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

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 order to monitor its growth in sea cages. Five classes of commercial size gilthead sea bream are characterized, comprising lengths from 20 to 25 cm, corresponding to weights between 160 and 270 g. A few specimens were introduced into a sea cage of 3 m in diameter and a height of 2.7 m. We measure TS directly using a Simrad EK60 echosounder with a 7^{\circ} split-beam transducer working at 200 kHz. The transducer was located in the center of the cage during measurements, at the bottom facing upwards for ventral recordings and on the surface facing downwards to perform dorsal recordings. Two analyses based on single echo detection were performed: the first one obtains compensated transducer directivity TS values from intensity and angular echosounder data; while the second one omit phase information, affording uncompensated TS values (TSu). Two algorithms have been applied to analyze single-beam-like data, that differ in the order in which threshold and echo length criteria are applied to detect single echoes. TS distributions obtained from the split-beam analysis are unimodal for both ventral and dorsal measurements, like TSu distributions obtained when the threshold criterion is applied first. A lineal relationship was found between mean TS for the ventral aspect and logarithm of total body length of fish, showing good correlations for both TS and TSu, even having a few detections. Note that the relationship between TSu and total body length allows monitoring the growth of gilthead sea bream using low cost equipment like single beam echosounders.