Comparing acoustic model predictions to in situ backscatter measurements of fish with dual-chambered swimbladders

Lavnun Mirogrex terraesanctae have a dual-chambered swimbladder and are the dominant fish species in Lake Kinneret, Israel. Pi-monthly acoustic assess ments are used to monitor lavnun abundance but the relation between the amo unt of reflected sound and organism morphology is not well described. Predi ctions from Kirchhoff-ray mode (KRM) backscatter models show a sensitivity of echo amplitude to fish length and fish aspect. Predicted mean KRM target strengths matched maximum in situ target strength measurements of eight te thered fish within 2.5 dB at 120 kHz and within 7 dB at 420 kHz. Tilt and r oll of lavnun during tethered measurements increased variance of backscatte r measurements. Accurate abundance and length frequency distribution estima tes cannot be obtained from in situ acoustic measurements without supplemen tary net samples. (C) 2000 The Fisheries Society of the British Isles.