Allocation of echo integrator output to small larval insect (Chaoborus sp.) and medium-sized (juvenile fish) targets

In acoustic sampling for fish, thresholding is normally applied to eliminate the unwanted contribution of noise to the integrator output. Since thresholding discriminates against small targets, this technique cannot be used for the quantitative study of these small targets in the presence of larger ones. When the integrator output (area backscattering coefficient, sa) due to one size class of targets is plotted vs. decreasing sv-threshold (volume backscattering strength) in 1 dB steps, the resulting curve can be described by an asymptotic Bertalanffy growth function. If the slope of the resulting curve decreases at some intermediate threshold level and then increases again before the final plateau is reached, the total integrator output can be allocated to two groups of targets according to the linearity principle in acoustics. The maximum sa value for the larger targets (sa-large) is estimated by fitting a Bertalanffy function to the lower part of the curve. The echo integral that corresponds to the smaller targets is then found by subtracting sa-large from the sa value that is reached at the lower threshold level. In a shallow (max. depth 8.5 m) mesotrophic lake of 24 ha surface area in northeastern Germany, this new technique was tested to estimate the abundance of insect (Chaoborus) larvae in spring. Hydroacoustic data were sampled during night, when juvenile roach occurred together with midge larvae in the pelagic zone. Chaoborus abundance was estimated by replicate vertical tows of a 1 m2 plankton net of 0.5 mm mesh size. The sa values that were allocated to Chaoborus larvae were significantly correlated to mean Chaoborus abundance. This relationship was then used to map the lakewide distribution of Chaoborus larvae on two dates in spring