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

ABSTRACT

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.