Target strength measurements of Atlantic bluefin tuna (Thunnus thynnus) from a live-bait fishing vessel in the Bay of Biscay

Despite its significant economic importance and intensive exploitation worldwide, the Atlantic bluefin tuna (ABFT) lacks the essential parameters required to estimate its abundance using acoustic methods at the most used 38 kHz frequency. This paper presents in situ measurements of target strength (TS; dB re 1 m²) for ABFT in dorsal aspect obtained from a baitboat in the Bay of Biscay, using a 38-kHz split-beam echosounder. A linear regression model was fit between TS and fork length (FL; cm) based on data from nine live-bait fishing operations. To mitigate potential bias due to double counting, TS measurements were filtered with fish tracking. The range of modal TS values per set was from −38 to −17 dB for modal tuna body fork lengths between 55 and 150 cm, that is, a rather thorough range of sizes including ABFT juveniles and adults. The linear regression resulted in a TS-length relationship characterised by a slope of 36.9 dB and an intercept of −101.6 dB, with a coefficient of determination R² = 87 %. When the slope of the TS-length relationship was forced to 20, the intercept (b₂₀) was −68.5 dB (R² = 79.5 %). The acoustic parameters presented in this work are key to providing an alternative to the uncertainties associated with the traditionally used fishery-dependent abundance indices of ABFT in the Bay of Biscay and abroad.