoustic technology combined with new mini-video cameras and hydrophones while diving with a UBA rebreather. Rebreathers and hydrophones have been around for more than 40 years but it has been only in the last decade that video cameras and hydrophones have become miniaturized. Most importantly, these video cameras are equipped with audio input enabling synchronous audio-video recording. Acoustic signal analysis is also greatly simplified by direct connections to advanced portable computers and new sound analysis software. Even so, the basic situation is the same now as when Steinberg and Koczy (1964) stated, "the principle accomplishment of the techniques for underwater observation is that of extending mans senses of sight and hearing underwater".
Being able to observe and record the sonic behavior of marine animals was recognized as an important scientific approach to understanding fish behavior ever since it was first revealed that sounds were integral to behavior in many species (Fish 1954, Griffin 1955, Moulton 1958, Tavolga 1960). The approach that I use in recording bioacoustics was pioneered in the 1960s at the Lerner Marine Laboratory in Bimini, Bahamas and during missions of the underwater habitat, TEKTITE. In 1963, a television and hydrophone system was deployed in about 20m depth and linked by cable to the Lerner lab with a room full of electronic equipment for recording (see figures in Kronengold et al. 1964). This system recorded a variety of sounds but the fixed camera did not usually catch the identity of the sound producer (Kumpf 1964). This early system did, however, clearly document several temporal patterns of distinctive sound production by marine animals (Cummings et al. 1964). This system was also used for the playback of sounds to determine the effectiveness of pulsed low frequency sounds for attracting sharks and other fishes (Richard 1968, Myrberg et al. 1969). During the TEKTITE missions of 1969, divers used rebreathers from an underwater habitat to record fish sounds using a 8mm film movie camera and a tape-recorder with hydrophone operated independently but simultaneously (Bright 1972). Bright clearly noted the benefit of the noiseless rebreather when recording fish behavior. The two major advances today are 1) advanced video and hydrophone technology enabling truly synchronous audio recording (in a much smaller package) and 2) the increased reliability of electronic rebreather systems.
Bubbles not only produce noisy sounds but also near-field vibrations in the water (Fig. 1, 2). This water disturbance is probably similar to the hydrodynamic disturbances produced by fast moving predators to which most fish are especially sensitive by means of their lateral line and sensory pore system. The common range of reef fish hearing is roughly between 20 and 1000 Hz although some hearing specialist species can hear in ranges up to 10 KHz. One herring species can hear at 180 KHz (Hawkins 1981, Mann et al.1997, Popper and Fay 1998). However, it is fair to note that some fish apparently habituate to boat and scuba noise, as is seen at some marine protected areas or other sites with high diver activities. For example, I recently recorded a situation where scuba and excessive boat noise did not affect the mating activity of the damselfish, Dascyllus reticulatus in a lagoon (Lobel pers. obs., Saipan, July 2000).
I started recording the acoustic behavior of free ranging fishes in 1988 using open circuit scuba and first generation 8mm camcorders connected to a hydrophone (Lobel 1992). Because of the scuba bubbles, I had to spend many hours per day in the water to allow the subject fish to habituate to my presence. This required remaining motionless on the bottom for long periods and carefully controlling my breathing so that only a trickle of tiny bubbles was slowly exhaled (Figure 3). This somewhat avoided the louder noise caused by a big burst of bubbles from a single exhalation. In order to obtain quality acoustic recordings without scuba bubble noise interference, acoustic measurements were edited in the lab from those portions of the video made between breaths. Thus, divers needed to be very disciplined in their respiratory pace and activity while recording.
We started using a rebreather three years ago and, without a doubt, it is the most successful method for obtaining fish behavioral and bioacoustic data (Lobel and Kerr, pers. obs). Beginning with my first field experience with the rebreather (after 31 years of scuba), I was greatly impressed with how differently fish behaved when there were no bubbles. The great advantage of the rebreather is to allow us to approach animals more closely. But this is also a bit more risky for the same reason. I have found that eels and grey reef sharks are more inquisitive and approach much closer without noisy bubbles. On the other hand, the only time I experienced a school of juvenile parrotfish actually swim toward and over me, as if I was just a rock, was when using a rebreather. This aspect of using rebreathers will have its most significant impact on the practice of conducting underwater transects for species census and abundance surveys. We have found that we see more individuals and a greater diversity, especially large fishes, while diving with the rebreather (Lobel and Kerr pers. obs.). Taking this one step further, we have recently acquired camouflage pattern wetsuits that are made to blend in with reef habitat. Thus, divers not only make no noise, but their silhouettes are less conspicuous and make the diver appear less like a large predator. Underwater photographers were among the first to use rebreathers routinely for the same reasons (e.g. Cranston 1993). Any type of observational data that marine biologists collect will clearly benefit from using the rebreather. Examples include: 1) conducting transects to determine species diversity and abundance, 2) quantifying fish feeding habits by observation, and 3) defining species habitat usage and behavior. Such field projects require a tool that provides the greatest degree of scientific accuracy possible and confidence in the results. The advantage of the rebreather is that it allows for: 1) noiseless operations, 2) the long bottom time necessary to allow fish to acclimate to the observers presence (overall, the use of a rebreather greatly reduces the time needed to habituate fishes compared to open circuit), and 3) sufficient dive time to record entire courtship and spawning activities. The unit only releases a limited amount of bubbles when ascending due to expansion and overflow of gases in the scrubber assembly.
for further details and literature cited see:
Lobel P. S. 2001 Fish bioacoustics and behavior: passive acoustic detection and the application of a closed-circuit rebreather for field study. Marine Technology Society Journal, 35:19-28
Funding for the field and laboratory studies is supported by the Army Research Office (DAAG55-98-1-0304). Support for this project was from the Army Program for Chemical Weapon Demilitarization, Aberdeen Proving Ground, Md. I am also very grateful for the tremendous help and advice from Carelton Technologies Inc. in applying the use of the rebreather for our scientific studies. Comparative bioacoustic study of fishes has been supported by grants from: the Island Foundation, the Kelley foundation of Cape Cod, World Wildlife Fund-US, Wildlife Conservation Society, The NOAA NURC Undersea Habitat Program, and the Office of Naval Research. I thank for help: Lisa Kerr Lobel, Gary McCloskey, David Shogren, Rich Pyle, John Earle, Bob Hennig, Matt Gavin, Scott Reuther, Scott Holt, Ingrid Kaatz, Steve Oliver, Justin Neviackas and David Mann.
My participation in this meeting was supported by the Woods Hole SEAGRANT office.