For a brief moment at the start of their lives, fish from different ocean ecosystems live side by side in gigantic fish nurseries, where surface waters converge and the prey is abundant.

Prey isn’t the only thing that’s abundant here. According to a paper published in PNAS this week, the same currents that make these regions appealing as nurseries mean that they’re awash with plastics. The consequences for commercial fisheries and the ocean's food webs are difficult to discern but could be significant.

Fish nurseries

Oceanographer Jamison Gove and his colleagues set out to understand more about how the features of the ocean affect the survival of larval fish—crucial information for the world’s fisheries. They didn’t expect to find a soup of microplastics in what looked like clear water.

The researchers focused on “surface slicks,“ which are long lines of smooth water on the ocean’s surface, formed by the convergence of oceans below the ocean’s surface. They look a bit like ribbons and accumulate the stuff that’s drifting about in the ocean, including plankton and larval fish.

To get a sense of what’s piling up in surface slicks, Gove and his colleagues trawled large nets through the surface water in slicks off the coast of Hawai’i Island. They found an incredible abundance of both plankton and larval fish compared to surrounding waters.

The slicks covered only around eight percent of the ocean's surface in a large coastal region off Hawai'i. Extrapolating from their findings, however, the researchers suggest that the slicks were home to nearly half of the region's larval fish. The larval fish in the slicks were bigger, and also stronger swimmers, compared to their non-slick neighbors.

Awash with plastics

Unfortunately, it's not only plankton and larval fish that end up in the slicks; microplastics find their way there, too. Much like the subtropical gyres that sweep floating plastic debris into huge plastic “islands” like the Great Pacific Garbage Patch, the slicks accumulate plastic on a more local level.

Gove and his colleagues found that there was 126 times as much plastic in the slicks compared to the surrounding ocean. Most of the plastic fragments were less than 5mm in length, and the majority of it was the kind of plastic used in single-use items like plastic bags and water bottles, as well as plastics from fishing equipment. Plastic particles outnumbered the larval fish seven to one in the slicks the researchers studied.

Extrapolating from the numbers in this study to a much larger area, Gove and colleagues suggest that 92% of the floating plastic in the region could be found in surface slicks. “To put this into context,” the researchers write, “plastic densities in slicks along West Hawai’i were 8.0- and 12.7-fold higher than the respective plastic densities recently sampled in the Great Pacific Garbage Patch.”

Plastic all the way up the chain

For larval fish, this is a potential crisis. These fish may be only a millimeter or two in length; to them, a piece of plastic of less than 1mm long is prey-sized and can look a lot like prey, too—especially the blueish threads of polyester and nylon microfibers. When the researchers dissected 658 of the bigger larval fish, they found that 8.6% of the ones found in slicks had eaten plastic. This was a much higher proportion than in surrounding non-slick waters.

At this crucial stage of development, nutrition is vital, much the same way that nutrition is vital for a human newborn. Most larval fish don't make it to adulthood; eating nutritionally void—and possibly toxic—plastic at this stage could substantially reduce the number of fish making it beyond the larval stage.

Because these nurseries are home to fish from coral reefs, the open ocean, and the ocean deeps, it also means that ingested plastics scatter from the ocean slicks to join the food webs in a range of ecosystems. Some of these larval fish are commercially important species, like swordfish. Some of them are vital prey for animals like tuna and seabirds. This means that as larval fish leave their nurseries to join the food webs in their various ecosystems, they’ll be taking their early-life plastic diet with them to pass up the food chain. And if their poor diet affects their survival rates, that effect will spin out across the oceans.

PNAS, 2018. DOI: 10.1073/pnas.1907496116 (About DOIs).