© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Diel patterns in pelagic fish behaviour and distribution observed from a stationary, bottom-mounted, and upward-facing transducer
Department of Systems Ecology, Stockholm University SE-106 91 Stockholm, Sweden
*Correspondence to Thomas Axenrot: tel: +46 86 747551; fax: +46 8158417. e-mail: thomas{at}ecology.su.se.
Diel variation in pelagic fish distribution influences hydroacoustic abundance estimates. To study and quantify diel patterns in behaviour and spatial distribution in pelagic fish without causing avoidance reactions or attraction to any floating equipment or vessel we used a bottom-mounted, upward-facing transducer. Light intensities were measured as skylight and underwater light (at 5-m depth). The study was performed in a coastal area in the Baltic Sea, late July to mid-August in 2001 and 2002. The results provided additional information on fish behaviour and distribution valuable for future survey planning and in the analyses of hydroacoustic data from regular surveys in this area. At night, the data on hydroacoustic backscattering (sA) were less variable, the vertical distribution of fish was more even, with fewer fish in the deepest layer, and the percentage of single-echo detections was higher. The tilt angle of fish seemed to differ day and night, but trawling and target-strength distribution results taken together also implied a partial diel change in the fish assemblage in the midwater layers. The processes of formation and disintegration of schools happened rapidly and coincided with day and night transition periods.
Keywords: diel variation, fish behaviour, stationary acoustics
Received 14 March 2003; accepted 12 May 2004.
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