Vinciguerria nimbaria (micronekton), environment and tuna: their relationships in the Eastern Tropical Atlantic

Micronekton is a major component of oceanic tuna diet. Within micronekton s pecies, Vinciguerria nimbaria, was found to constitute the main forage fish for tuna in the [10-20 degrees W, 0-5 degrees N] area where a large season al tuna fishery occurs. The relationships linking the Vinciguerria abundanc e, its spatial distribution and behaviour to its dynamical or biological en vironment, were investigated as part of the Picolo program, devoted to the study of the mechanisms leading to the high seasonal tuna concentration in that area. During the Picolo 1 cruise, in January-February 1997, the 1 degr ees S-4 degrees N transect was sailed nine times back and forth along 15 de grees W. Micronekton and Vinciguerria were acoustically surveyed, hydrologi cal conditions sampled, phytoplankton and zooplankton biomasses measured. S outh of 0 degrees 30 degrees N, a marked divergence (upwelling) was found, with high abundance of zooplankton and micronekton. From there to 4 degrees N a stable situation occurred with a well mixed surface layer, a strong De ep Chlorophyll Maximum (DCM) but less zooplankton and micronekton. While mo st micronekton pet-formed large diel vertical migrations, schools of Vincig uerria remained at the surface by day in the stable zone, therefore becomin g vulnerable to tuna in contrast to the upwelling area. It is concluded tha t Vinciguerria fits its behaviour according to zooplankton abundance, havin g to spend more time in the surface layer for feeding in poor areas. More p recisely, they remain during the day near the strong DCM where they find ag gregated zooplankton, and they become available for tuna. This could explai n why a rather poor area may hold and sustain a high biomass of tuna. (C) 2 000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.