A study by a team of researchers from the California Academy of Sciences, Nekton and the universities of São Paulo, Oxford and Exeter has found that large pieces of plastic waste are now present in almost all coral reefs across the world.

The study, which was published in the journal Nature, surveyed 84 shallow and deep reefs at 25 locations across the Pacific, Atlantic and Indian oceans for human-made debris that is more than 5 cm in size.

Debris was found at 77 of the 84 reefs, including at near-pristine locations such as unhabituated atolls in the central Pacific. Most of the debris comprised plastic and almost three-quarters of items identified were fishing gear.

The researchers found that deeper reefs were more polluted than shallow ones.

“Plastic pollution is one of the most pressing problems plaguing ocean ecosystems, and coral reefs are no exception. From plastics that spread coral diseases, to fishing lines that entangle and damage the structural complexity of the reef, reducing both fish abundance and diversity, pollution negatively impacts the entire coral reef ecosystem,” explains University of São Paulo PhD student and California Academy of Sciences research fellow Hudson Pinheiro.

California Academy of Sciences Hope for Reefs initiative codirector Luiz Rocha says it was surprising to find that debris increased with depth, since deeper reefs are generally farther from the sources of plastic pollution. It puts the effect we have had on the planet into perspective.”

Comoros, an island chain off the south-east coast of Africa, had the highest density of pollution, with nearly 84 500 items per square kilometre. Reefs off the Philippines and Brazil were also highly polluted.

The only plastic-free reefs recorded in the study were in the Seychelles Outer Islands – but even here the researchers spotted plastic outside surveyed areas.

Commenting on the scale of debris in deep reefs, meaning between 30 m and 150 m deep, California Academy of Sciences Steinhart aquarium director Bart Shepherd adds these are “not a refuge for shallow reef species in a changing climate as we once thought”.

“Rather, these reefs face many of the same pressures from human society as shallow reefs, and have a unique and poorly studied fauna. We need to protect deeper reefs and make sure that they are included in the conservation conversation.”

The researchers found consumer debris such as water bottles and food wrappers, but these were not the main form of plastic pollution at reefs.

Conservation charity Nekton principal scientist and University of Exeter Associate Professor Lucy Woodall states the large amount of fishing gear at deep reefs can entangle marine animals in so-called “ghost fishing” and that fishing gear debris is often not reduced by general waste management interventions.

“Therefore, specific solutions related to the needs of fishers should be considered, such as no-charge disposing of damaged gear in ports, or individually labelling gear to ensure fishers take responsibility for misplaced equipment,” she suggests.

Moreover, the researchers found that reef pollution increased near densely populated cities, local markets, and – counterintuitively – marine protected areas.

Since most marine protected areas allow some fishing within or near their borders and are typically rich in marine life, they are often heavily frequented by fishers.

The researchers stress the need to expand the depth of marine protected areas to include deep reefs.

“The results of our global study shine a light on one of the many threats that deep reefs face today. Because these ecosystems are ecologically and biologically unique, much like their shallow-water cousins, they need to be conserved and explicitly considered in management plans,” Nekton research scientist Paris Stefanoudis notes.

Shepherd adds that, despite the disturbing overall trend, there were some places where the researchers found relatively little debris, which shows us that there are effective strategies for preventing plastic pollution.

“If we act fast and employ science-based solutions, there absolutely is hope for coral reefs,” the researchers agree.