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The aim of this study is to establish a relationship between target strength(TS) and total body length of the gilthead sea bream (Sparus aurata), in orderto monitor its growth in sea cages. Five classes of commercial size giltheadsea bream are characterized, comprising lengths from 20 to 25 cm, correspondingto weights between 160 and 270 g. A few specimens were introduced into a seacage of 3 m in diameter and a height of 2.7 m. We measure TS directly using aSimrad EK60 echosounder with a 7^{\circ} split-beam transducer working at 200kHz. The transducer was located in the center of the cage during measurements,at the bottom facing upwards for ventral recordings and on the surface facingdownwards to perform dorsal recordings. Two analyses based on single echodetection were performed: the first one obtains compensated transducerdirectivity TS values from intensity and angular echosounder data; while thesecond one omit phase information, affording uncompensated TS values (TSu). Twoalgorithms have been applied to analyze single-beam-like data, that differ inthe order in which threshold and echo length criteria are applied to detectsingle echoes. TS distributions obtained from the split-beam analysis areunimodal for both ventral and dorsal measurements, like TSu distributionsobtained when the threshold criterion is applied first. A lineal relationshipwas found between mean TS for the ventral aspect and logarithm of total bodylength of fish, showing good correlations for both TS and TSu, even having afew detections. Note that the relationship between TSu and total body lengthallows monitoring the growth of gilthead sea bream using low cost equipmentlike single beam echosounders.

Dorsal and ventral target strength measurements on gilthead sea bream (Sparus aurata) in sea cages