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 spawning herring (Clupea harengus) wintering in the Ofotfjord-Tysfjord-Vestfjord system during late autumn 1995 and early winter 1996. The primary instruments of acoustic data collection and processing were the SIMRAD EK500/38-kHz echo sounder and the Bergen Echo Integrator. Biological sampling was effected by means of a so-called MultiSampler pelagic trawl in addition to standard pelagic trawls. Compensation was made during postprocessing for the effect of acoustic extinction. The major complication of the survey and challenge of the analysis has been stratification. This is discussed in the context of (1) mixing of immature and mature year classes, each with its own behavioural characteristics apropos of diurnal vertical migration and outwards spawning migration, (2) degree of achieved survey coverage, depending on fjord geometry, navigational hazards, available time, and fish distribution, and (3) ongoing spawning migration. Because of various uncertainties, a series of abundance estimates is presented. These are accompanied by fitted variogram models and geostatistical variance estimates