Vinciguerria nimbaria (micronekton), environment and tuna: Their relationships in the eastern Tropical Atlantic

Micronekton is a major component of oceanic tuna diet. Within micronekton species, Vinciguerria nimbaria, was found to constitute the main forage fish for tuna in the [10-20°W, 0-5°N] area where a large seasonal tuna fishery occurs. The relationships linking the Vinciguerria abundance, its spatial distribution and behaviour to its dynamical or biological environment, 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°S-4°N transect was sailed nine times back and forth along 15° W. Micronekton and Vinciguerria were acoustically surveyed, hydrological conditions sampled, phytoplankton and zooplankton biomasses measured. South of 0°30' N, a marked divergence (upwelling) was found, with high abundance of zooplankton and micronekton. From there to 4°N a stable situation occurred with a well mixed surface layer, a strong Deep Chlorophyll Maximum (DCM) but less zooplankton and micronekton. While most micronekton performed large diel vertical migrations, schools of Vinciguerria remained at the surface by day in the stable zone, therefore becoming vulnerable to tuna in contrast to the upwelling area. It is concluded that Vinciguerria fits its behaviour according to zooplankton abundance, having to spend more time in the surface layer for feeding in poor areas. More precisely, they remain during the day near the strong DCM where they find aggregated zooplankton, and they become available for tuna. This could explain why a rather poor area may hold and sustain a high biomass of tuna. (C) 2000 Ifremer/CNRS/IRD/Editions scientifiques et medicales Elsevier SAS.