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11 th International Symposium for GIS and Computer Cartography for Coastal Zones ManagementAn AZFP instrument was deployed looking downward from a drifting surface float, and acoustic backscatter dataat four frequencies were collected from late afternoon until darkness on February 23, 2012. The ascent of the scatteringlayer from approximately 100 m depth to the surface was clearly evident in the three lower frequency channels,but was detectable in the 770 kHz channel only after it passed through 40 m because of the limited detection range atthe higher acoustic frequency. The volume backscatter in the migrating layer was higher at 200 kHz than at 125kHz, which in turn was higher than at 455 and 770 kHz. There was also another weaker, more diffuse layer continuallypresent near the surface, in which backscatter appeared highest at 770 kHz. The differences in scatteringstrength with frequency were interpreted using the scattering model developed by Stanton et al. (1994). The scatteringspectrum suggested that the zooplankton species in the diurnally-migrating layer between 0 and 40 m in SaanichInlet was Euphausia pacifica. It was not possible to make net tows and microscopic observations, but many previousstudies have shown E. pacifica to be the dominant zooplankton in this area (Peiper, 1971). The zooplankton with thestrongest scattering at 770 kHz appeared to have been a smaller organism, likely Calanus plumchrus, commonlyfound in the adjacent waters of the Strait of Georgia (Harrison et al., 1983).ConclusionLow power, multi-frequency scientific echo sounders, with their ability to collect long, high-resolution times seriesof acoustic backscatter strength, can be a valuable tool in furthering understanding of zooplankton and fish populationsand behaviour. When used in conjunction with other resources, such as satellite remote sensing and vesselbasedacoustic surveys and net tows, they can contribute significantly to understanding oceanic ecosystem dynamics.AcknowledgmentThe data in Figure 1 were provided by the VENUS project at the University of Victoria, Victoria, BC, Canada.ReferencesBorstad, G.A., L. Brown., M. Sato, D. Lemon, R. Kerr, and P. Willis. (2011), “Analysis of zooplankton time series from an upwardlooking sonar: the data cube concept”. 5 th ICES/PICES zooplankton symposium, Pucon, Chile 14-18 March 2011.Harrison, P., J. Fulton, F. Taylor, and T. Parsons. (1983), “Review of the biological oceanography of the Strait of Georgia: pelagicenvironment”. Canadian Journal of Fisheries and Aquatic Sciences, 40:1064–1094.Holliday, D.V. and R. E. Peiper (1980), “Volume scattering strengths and zooplankton distributions at acoustic frequencies between0.5 and 3 MHz”. Journal of the Acoustical Society of America, 67(1):135–146.Peiper, R. (1971), A study of the relationship between zooplankton and high frequency scattering of underwater sound. Ph.Ddissertation, University of British Columbia, Vancouver, BC, Canada.Stanton, T.K., D. Chu, M.C. Benfield, L. Scanlon, L. Martin, and R.L. Eastwood. (1994), “On acoustic estimates of zooplanktonbiomass”. ICES Journal of Marine Sciences, 51:505–512.Trevorrow, M.V. and Y. Tanaka (1997), “Acoustic and in situ measurements of freshwater amphipods (Jesogammarus annandalei)in Lake Biwa, Japan”. Limnology & Oceanography, 42(1):121–132.Xie, Y. and R. Dewey. (2011), “Fraser River Salmon Run”.VENUS Newsletter, 31:4.208