Experiences in making acoustic measurements in a mesocosm with Calanus finmarchius
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In connection with a larger study of the effects of nutrification and turbulence on primary and secondary production, acoustic measurements have been made in the water volume defined by floating, vertically oriented, cylindrical enclosures, with flexible but impermeable walls, called mesocosms. Nominal diameters and greatest depths in the tapered bottom region are 2 and 9 m, respectively. The mesocosms contain rather small quantities of zooplankton, of which Calanus finmarchicus is the organism of primary interest in the acoustic work. In fact, a range of scatterers were visualized, including krill, smaller organisms, and, most likely, hydrographic features as well as the wall of the mesocosm, complicating recognition and quantification of Calanus finmarchicus. The density of this copepod in stages I-VI, reckoned in orders of magnitude, is 10-1000 animals per cubic meter, the precise figure depending on the particular mesocosm. An attempt was made to quantify the scatterers by echo integration. Measurements were made with the SIMRAD EK500 echo sounder, with narrowband transducers operating at 119, 200, and 714 kHz. Postprocessing was accomplished with the Bergen Echo Integrator. While preliminary, limited results are not demonstrably unambiguous, the observations and associated measurement experiences may prove valuable in like future investigations.