Variation in hydroacoustic abundance of pelagic fish

Abstract

The variation in hydroacoustic abundance of pelagic fish was studied in Baltic coastal areas. Comparisons between zigzag transects and straight transects in the middle of a study area showed no consistent differences and neighbouring areas had generally similar abundances. The layout of transects and the exact boundaries chosen for a study in our open area are thus not crucial to the result. However, increasing the length of hydroacoustic transects within an area (increased degree of coverage) decreases the coefficient of variation between replicates. The average coefficient of variation (CV) between replicates from one night along a single transect was 20%, while for replicates from different nights the CV value was on average 29%. This implies that day-to-day migration between the study area and its surroundings influences hydroacoustic assessments less than the factors causing variation between replicates from the same night. These relations, however, differ between areas. In a sheltered bay, the CV between nights was 9% at a high degree of coverage. In a more open area the value was 20–30% and increased coverage had small effects on the variation between nights. Thus, in a relatively closed area, a high degree of coverage increases the reliability of the assessment, while in a more open area it might be more important to increase the study area (decreases effects of migration) or to record data on several nights. The precision in hydroacoustic assessment of fish is as good as or even better than recorded assessments for zooplankton, pelagic mysids, macrobenthos, meiobenthos, shallow water fish and gill net catches of littoral fish.