Estimation of Bluefin Tuna (Thunnus thynnus) mean length in sea cages by acoustical means.

• Estimation of Bluefin Tuna mean length in sea cages has been performed using underwater acoustic tools. • The combined use of optical and acoustical techniques has revealed a strong relationship between TS and ABFT length measured in fattening cages. • Good agreement was found between TS and the fish fork (without the need to know the tuna's tilt angle), even considering uncompensated TS variable. This paper proposes an indirect method to estimate Bluefin Tuna (Thunnus thynnus) biomass in cages using acoustic techniques. Two Simrad EK60 echosounders working at 120 and 200 kHz and a stereo camera were used to obtain target strength (TS) to fork length (FL) relationships for both operating frequencies. The equipment was placed at the bottom of a floating cage, facing towards the surface to record the ventral aspect of fish. The acoustic and optical recordings were automatically analysed and the combination of acoustic and optical results provided unequivocal TS-FL assignments. Good relationships between TS and FL were obtained for both frequencies even without discriminating data from different fish tilts and without using beam directivity compensation. Stronger correlations were obtained for compensated TS at 200 kHz when reduced tuna swimming tilt was considered. TS measurements were compared to MFS numerical predictions for a Bluefin swimbladder model, with the simulation results showing good agreement with experimental measurements. The results allow the mean tuna length to be predicted in growing or fattening cages from acoustic data raising the possibility of improving production management and of providing a useful tool for catch control estimations made by international organisations (like ICCAT) and government bodies. [ABSTRACT FROM AUTHOR]