Acoustic abundance estimation of wintering Norwegian spring spawning herring, with emphasis on methodological aspects
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The spawning component and part of the juvenile stock of Norwegian spring spawning herring have been wintering in a fjord system in northern Norway in recent years. Conditions for measuring the stock size have been excellent. Here, results are presented from the December 1993 and January 1994 cruises with RIV "Johan Hjort", supplemented by measurements with RIV "Michael Sars" in December 1993. Following a review of the biology, principal elements of the survey are described, namely design, acoustic measurements, and biological sampling. A variety of auxiliary studies, some ongoing, are breifly reviewed. The subjects of these are ( 1) acoustic extinction, (2) fish orientation in situ, (3) fish target strength in situ, ( 4) geographic differences in size distribution and associated dynamics, ( 5) herring fat content, ( 6) hydrography with respect to herring distribution, (7) inter-ship calibration of echo integrators, and (8) multi-frequency echo integration. Methods of analysis: criteria for echogram interpretation, stratification, global averaging, and geostatistical estimation of variance, are described. Results of the acoustic survey are expressed through absolute numbers for abundance, composite size distribution, and estimates of confidence associated with the degree of coverage based on geostatistics. The results are placed in the context of other methods of assessment and prognoses. Error sources in the acoustic abundance estimate are enumerated and critiqued. Outstanding problems, if obvious, are mentioned.