Page 26 - Acoustics Today

COUNTING CRITTERS IN THE SEA USING ACTIVE ACOUSTICS
Joseph D. Warren
Stony Brook University, School of Marine and Atmospheric Sciences Southampton, New York 11968
Introduction
“Alex Trebek asked me a
“fancy fish-finder.” While this is true in a very broad sense, there are significant differences between scientific echosounders and the fish-finders on most fishing boats.
The first recorded incident (that I am aware of) of active acoustic detec- tion of biological organisms was in the “deep scattering layer” (DSL) (Dietz, 1948; Johnson, 1948). Early depth- measuring systems used paper-charts to record the strength of the echoes that were detected. The seafloor produced a very strong echo, however the chart-
Afew years after I finished gradu-
ate school, I was a contestant on
the TV game show “Jeopardy,”
where my performance could gener-
ously be described as terrible. During
the between-rounds Question and
Answer (Q&A) segment with host Alex
Trebek, we talked about my research on
Antarctic krill and he asked me a sim-
ple question. “How many krill does a
whale eat when it opens its mouth and
takes a gulp?” I froze and realized I had
no idea what the answer was to his
question. Having attended a few scientific conferences at this point in my career, I knew how to respond to a question like this: sound knowledgeable, speak confidently, and answer a different question. So I told him that each gulp is about the size of a small car like a Volkswagen bug. A decade later, I still can’t answer Trebek’s question, but I can at least make an educated estimate based on field-collected data from my research. And it’s all because of underwater acoustics.
Terrestrial acousticians (my name for those whose research doesn’t occasionally involve seasickness) may be unfamiliar with the properties of sound in the marine envi- ronment. It travels faster (by a factor of 5) and more effi- ciently (with less energy dissipation) than in air. As a result, while visual systems are limited to ranges of a few hundred meters (at best); acoustic transmission and detection systems can reach halfway around the globe (literally) (Munk et al., 1994; Baggeroer et al., 1994). The term “active acoustics” is often used to describe those who both transmit and receive acoustic signals in the ocean; whereas “passive acoustics” refers to those who just listen to sounds. As acoustic waves propagate through the ocean, they are scattered by inhomo- geneities in the water column. The seafloor and sea surface are very strong reflectors due to differences in acoustic impedance between seawater, sediment, and air. By transmit- ting a short acoustic pulse downward, detecting the echo reflection from the seafloor, and tracking the time delay, one can calculate the distance from the transmitter to the seafloor. This is how the depth-sounder on a boat can tell you how deep the water is underneath the vessel. For more detailed information on underwater acoustics, see Urick (1983), Medwin and Clay (1998), or Simmonds and MacLennan (2005). Similarly, biological organisms (as well as many other inhomogeneities in the water column) can reflect acoustic energy. Thus many of those depth-finders on the boat will also work as fish-finding sonars. When speak- ing to a non-scientist, I often describe my research as using a
simple question. How many
krill does a whale eat when it
opens its mouth and takes a
gulp? I froze and realized I
had no idea what the answer
was to his question.”
recorder also showed weaker reflections occurring several hundred meters deep in the ocean that were definitely not the seafloor. The cause of the DSL was correctly hypothesized to be biological in origin, but it was not until the first sub- mersible traveled to these depths that the specific sources (small fish and zooplankton) were identified. (Marshall, 1951; Hersey and Backus, 1954; Barham, 1963, 1966; Bary, 1966; Bary and Pieper, 1971; Hansen and Dunbar, 1971; Castile, 1975). If you are trying to create bathymetric maps using acoustic methods, non-seafloor reflections (like the DSL) are noise or unwanted signals. However, as many peo- ple have said, one person’s noise can be another person’s sig- nal. Those of us who are interested in the biological organ- isms in the ocean have found this “noise” to be one of the best methods possible to study these animals in their natural habitat.
Nekton and zooplankton are the names given to two broad categories of animals in the marine environment. Nekton are animals that typically are actively moving within the marine environment, whereas zooplankton are usually passively advected or moved by ocean currents. These defini- tions are somewhat crude as many organisms called zoo- plankton can actively move. Some do so regularly, traveling hundreds of meters up and down in the water column every day—a process called diel vertical migration (Miyashita, 2003; Benoit-Bird et al., 2009; Kaltenberg and Benoit-Bird, 2009). On the other hand, numerous nekton species (includ- ing most fish) have larval or juvenile forms that can be con- sidered zooplankton as they are unable to move against a cur- rent until they reach a later life stage. Regardless of whether we are discussing nekton or zooplankton, both types of ani- mals are incredibly important both economically (as com- mercial or recreational fisheries) and ecologically. Humans and many other higher-trophic levels rely on nekton and zooplankton as their primary energy source (Sinclair, 1994; Honkalehto et al., 2009). Zooplankton are, in most oceanic environments, the link between the primary producers (phy-
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