Abstract: This paper describes the results of 24 h acoustic experiment based on three-dimensional measurements of the S; distribution of herring and sprat in the southern Baltic in October 2001. The experiment was based on a continuous (24h day) integration offish echoes using an EY500 echo sounder from a vessel with a constant speed of 8 knots, moving along the sides of a square equal to 4 n.mi., localized inside the specified area. Duration of the experiment was limited by deterioration of the weather conditions but nearly 300 ESDU samples were collected. These are correlated with values of coincident environmental factors: time, depth, water temperature, salinity and oxygen levels; estimated on the basis of survey data. Fish behaviour vs environmental factors is described by different macrosounding visualizations, statistical, and mathematical models. The purpose of the paper is to compare results of the experiment to the average characteristics of fish behaviour in the same basin, based on autumn studies over the period 1995-2001. General analysis was provided for the selected area of the Polish EEZ (south Gotland Deep), characterized by the greatest depth movements of fish diel migration. The most significant differences were found between fish migration pattern and diel stability of the acoustic response between these situations, specially during the sunset period.