LAUSR

Abstract Using scientific echosounders, we investigate the distribution and in situ target strength (TS, dB) of skipjack tuna Katsuwonus pelamis and juvenile yellowfin tuna Thunnus albacares associated with light fish-aggregating devices in the South China Sea. We analyze the volume backscattering strength (SV, dB), TS, and density (the number of fish in the effective reverberation volume, fish/sampling volume) of fish. In natural conditions (without light), tuna occur at depths exceeding 50 m. After luring lamps are turned on, SV and TS increase, especially from 21 to 50 m, with SV manifesting time and depth-related differences. Skipjack and yellowfin tunas occur mainly from 31 to 50 m between 24.0 °C and 28.0 °C, and 21–30 m between 28.2 °C and 28.7 °C, respectively. From 18:30 to 21:30, mean TS tends to increase with density when density exceeds a cut-off value, indicating that overlapping echoes contribute to increased TS. From 21:30 to 22:00, there was no significant relationship between increased mean TS and density. Mean TS values for skipjack tuna of mean fork length 36.1 cm are −50.77 dB at 70 kHz and −52.29 dB at 120 kHz. Mean TS values of yellowfin tuna of mean fork length 41.2 cm are −38.01 dB at 70 kHz and −38.30 dB at 120 kHz. These results can be used to estimate tuna abundance, and monitor and identify their behavior.