The aim of this study was to find the location of a breakpoint in the weight–length relationship using a linearized biexponential model (LinBiExp). The analysis was performed using weight and length measurements of bleak (Alburnus alburnus) individuals captured in freshwaters of the Province of Ferrara (Italy). The LinBiExp function identified the breakpoint coordinates in the data set. The level of uncertainty in the breakpoint coordinates was analysed using bootstrap regression. Bilinearity was verified by comparing slopes and intercepts (ANCOVA) of the two linear sections. The study confirmed that LinBiExp can be a valuable tool for detecting the coordinates of a breakpoint in weight–length data.