Climate change and fisheries are transforming the oceans, but we lack a complete understanding of their ecological impact [1; 2 ; 3]. Environmental degradation can cause maladaptive habitat selection, inducing ecological traps with profound consequences for biodiversity [4; 5 ; 6]. However, whether ecological traps operate in marine systems is unclear . Large marine vertebrates may be vulnerable to ecological traps , but their broad-scale movements and complex life histories obscure the population-level consequences of habitat selection [8 ; 9]. We satellite tracked postnatal dispersal in African penguins (Spheniscus demersus) from eight sites across their breeding range to test whether they have become ecologically trapped in the degraded Benguela ecosystem. Bayesian state-space and habitat models show that penguins traversed thousands of square kilometers to areas of low sea surface temperatures (14.5°C–17.5°C) and high chlorophyll-a (∼11 mg m−3). These were once reliable cues for prey-rich waters, but climate change and industrial fishing have depleted forage fish stocks in this system [ 10 ; 11]. Juvenile penguin survival is low in populations selecting degraded areas, and Bayesian projection models suggest that breeding numbers are ∼50% lower than if non-impacted habitats were used, revealing the extent and effect of a marine ecological trap for the first time. These cascading impacts of localized forage fish depletion—unobserved in studies on adults—were only elucidated via broad-scale movement and demographic data on juveniles. Our results support suspending fishing when prey biomass drops below critical thresholds [ 12 ; 13] and suggest that mitigation of marine ecological traps will require matching conservation action to the scale of ecological processes .