Acoustic abundance estimation of the stock of Norwegian spring spawning herring, winter 1995- 1996

Standard echo integration methodology has been applied to the stock of Norwegian spring\nspawning herring (Clupea harengus) wintering in the Ofotfjord-Tysfjord-Vestfjord system\nduring late autumn 1995 and early winter 1996. The primary instruments of acoustic data\ncollection and processing were the SIMRAD EK500/38-kHz echo sounder and the Bergen\nEcho Integrator. Biological sampling was effected by means of a so-called MultiSampler\npelagic trawl in addition to standard pelagic trawls. Compensation was made during\npostprocessing for the effect of acoustic extinction. The major complication of the survey and\nchallenge of the analysis has been stratification. This is discussed in the context of (1) mixing\nof immature and mature year classes, each with its own behavioural characteristics apropos of\ndiurnal vertical migration and outwards spawning migration, (2) degree of achieved survey\ncoverage, depending on fjord geometry, navigational hazards, available time, and fish\ndistribution, and (3) ongoing spawning migration. Because of various uncertainties, a series of\nabundance estimates is presented. These are accompanied by fitted variogram models and\ngeostatistical variance estimates.