Small Scale Vertical Behaviour of Juvenile Albacore in Relation to Their Biotic Environment in the Bay of Biscay

The goal of the present study is to analyze the small scale vertical behaviour of juvenile albacore tuna (Thunnus alalunga) in relation to the abundance and distribution of their main prey, which has particular importance regarding catchability by surface fishing gears, such as trolling. A total of six juvenile albacore were tracked in the south east Bay of Biscay in July and August 2005, using ultrasonic transmitters. Two echosounders working at 38 and 120 kHz on the tracking vessel were used to collect data on the biotic environment (krill, small pelagic fish and planktonic layers) between the surface and 200 m depth. These data were echo-integrated in order to relate tuna vertical movements to food availability. The stomach contents of 97 albacore caught during the surveys were analyzed, the comparison of prey occurrences respectively in the stomachs and on the echograms showed selectivity for blue whiting. However, the biotic factors considered in this study had no significant influence on the depth of albacore, which possibly feed during night-time in surface waters. The tracked albacore had a shallow depth distribution and did not exhibit any regular deep-diving behaviour. A significant effect of time of day and body size on albacore depth was shown, all fish remaining deeper during daytime, and smaller fish having a shallower vertical distribution.