Despite holding most of the global fish biomass, the open ocean twilight zone (200-1,000 m) is poorly understood due to the difficulty of measuring subsurface ecosystem processes at scale. We demonstrate that a wide-ranging carnivore - the northern elephant seal - serves as an ecosystem sentinel for the open ocean. We linked ocean-basin-scale foraging success with oceanographic indices to estimate twilight zone fish abundance five decades into the past, and into the future. We discovered that small variation in seal foraging success amplifies into large demographic changes in offspring body mass, first-year survival, and recruitment. Further, worsening oceanographic conditions could shift predator population trajectories from current growth to sharp declines. As ocean integrators, wide-ranging predators can reveal the impacts of past and future anthropogenic change on open ocean ecosystems.

Funding provided by: National Science FoundationROR ID: https://ror.org/021nxhr62Award Number: