Immense but not immutable: A sudden and sustained decline in deep-pelagic life in a large oceanic ecosystem.

The deep-pelagic habitat of the Gulf of Mexico comprises 90.4% of its volume and houses an exceptionally diverse deep-pelagic animal community that provides numerous ecosystem services, including carbon cycling and prey provisioning for oceanic predators. Here, we report sudden, severe, and persistent declines in the abundance of deep-pelagic fishes, crustaceans, and cephalopods that have persisted for over a decade. Over a 3.5-y period, animal abundances declined by 60 to 85%, taxon depending. Particularly hard-hit were ecologically foundational taxa such as euphausiids and lanternfishes. The magnitude of population-level decline that occurred during this 3.5-y period was greater than any observed over a 60-y period in the California Current system, the only comparable dataset, making this the fastest and most severe faunal decline reported for Earth's largest ecosystem type. These declines were detected both via direct sampling and acoustic sensing, providing independent corroboration. Genomic analyses indicate reductions in genetic diversity among dominant taxa, both a corroboration and a consequence of the decline. Furthermore, resource managers report that these trends parallel those of managed or protected Gulf species that are dependent on deep-pelagic prey, including endangered species (e.g., beaked whales, Rice's whale). No naturally occurring factors (e.g., temperature, oxygen, food supply) correlated with the observed declines, suggesting an anthropogenic proximate cause. That the declines began shortly after the Deepwater Horizon oil spill points to that event as the most parsimonious explanation. These findings suggest that despite its immense size the deep-pelagic biome is vulnerable to human-induced disturbance and possesses limited resilience to facilitate recovery.