Hydroacoustic target strength vs. fish length revisited: Data of caged, free-swimming European whitefish (Coregonus lavaretus L.) suggest a bi-phasic linear relationship under a limited range of tilt angles

Abstract

To develop specific target strength (TS) - fish length relationships using 38 kHz and 120 kHz split beam echosounders, we measured TS (dB) in five size groups (5−59 cm total length) of European whitefish (Coregonus lavaretus) swimming freely in a net cage of 4 m diameter and 8 m height. For 120 kHz, we deployed down-looking (dorsal aspect) as well as upwards-looking (ventral aspect) transducers, whereas 38 kHz measurements were taken for the dorsal aspect only. The ventral aspect dataset revealed stronger TS values especially for the larger length classes. The dorsal aspect datasets of both frequencies were not significantly different across all lengths and both showed a steep increase of the relationship in small sizes (5−20 cm) which levelled off in the larger sizes (>20 cm). For 120 kHz the slope of a linear regression of TS vs. log fish length was 29.8 for fish between 5−20 cm (much higher than the generally proposed slope of 20), while in fish >20 cm the slope was 10.7 which is far below the generally proposed value. We advocate considering a potential bi-phasic linear regression of the log length – TS relationship for all species whenever a large size range of fish is studied, encompassing significant proportions below and above approximately 20 cm total length.