Page 32 - Acoustics Today

animal’s material properties was where they were caught. We believe this is the result of differences in food (phytoplank- ton) abundance for the krill and that well-fed animals (pre- sumably with larger lipid reserves) differed physically from krill in regions with less food. The differences that we meas- ured in these animals are important when trying to interpret acoustic survey data. Colleagues at the National Marine Fisheries Service who conduct acoustic surveys to measure pollock (the largest US fishery in terms of landing biomass) stocks in the Bering Sea (Figs. 3 and 4) have also used the acoustic survey data to develop a measure of euphausiid bio- mass in this region (Ressler et al., 2012). The krill biomass estimate is very important in understanding the ecosystem dynamics of this region as pollock’s preferred prey are euphausiids. The results from our work (Smith et al., 2010) showed that regional differences in the euphausiid’s physical state (and thus their acoustic scattering efficiency) would lead to a doubling (or halving) of the krill biomass estimate. The results of this study are currently being applied to acoustic data collected at a mooring site in the Bering Sea which may allow us to monitor seasonal differences in the zooplankton and fish components of the ecosystem over annual cycles which may provide opportunities to examine how climate change (and the changes in sea ice in this region) will affect the fish and zooplankton.
The future?
In many marine systems, the ability to use acoustics to remotely monitor over long time periods and identify key components of the ecosystem is a distant goal. However in some locations, at certain times, and under certain condi- tions, acoustic techniques provide a remarkable ability to col- lect information about the fish and zooplankton in the ocean that cannot be collected in any other way. The spatial and temporal resolution of acoustic systems combined with the ability to ensonify large volumes of water rapidly allows acoustic data to provide insights into different aspects of marine ecology in addition to just producing abundance and distribution data for different species of animals. Recent advances in technology will increase our capabilities and help us to reduce the uncertainties in our indirect measurements of biology. However, the constant confounding factor will be our lack of knowledge about the biological organisms in their natural state. These data are the key to improving our ability to study marine life using active acoustic systems. A decade later, I think I finally may have an answer for Alex Trebek (several thousand Antarctic krill in a gulp).AT
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