Measuring distribution and density of sprat in Årdalsfjorden with a kayak drone - 15-16 August 2020

Abstract

Sprat is a key species in several Norwegian fjords, as an important prey for fish, sea mammals and sea birds. The sprat populations are monitored with routine acoustic-trawl surveys using a large research vessel, which provide the main scientific basis for sprat quota advices. Previous survey observations indicate that a significant fraction of the sprat population can be found close to the sea surface and thereby in the acoustic blind zone of the echosounder mounted below the vessel. In addition, fish may avoid the approaching vessel, and thereby measure less acoustic density. Furthermore, a large research vessel cannot operate in shallow waters and is difficult to maneuver close to the shore. In an experiment carried out in Årdalsfjorden in the evening 15 August and during daytime 16 August, the distribution of sprat was measured with a kayak drone installed with a 200 kHz echosounder. The geographical distribution and the mean acoustic density were similar between the two coverages. The experiment showed that 63% of the fish density was observed closer to the surface than 8 meters, which is the acoustic blind zone of research vessel. At daytime, the fish was distributed deeper, but 24% of the fish was still distributed closer to the surface than 8 m. This experiment show that future sprat surveys should be carried with silent vehicles where the echo sounders are mounted near the surface. A larger vessel is though also needed for trawling for biological samples.