Size, distribution, and abundance of pelagic fish by deconvolution of single-beam acoustic data
We analyzed single-beam acoustic data using a deconvolving filter technique. Our purpose was to assess the ability of our acoustic methodology to estimate the size, distribution, and abundance of pclagic fish. Acoustic data were collected on Trout Lake, Vilas County, Wisconsin, USA, in 1983 and 1985 using a single-beam 70 kHz Simrad EY-M echo sounder. The data were recorded on cassette tapes and sub-sequently processed in the laboratory using a microcomputer-controlled sonar echo processor. The beam-pattern effect was removed using a deconvolving filter technique, and backscattering cross-section was estimated by fitting Rician PDFs to the scattering data. Fish lengths estimated using Love’s method ranged from 44 mm to 426 mm, exceeding the range of fish lengths captured in gillnets. Within the size range where the two gear types overlapped, the estimates of fish length matched. High fish densities produced overlapping echoes at some depths. The shape of the echo-peak PDF was used as an indicator of overlapping echoes, and blocks of data with overlapping echoes were excluded from the data to be deconvolved. The linearity of integrated echo square with respect to fish density was used to estimate fish density in the regions with overlapping echoes. Fish densities of up to 105 individuals/1000 m3 were observed. Transects repeated on the same night gave abundance estimates that differed by less than 6 \%, while perpendicular transects differed by 52 \%. Our acoustic estimates of total pelagic fish abundance declined by 54 \% from 1983 to 1985. Gillnet catch per unit effort declined by 54\% during the same period.