Benefits from horizontal beaming during acoustic survey: application to three Norwegian lakes

摘要

The abundance of fish in three contrasted Norwegian freshwater lakes (different species, depth, habitat) was measured in August and September 1999 by echo sounding. All three lakes were surveyed by day and night with the echo sounder acoustic beam oriented both vertically and horizontally. Vertical beaming, due to the hydroacoustic blind-zone, detected little fish activity close to the surface of the lakes, When beaming horizontally, particularly at night, large number of fish were observed near the surface of all the lakes. The range obtained when beaming horizontally was 50-100 m. Vertical beaming was shown to underestimate the presence of fish by 20-100%, supporting the conclusion that horizontal beaming is a critical factor in compiling accurate fish stock assessments in lakes. Echo sounding data were in agreement with catch data with respect to vertical distribution and fish density. Other valuable information was obtained on diurnal and nocturnal fish distribution and noise masking of fish echoes due to shallow water and wind.