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Use of horizontal guided sonar in addition to vertical echo sounder may improve biomass estimation of pelagicfish schooling near the surface. But proportions of horizontal guided sonar to vertical echo integration estimates have beenshown to be very variable, and it is necessary to provide better knowledge on factors that influence acoustic recordings offish near surface before a reliable method of combining sonar and echo integration estimates can be established. Toexplore factors that influence horizontal sonar recordings of fish biomass near surface, we collected and analysed datafrom two acoustic surveys on Norwegian spring-spawning herring (Clupea harengus L.) in the Norwegian Sea in 1997and 1998, and two acoustic surveys off the southwest coast of Africa, investigating pilchard (Sardinops sagax), anchovy(Engraulis capensis) and round herring (Etrumeus whiteheadi) off the coast of Namibia in 1994, and sardinella(Sardinella sp.) off the coast of Angola in 1995. The ratios of fish densities obtained by sonar to those obtained by echosounder varied both with respect to location and between different years within a specific location. For three of the foursurveys, the biomass estimated by sonar was significantly higher than the estimates by echo sounder, whereas in theremaining survey there was no significant difference in estimated biomass. Periods of bad weather, shallow, or lowdensitypatchy fish distributions and mixture of the target species with other species contributed to the higher and morevariable sonar estimates. Still a goal should be to improve knowledge about the factors that contributes to variationsbetween simultaneously recorded echo sounder and sonar data. An aim should be to combine the recordings from bothmethods in the biomass estimations, or at least assess which of the estimates that are less biased under the prevailingenvironmental conditions and actual school distribution

Influence of Different Factors on Abundance Estimates Obtained from Simultaneous Sonar and Echo Sounder Recordings