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

• Published in: Fisheries research Vol. 35; no. 1; pp. 107 - 113
• Main Author: Eckmann, Reiner
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Amsterdam Elsevier B.V 01-05-1998; Elsevier
• Subjects: Acoustic size discrimination; ALEVINES; Animal, plant and microbial ecology; Biological and medical sciences; Chaoboridae; Chaoborus; Chaoborus larvae; DETECCION DE PECES; DETECTION DU POISSON; FISH DETECTION; FISH LARVAE; FORMAS JUVENILES; Fundamental and applied biological sciences. Psychology; General aspects. Techniques; INSECTA; Integrator output; JUVENILE; Juvenile roach; JUVENILES; LARVE DE POISSON; Methods and techniques (sampling, tagging, trapping, modelling...); RUTILUS; Use of Bertalanffy function; Chaoborus larvae; Juvenile roach; Acoustic size discrimination; Integrator output; Use of Bertalanffy function; Insecta; Interspecific comparison; Acoustic sounder; Acoustic method; Young animal; Acoustic detection; Vertebrata; Chaoborus; Spatial distribution; Chaoboridae; Larva; Arthropoda; Pisces; Body size; Echointegration; Invertebrata; Diptera
• ISSN: 0165-7836; 1872-6763
• DOI: 10.1016/S0165-7836(98)00065-4

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,
s
a) due to one size class of targets is plotted vs. decreasing
s
v-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
s
a value for the larger targets (
s
a-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
s
a-large from the
s
a 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 m
2 plankton net of 0.5 mm mesh size. The
s
a 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.