| Multi-frequency echo sounder samples Small organisms like many plankton species will normally have the strongest sound reflection in the higher end of this frequency range. The echograms below are kindly provided by NOAA. Using 18 and 200 kHz, they clearly demonstrate that the reflective properties of aquatic animals can be strongly frequency dependent. These dual frequency echo sounder samples shows simultanous recordings of the same water column using two different frequencies. The echograms illustrate the detection of age-0 walleye pollock and capelin (A) and adult pollock (B) at a relatively low frequency (18 kHz), and the detection of age-0 pollock and capelin (C) and euphausiids and adult walleye pollock (D) at a higher frequency (200 kHz). These species-specific frequency-dependent differences in the acoustic data are useful in identifying the organisms which are responsible for the backscatter. Researchers are currently refining methods to discriminate species of interest based on their multi-frequency signatures. The data is provided from a NOAA Alaska Fisheries Science Center survey conducted off Kodiak. The bottom depth is about 140 meters. |