A high variability in fish’s aspect, causing strong target-strength (TS) fluctuations, was observed in nase Chondrostoma nasus (L.) during Upstream spawning migration through a horizontal, bottom-aligned beam in the Fischa River, a tributary of the Danube. Size estimates based on traditional mean target strength Underestimated the total length of nase. Applying maximum TS from side-aspect detections overestimated size due to signal amplifications of Lip to 10 dB in the single-echo detections of tracked fish. Based on Lilia’s simulations, the strong signal amplifications probably originated from flash-effects. Inhomogeneous sound-propagation due to cylindrical spreading and the maturity state of spawning nase probably contributed to signal amplification of Lip to 6dB. In order to find better size estimators than the mean and max size estimator, we tested percentile estimators, and we developed a new statistical method-the Maximum Excess Test and Maximum Isolation Test (MET-MIT). This method links information from catch data with the TS-distribution from tracked fish in order to exclude outliers. The results show that both the MET-MIT and the percentiles gave reasonably similar results and that both were better size estimators than the traditional mean-target-strength estimator (C) 2008 Elsevier B.V. All rights reserved.