Future physical and chemical changes to the ocean are likely to significantly affect the distribution and productivity of many marine species. Tuna are of particular importance in the tropical Pacific, as they contribute significantly to the livelihoods, food and economic security of island states. Changes in water properties and circulation will impact on tuna larval dispersal, preferred habitat distributions and the trophic systems that support tuna populations throughout the region. Using recent observations and ocean projections from the CMIP3 and preliminary results from CMIP5 climate models, we document the projected changes to ocean temperature, salinity, stratification and circulation most relevant to distributions of tuna. Under a business-as-usual emission scenario, projections indicate a surface intensified warming in the upper 400 m and a large expansion of the western Pacific Warm Pool, with most surface waters of the central and western equatorial Pacific reaching temperatures warmer than 29 °C by 2100. These changes are likely to alter the preferred habitat of tuna, based on present-day thermal tolerances, and in turn the distribution of spawning and foraging grounds. Large-scale shoaling of the mixed layer and increases in stratification are expected to impact nutrient provision to the biologically active layer, with flow-on trophic effects on the micronekton. Several oceanic currents are projected to change, including a strengthened upper equatorial undercurrent, which could modify the supply of bioavailable iron to the eastern Pacific.
Warm Pool Euphotic Zone Cold Tongue Online Figure South Pacific Convergence Zone
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Most of the analyses were based on the international modeling groups participating in IPCC AR4 and AR5, the Program for Climate Model Diagnosis and intercomparison (PCMDI). The IPCC Data Archive: allowed efficient analysis of the massive numerical model outputs. Much of the work described in this paper was commissioned by the Secretariat of the Pacific Community to help assess the vulnerability of fisheries and aquaculture in the tropical Pacific to climate change. J. Lefèvre’s technical help was most valuable. We thank Johann Bell for his guidance on how to present the analyses to make them easy for fisheries scientists to use. K. Richard and N. Holbrook also provided valuable feedback at early stages of this work.
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