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When small changes make huge differences

Acoustic surveys of krill, fish, and other marine animals requires a fundamental understanding of how sound transmitted from a transducer mounted on the ship's hull is reflected back from a target organism as it goes about living at some subsurface depth below the sea surface. The intensity of the returning echo is a measure of the target strength of a given individual at a particular sound frequency. But what determines an individual’s target strength?

The two sound speed tubes side by side in the aluminum reservoir filled with seawater. At one end of each tube is a transmitting transducer and at the other end is a receiving transducer. In the middle of each tube is a compartment in which krill can be placed so that they are in the path of the transmitted sound. Normally one chamber is kept empty to serve as a control. Photo: Peter H. Wiebe

Size and orientation are two components that are very important. In general the larger animals have larger target strength. An animal broad-side to the emitted sound will produce a larger echo than one angled obliquely. Equally important to the determination of target strength are what we call the "material properties" of the animal. These are its sound speed contrast (h), the speed of sound through the animal’s body relative to the speed of sound through the surrounding seawater, and its density contrast (g), the difference in its density and the density of the surrounding seawater. If the animals sound speed contrast and density contrast were unity, there would be no echo. Fish are strong acoustic targets because they are large and usually have high sound speed and density contrast values. Zooplankton including krill, however, is much smaller and the material properties of their bodies are more nearly those of seawater. Thus, most zooplankton weakly backscatter sound and to survey them requires high-resolution precisely calibrated echosounders that can detect very small acoustic targets.

APOP with live krill being deployed for a 200 m profile cast to measure their sound speed contrast as a function of depth. Note the aluminum reservoir within the pipe framework that the sound speed chambers are contained in. (Photo: Peter H. Wiebe)

Knowledge about zooplankton material properties, however, is scant primarily because of the difficulty of making such measurements on living zooplankton. On this expedition, we are using a specially designed device dubbed APOP, short for "Acoustic Properties of Plankton". APOP consists of two parallel sound tubes or chambers with a transmitting transducer at one end of the tube and a second receiving transducer at the other end (Figure 1). Both tubes have a central compartment in which animals can be placed and held alive for the duration of an experiment. During an experiment the time difference is measured for acoustic waves or sounds traveling directly from one acoustic transducer (the transmitter) to another transducer (the receiver) with and without animals in the acoustic path. If sound travels faster in animal bodies than in water, the travel time with animals present in the acoustic path will be shorter and vice versa. The ratio of the sound speed in animals to that in seawater can be computed using the difference in sound speeds between the two tubes. Each acoustic chamber contains two identical broadband transducers with a center frequency around 500 kHz and a bandwidth of about 300 kHz. The two chambers are mounted next to each other on a frame and the frame suspended in an aluminum reservoir to keep the chambers surrounded by seawater. The sound speed chambers and reservoir are mounted in an aluminum pipe frame for deployment off the side of the ship for profiles down to 200 m (Figure 2). A cable (~220 m) with electrical conductors connects the broadband transducers and the surface data acquisition system.

Dezhang Chu recording the sound-speed data from APOP during an experiment. (Photo: Peter H. Wiebe)

The standard procedure on this cruise is to do an initial set of measurements on the deck of the ship with both sound speed chambers empty. Usually between 15 and 25 individuals of living Antatarctic krill, Euphausia superba, are then put into one of the sound speed chambers (Figure 3). After another set of measurements on the deck with the animals sealed in the chamber, the system is deployed over-the-side and sound speed measurements are made on both chambers at 20 m intervals down to 200 m and again on the way back to the surface to see what effect pressure has on their sound speed contrast. The animals, still alive, are then removed from the sound speed chamber and held in seawater for the next step in the procedure. A volume estimate is used to estimate what fraction of the APOP volume was filled by the animals when the sound speed contrast measurements were made. Needless to say, all of these measurements take time and patience to be completed. What always amazes us is how hardy these krill are. They remain alive and energetic in spite of the handling and the trip down to depth in a small experimental chamber. And they mostly survive the process of measuring their density in the laboratory and even the process of measuring their lengths and displacement volumes.

Peter Wiebe putting krill into the weighing vessel prior to making the first weight measurement to estimate krill density and volume. (Photo: Dezhang Chu)

 The results thus far on this cruise are preliminary, but the krill sound speed and density data are quite consistent and so far the values are somewhat higher than found for krill collected in the Western Antarctic Peninsula shelf waters back in 2002. We hypothesized that krill collected in the summer would be in better condition and might have higher density and sound speed. The data seems to support that contention (Figure 4).



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Peter H. Wiebe (right), Ph.D. in biological oceanography from the University of California, San Diego in 1968. An Emeritus Senior Scientist at the Woods Hole Oceanographic Institution, his interests have most recently been focused on the dynamics of zooplankton populations on Georges Bank and on krill living on the continental shelf region of the Western Antarctic Peninsula.

Dezhang Chu is the supervisory physical scientist with National Marine Fisheries Service (NMFS), National Oceanic and Atmospheric Administration (NOAA) of the United States. He got his PhD in geophysics from the University of Wisconsin-Madison, WI, USA in 1989 and has been working on fisheries acoustics since then. His research focus is on acoustic scattering by marine targets.