Hydroacoustic measurements of fish community size spectra in a boreal reservoir

Size-based processes largely drive the structure and function of aquatic ecosystems, making ecological indicators based on body size especially meaningful. Hydroacoustic surveys in 2011(n = 6) and 2012 (n = 5) (BioSonics DTX, split-beam 200 kHz) were used to derive length-frequency spectra of fish communities within 3 basins of Lac du Bonnet, a hydropower reservoir along the Winnipeg River, Manitoba, Canada. Acoustic data attributed to single fish echoes comprised the majority of the acoustic backscatter. Fish echoes tracked and converted to length (5-50 cm) formed significant spectra in all cases (p values < 0.05, R² values from 0.77-0.99). Spectra height within the reservoir was greater in 2011 than in 2012, varied consistently among basins, and decreased through the sampling season in both years. Spectral slopes did not differ between years, but were related to physical habitat characteristics with steeper slopes in areas characterized by shallow waters and large littoral zones. Acoustically derived size spectra provide an efficient means to detect and monitor fish community dynamics over varied spatial and temporal scales in boreal (and other) aquatic ecosystems where single acoustic targets predominate.