Target strength (TS) as a function of fish length (L) must be known to estimate fish density in acoustic survey. This TS-length function may be estimated by comparing in situ acoustic measurements with the fish size composition from trawl catches and should be accompanied with accuracy indices characterizing the uncertainty of its parameters. We show that the bootstrap–method is a highly efficient method of estimating the parameters of TS-length function and emphasize that calculation results depend on the adopted mapping rule of fishing data into acoustic data. The bootstrap methods has been applied to data from icefish measurements. The TS-length functions based on full probability density function (PDFs) of fish length and target strength at 38 kHz and truncated PDFs are presented. We reveal that it is probably necessary to use different regression equations for different length ranges.