Acoustic measures of lake community size spectra

Abstract

Acoustic methods were tested as an alternative to catch data to measure size spectra of freshwater fish communities in Lac du Bonnet (LdB), Manitoba, Canada. Eleven daytime hydroacoustic surveys conducted in 2011 and 2012 enabled length-frequency spectra of fish communities to be calculated within LdB. Fish echoes tracked and converted to length (>5 cm) formed size spectra with significant linear regressions in all cases (p values < 0.05, R2 values from 0.87 to 0.99). Fish abundance (spectral height) was greater in 2011 (mean ± SD = 0.19 ± 0.15) than in 2012 (–0.19 ± 0.20), decreasing through the summer sampling season. Both abundance and size composition (spectral slope) of fish communities differed between three basins of varying bathymetry, with steepest slopes and lowest heights associated with the shallowest waters. We conclude that acoustically derived size spectra provide an efficient means to detect and monitor fish community dynamics over varied spatial and temporal scales in aquatic ecosystems where fish are not highly aggregated and hence single acoustic targets predominate.