This report gives the findings of a study commissioned by WWF to estimate the carbon footprint of tuna fisheries. Carbon footprint, which indicates the carbon dioxide emissions generated by a product system or supply chain per unit of output on a life cycle basis, provides a quantitative index of potential adverse impacts with respect to climate change. The carbon footprint can be computed using extended Leontief input-output models based on “top-down” or “bottom-up” data. Initial attempts using top-down data appear to underestimate the total carbon footprint. The initial estimates were revised using activity- and process-based (“bottom-up”) data for different types of fishing gear, as well as various alternative scenarios for downstream processing and logistics. One of the main findings is that fishery fleet fuel consumption is typically the largest contributor to overall carbon footprint. Purse seine fishing gives the lowest carbon footprint per kg of landed catch, while long line gear has the largest footprint. Furthermore, the footprint figures are highly sensitive to yields, which then implies that carbon footprint reduction is compatible with increasing fishery profitability. The other major contributors to the overall carbon footprint are cannery operations and transportation by air. Cold storage, on the other hand, has a relatively minor contribution.