Predicting skipjack tuna forage distributions in the equatorial Paci®c using a coupled dynamical bio-geochemical model PATRICK LEHODEY, 1,* JEAN-MICHEL ANDRE, 2 MICHEL BERTIGNAC, 1 JOHN HAMPTON, 1 ANNE STOENS, 3 CHRISTOPHE MENKES, 3 LAURENT MEMERY 3 AND NICOLAS GRIMA 3 1 Secretariat of the Paci®c Community, PO Box D5, 98848 Noumea cedex, New Caledonia 2 ORSTOM, Noumea, New Caledonia 3 LODYC (UniversiteÂParis 6, ORSTOM, CNRS, IFREMER), Paris, France ABSTRACT Skipjack tuna (Katsuwonus pelamis) contributes »70% of the total tuna catch in the Paci®c 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 Paci®c. Analysis of catch and effort data for US purse seine ®sheries in the western Paci®c has demonstrated that one of the most successful ®shing grounds is located in the vicinity of a convergence zone between the warm (>28±29°C) low-salinity water of the warmpool and the cold saline water of equatorial upwelling in the central Paci®c (Lehodey et al., 1997). This zone of convergence, identi®ed by a well-marked salinity front and approximated by the 28.5°C isotherm, oscillates zonally over several thousands of km in correlation with the El Nin Ä o±Southern Oscillation. The present study focuses on the prediction of skipjack tuna forage that is expected to be a major factor in explaining the basin-scale distribution of the stock. It could also ex- plain the close relation between displacements of skipjack tuna and the convergence zone on the eastern edge of the warmpool. A simple bio-geochemical model was coupled with a general circulation model, allowing reasonable predictions of new primary pro- duction in the equatorial Paci®c from mid-1992 to mid-1995. The biological transfer of this production toward tuna forage was simply parameterized according to the food chain length and redistributed by the currents using the circulation model. Tuna forage ac- cumulated in the convergence zone of the horizontal currents, which corresponds to the warmpool/equato- rial upwelling boundary. Predicted forage maxima corresponded well with high catch rates. Key words: advection, convergence, coupled models, equatorial upwelling, Katsuwonus pelamis, Paci®c, secondary production, skipjack, tuna, warmpool INTRODUCTION Since 1991, the annual catch of tuna and tuna-like species in the Paci®c Ocean has approached 70% of the global catch of these species (Shomura et al., 1994). Skipjack tuna (Katsuwonus pelamis) is the most important species in terms of catch weight and more than 90% of the Paci®c skipjack tuna catch is taken west of 150°W longitude within the so-called `warm- pool' (Lehodey et al., 1997). Despite large catches (almost 1 million tonnes, SPC, 1995), skipjack tuna exploitation rates are estimated to be low to moderate (Kleiber et al., 1987), and the western Paci®c stock is believed to be capable of sustaining even higher catches. This high productivity is sustained by a high growth rate (Wild and Hampton, 1994), rapid turn- over (Kleiber et al., 1987) and early onset of repro- ductive maturity (»1 year, »45 cm fork length). Such a rapid development demands very high energetic re- quirements. Several studies (e.g. Kitchell et al., 1978) suggest that skipjack tuna consume 10±15% of their body weight each day. Thus, being opportunistic feeders, skipjack tuna would be expected to be con- centrated in areas where suitable food is abundant. As direct observations of zooplankton and micronekton distributions at the scale of the region are not avail- able, it is worth attempting to test this hypothesis through a modelling approach. Skipjack tuna are known to consume epipelagic prey items ranging from larger zooplankton, such as euphausiids, amphipods and other small crustaceans, to bait®sh, such as the oceanic anchovy Stolephorus buccanneeri (Hida, 1973, later called Encrasicholina punctifer). Ideally the simu- lation of skipjack tuna forage would integrate the * Correspondence. Fax (687) 26 38 18. e-mail: PatrickL@spc.org.nc Received for publication 5 April 1998 Accepted for publication 3 June 1998 FISHERIES OCEANOGRAPHY Fish. Oceanogr. 7:3/4, 317±325, 1998 Ó 1998 Blackwell Science Ltd. 317