Tuna associated with Fish Aggregating Devices (FADs) often exhibit higher proportions of empty stomachs and lower condition factors. However, it remains unclear whether this lower condition is a cause or a consequence of FAD association and whether the FAD-associated tuna condition is good or bad, due to the lack of fundamental knowledge on tuna physiology. This study aimed to quantify the physiological condition of tuna during food deprivation. 25 captive tuna individuals (FL: 31-71 cm) were initially fed ad libitum for 2 weeks and then subject to a controlled fasting period of 36 days. During fasting, 3 to 7 individuals were sampled at regular intervals (every 3-6 days) to assess various condition factors, including biometric parameters (e.g., Le Cren condition factor (Kₙ), hepatosomatic index (HSI), and girth length index (GL)), bioelectrical impedance analysis (BIA), body composition, and blood parameters. The results indicate that tuna initially mobilized the energy stored in the liver and then in the muscles. Three distinct phases of fasting were identified: Phase 1 (high HSI and Kₙ during ad libitum feeding and the early fasting days), Phase 2 (low HSI and high Kₙ, from days 4 to 14), and Phase 3 (low HSI and low Kₙ after 14 days of fasting). When comparing these results with condition factors measured on wild tuna associated with drifting FADs, it appears that all FAD-associated tuna fall within Phases 1 and 2. These results suggest that FAD-associated tuna do not attain the low Kn values observed during extended periods of fasting.

Fabien Forget

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