Decade of Data Shows Rapid Decline in Pelagic Fish and Invertebrates in Deep Gulf

Following the Deepwater Horizon oil spill, scientists across the region jumped into action to monitor its impacts. One particular effort focused on pelagic nekton-the small critters like fish, shrimp, and cephalopods that swim freely in the ocean and serve as the base of pelagic food webs. The Deep-Pelagic Nekton Dynamics of the Gulf (DEEPEND) Consortium led this project to quantify long-term trends in the offshore fauna of the Gulf. While observed declines in these critical prey groups can't be tied directly to the spill, scientists have not identified another likely explanation.

Tracking the Shift: A Decade of Deepwater Science

The first pelagic nekton sampling effort took place as soon as possible following the spill, with sampling occurring almost continuously for 9 months in 2011. These data represent the benchmark of pelagic nekton to which past and future numbers can be compared. The numbers were also similar to data collected during the 1980s and 1990s.

The next sampling involved six efforts across four consecutive years (2015-2018), followed by three more in 2021-2023. Overall, these efforts comprise more than a decade of abundance and composition data for pelagic nekton in the Gulf of America.

During each of these events, the project team sampled discrete depths from 0 to 1,500 meters in the Gulf using a 10-square-meter Multiple Opening/Closing Net and Environmental Sensing System, or MOCNESS, with 3 millimeter mesh. Once net tows were complete, the science teams identified and counted each of the animals collected in each sample.

"We had few measurements of what was really going on in the deep open Gulf before the oil spill," recalled John Quinlan, fisheries biologist at the Southeast Fisheries Science Center and co-author of the paper. "This event really showed us where we lacked critical information. So scientists developed a program specifically designed to fill those gaps."

In addition to traditional net-sampling methods, the team also used echosounders to measure acoustic backscatter in the water column during the day and at night. This allowed them to independently estimate relative abundance of animals in fine spatial detail. Scientists also collected tissue samples from three dominant lanternfish species to monitor changes in genetic diversity.

Significant Declines for Significant Prey

By comparing the baseline data to abundances measured in 2015-2023, the team found severe declines across major faunal groups:

• All fish: 63.5 percent
• Lanternfish: 62.9 percent
• Bristlemouth fish: 72.8 percent
• All macrocrustaceans (krill, decapod shrimps, lophogastrids): 62 percent
• Krill: 72.8 percent
• All cephalopods: 59.7 percent

While these animals may be small in size, they are not insignificant. These fauna are the foundation of pelagic food webs-consuming smaller nekton and plankton and serving as prey for important Gulf predators such as tuna, billfish, oceanic dolphins, and many whale species. In fact, the decrease of deep-pelagic prey parallels observed decadal declines of up to 80 percent in deep-diving marine mammals (e.g., sperm whales , beaked whales , and Rice's whales ). This evidence may point to a system-wide ecological regime shift.

"This represents the largest faunal decline recorded for Earth's largest ecosystem type. These data show that despite their immense size, open-ocean ecosystems have become finely tuned over millennia with little disturbance, and thus are not equipped to handle sudden, large-scale changes. In essence, the deep-pelagic domain appears to be much more fragile than we had imagined," stated Tracey Sutton, the founder and chief scientist of the DEEPEND study team.

The acoustic data corroborates these findings, showing a significant decline in acoustic backscatter-a proxy of pelagic biomass. The greatest difference was observed in the epipelagic zone at night, where total backscatter declined by 40 percent across the study.

Not only did abundance decline, but genomic monitoring of important lanternfish species revealed significant reductions in genetic diversity. This could lead to population bottlenecks and reduced ecological resilience-further eroding prey availability for the Gulf's megafauna.

By examining numerous environmental variables, the team ruled out marine heatwaves, hypoxia, or food supply limitations as the cause of the decline. Since the declines followed the Deepwater Horizon oil spill, the authors suggest the spill as the most likely cause.

Read the paper in the Proceedings of the National Academy of Sciences >

Learn more about NOAA's role in Deepwater Horizon response and restoration

These efforts were possible due to funding from: