In 1995, the SPC Oceanic Fisheries Programme (OFP) initiated the development of a Spatial Ecosystem and Population Dynamics Model (SEAPODYM), in which tuna stock distribution and abundance is modelled in relation to the biological characteristics of the stock and their interaction with environmental conditions (e.g., water temperature, currents, oxygen concentration, primary productivity, etc). The objective of this work was to better understand the impact of oceanographic variability, in particular that associated with ENSO, on tuna stocks and fisheries. The development of the model has continued at SPC, in collaboration with the University of Hawaii Pelagic Fisheries Research Program, and more recently, with Collecte Localisation Satellites (CLS), a subsidiary of the French CNES and IFREMER institutes. The current version of the model has reached a high degree of sophistication, with the main features being (i) forcing by environmental data (either observed or modelled); (ii) prediction of the temporal and spatial distribution of age-structured tuna populations; (iii) prediction of the total catch and size frequency of the catch by fishing fleet; and (iv) parameter optimization based on fishing data assimilation techniques. The model has been extensively described and published in the peer-reviewed scientific literature.

SEAPODYM has recently been used to investigate the potential impacts of global warming on tuna populations. Preliminary optimized parameterizations have been obtained for bigeye and skipjack tuna in the Pacific Ocean at a spatial resolution of 1 degree latitude by 2 degrees longitude, using historical catch data for the past 50 years. These models were then used to forecast the future of the bigeye and skipjack tuna populations in the Pacific Ocean using physical-biogeochemical data fields predicted from a global marine biogeochemistry – climate simulation. This global simulation was performed with the IPSL climate model version 4 (IPSL-CM4) coupled to the oceanic biogeochemical model PISCES, and forced by (i) atmospheric CO₂ from historical records over the period 1860-2000, and (ii) under the SRES A2 IPCC scenario for the 21^st Century (i.e. atmospheric CO₂ concentration reaching 850 ppm in the year 2100). The model therefore provides a replication of historical tuna fisheries data through the estimation of population dynamics parameters, and applies this model to predict outcomes under a plausible climate change scenario.

Preliminary modelling results for bigeye and skipjack tuna are shown in Figure 1. For both species, eastwards shifts of population density in the Pacific are predicted under the SRES A2 IPCC scenario. This shift is particularly strong in the case of bigeye tuna. The mechanisms involved in the shift appear to be related mainly to reduced equatorial upwelling and the resulting reduced infusion of primary productivity into the western tropical Pacific due to both reduced westward surface currents and reduced nutrient influx from the ocean depths.