||ABSTRACT: We performed a global assessment of how fish biomass has changed over the last 100 yr, applying a previously developed methodology using ecological modeling. Our assessment built on more than 200 food web models representing marine ecosystems throughout the world covering the period from 1880 to 2007. All models were constructed based on the same approach, and have been previously documented. We spatially and temporally distributed fish biomasses delivered by these models based on fish habitat preferences, ecology, and feeding conditions. From these distributions, we extracted over 68000 estimates of biomass (for predatory and prey fishes separately, including trophic level of 3.5 or higher, and trophic level between 2.0 and 3.0, respectively), and predicted spatial-temporal trends in fish biomass using multiple regression. Our results predicted that the biomass of predatory fish in the world oceans has declined by two-thirds over the last 100 yr. This decline is accelerating, with 54% occurring in the last 40 yr. Results also showed that the biomass of prey fish has increased over the last 100 yr, likely as a consequence of predation release. These findings allowed us to predict that there will be fish in the future ocean, but the composition of fish assemblages will be very different from current ones, with small prey fish dominating. Our results show that the trophic structure of marine ecosystems has changed at a global scale, in a manner consistent with fishing down marine food webs.