Acoustic assessment of Norwegian spring spawning herring in the wintering area, December 1995 and January 1995
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The abundance of the mature component of the stock of Norwegian spring spawning herring has been estimated based on a series of echo integration surveys. These were performed in the wintering area of Ofotfjord, Tysfjord, and Vestfjord in December 1994 and January 1995 using the SIMRAD EK500/38 kHz echo sounder and Bergen Echo Integrator postprocessing system. Compensation has been made for extinction, assuming that the ratio of extinction and backscattering cross sections is 2.41. Three considerations have dictated the form of stratification of the wintering area: coastline compexity, presence of herring, and herring size distribution as determined by pelagic trawling. The variogram has been computed for each stratum to determine the characteristic scale size of aggregation. Geostatistical theory has been applied to estimate the variance associated with abundance estimates due to the degree of survey coverage in relation to observed aggregation properties and fjord geometry.