A recent study has revealed a significant decline in marine life within the Gulf of Mexico’s “twilight zone” in the years following the 2010 Deepwater Horizon oil spill. The research, published Monday in the journal Proceedings of the National Academy of Sciences (PNAS), found that populations of fish, crustaceans, and larger animals such as squid and octopuses fell by approximately 60% in the area situated between the ocean’s surface and depths of up to 5,000 feet (1,500 meters).

The investigation focused on the midwater zone above the seafloor, a critical and largely underexplored ecosystem that supports a variety of species, including those commonly preyed upon by marine mammals like dolphins and whales. Using consistent sampling methods—including net collections at various depths and acoustic sonar—the study compared data collected in 2011, one year after the deadly rig explosion, with data from 2015 and subsequent years. Results showed fish numbers decreased by 63.5%, crustaceans such as shrimp and krill dropped 62%, and larger species declined nearly 60%. These trends persisted through 2022 and 2023, with only marginal, statistically insignificant increases.

Lead author Tracey Sutton, a biological oceanographer at Nova Southeastern University, indicated that the researchers ruled out natural variables such as temperature, oxygen levels, and food availability as causes for the decline. The timing and scale of the drop aligned most closely with the Deepwater Horizon spill, which released approximately 134 million gallons of oil due to the failure of the BP-operated well. Sutton said the damage likely resulted not only from immediate mortality following the explosion but also from lasting impacts on surviving populations, as pollutants accumulated in the eggs of affected marine species and compromised subsequent generations.

The study also highlighted the distinct but complementary harms caused by both the spilled oil and the chemical dispersants used in the cleanup efforts. Co-author Isabel Romero, a chemical oceanographer at the University of South Carolina, explained that while oil presents a direct toxic threat, dispersants fragment the oil into smaller droplets, increasing exposure for animals inhabiting deeper waters. Former National Science Foundation Director Rita Colwell, who led a decade-long research initiative funded by BP to study the spill’s effects, noted that dispersants have caused significant issues and recommended future spill responses prioritize substances that boost oil-degrading microbes instead.

BP acknowledged the spill’s enduring legacy, with spokesperson Paul Takahashi stating the company has implemented numerous changes aimed at preventing similar disasters.

Additional findings underscored how integral species such as krill, lanternfish, and dragonfish are to the Gulf’s food web. These key species declined sharply—krill by 72%—contributing to broad alterations throughout the ecosystem. Sutton emphasized the interconnectedness of the Gulf’s ocean system, where many animals move vertically between depths, creating a dynamic environment rather than a simple oceanic layering.

Outside experts praised the study for shedding light on a previously under-investigated part of the ocean. Scripps Institution of Oceanography marine scientist Kaitlin Frasier, who was not involved with the study, remarked on the gulf’s poorly understood inner depths and cited her own research confirming severe population drops in whales and dolphins in the region. Dalhousie University marine scientist Boris Worm, also unaffiliated, described the long-lasting and extensive damage revealed, cautioning that such impacts have often gone unnoticed.

Both Worm and Colwell urged that the environmental consequences of Deepwater Horizon remain a critical consideration as global discussions on deep-sea mining and offshore drilling continue.