Seasonal and interannual variability of fat content of juvenile albacore (Thunnus alalunga) and bluefin (Thunnus thynnus) tunas during their feeding migration to the Bay of Biscay

The fat content of 2945 juvenile albacore and 618 juvenile bluefin tunas caught in the Bay of Biscay was measured. Individuals were caught in 2004, 2005, 2006 and 2007 from June to early November by pelagic trawling, trolling and baitboat gears. The results for the two species show different seasonal trends. The fat content of albacore tuna increased linearly throughout the fishing season, which reflects their feeding migration. The seasonal trend of bluefin tuna showed a minimum in early August, which may be related to a different behaviour, physiology or feeding strategy. An interannual increase of fat content was observed in albacore tuna and in age-2 to age-5+ bluefin tuna, which is possibly related to a density-dependence phenomenon. The seasonal increase of fat content was strongest and appeared in the four years studied for age-3 and age-4 albacore tuna, which can be related to a different vertical habitat or a more efficient use of their ecological niche by the individuals of these age-groups, relatively to the younger age-groups. Condition factor and girth/length ratio do not appear to be relevant indicators of fat content.