Predicting skipjack tuna forage distributions in the equatorial Pacific using a coupled dynamical bio-geochemical model

Skipjack tuna (Katsuwonus pelamis) contributes approximate to 70% of the to tal tuna catch in the Pacific Ocean. This species occurs in the upper mixed -layer throughout the equatorial region, but the largest catches are taken from the warmpool in the western equatorial Pacific. Analysis of catch and effort data for US purse seine fisheries in the western Pacific has demonst rated that one of the most successful fishing grounds is located in the vic inity of a convergence zone between the warm (>28-29 degrees C) low-salinit y water of the warmpool and the cold saline water of equatorial upwelling i n the central Pacific (Lehodey et al., 1997). This zone of convergence, ide ntified by a well-marked salinity front and approximated by the 28.5 degree s C isotherm, oscillates zonally over several thousands of km in correlatio n with the El Nino-Southern Oscillation. The present study focuses on the p rediction of skipjack tuna forage that is expected to be a major factor in explaining the basin-scale distribution of the stock. It could also explain the close relation between displacements of skipjack tuna and the converge nce zone on the eastern edge of the warmpool. A simple bio-geochemical mode l was coupled with a general circulation model, allowing reasonable predict ions of new primary production in the equatorial Pacific from mid-1992 to m id-1995. The biological transfer of this production toward tuna forage was simply parameterized according to the food chain length and redistributed b y the currents using the circulation model. Tuna forage accumulated in the convergence zone of the horizontal currents, which corresponds to the warmp ool/equatorial upwelling boundary. Predicted forage maxima corresponded wel l with high catch rates.