Plenty of heartbreaking stories about turtles, seabirds and whales found dead with dozens of plastic bits in their stomachs have surfaced in recent years. But a new study reveals that it’s not just adult sea animals that are getting a gullet full of plastic. Larval fish are inundated with plastic fragments in their nursery habitats and they’re eating those pieces along with their natural food sources, according to the paper published in the journal PNAS.
The finding comes from a recent study looking at where baby fish spend their time. An international team of scientists joined up with NOAA's Pacific Islands Fisheries Science Center to study an ocean habitat called surface slicks, or long lines of smooth water found paralleling coastlines that are created when internal ocean waves converge.
To do that, the team used remote sensing data to identify slicks along the coast of Hawaii then used tow surveys to scoop up plankton and larval fish in them. They found that larval fish prefer to congregate in slicks, which have lots of tasty zooplankton.
The team found the slicks have more eight times as many larval fish as surrounding waters and act as de facto fish nurseries for the first few months of a fish's life cycle.
“We found that surface slicks contained larval fish from a wide range of ocean habitats, from shallow-water coral reefs to the open ocean and down into the deep sea—at no other point during their lives do these fish share an ocean habitat in this way,” says study co-author Jonathan Whitney, a NOAA marine ecologist, says in a press release. “Slick nurseries also concentrate lots of planktonic prey, and thereby provide an oasis of food that is critical for larval fish development and survival.”
Not only do surface slicks have lots of plankton, however, they also trap microplastics. The team found that microplastics were 126 times more concentrated in surface slicks than in the surrounding waters. In fact, there was seven times more plastic present than larval fish. Most of that plastic was smaller than one millimeter, which is the perfect size tiny fish to eat.
The study estimates that while the surface slicks make up less than 10 percent of the ocean surface habitat, they contain 42.3 percent of surface-dwelling larval fish and 91.8 percent of all floating plastic. (The team calculates, for example, that plastic concentration in surface slicks is eight times greater than the density of plastic found in the Great Pacific Garbage Patch, a debris-choked area of the Pacific Ocean known for its stew of microplastics.)
“We hadn’t anticipated how much plastics we would find,” study coauthor Jamison Gove, a NOAA oceanographer, tells Timothy Hurley at the Honolulu Star Advertiser. “Once we started sampling and finding a lot of plastics, there was no way to ignore them. The fact that larval fish are surrounded by and ingesting non-nutritious plastics, at their most vulnerable life stage, is certainly cause for alarm.”
Matt Simon at Wired reports that the team also dissected 600 larval fish, finding that 48 of them, or 8.6 percent, contained plastics—twice the rate of larval fish outside the slick. It’s possible that rate could be higher.
“One possibility is that because larval stages are so vulnerable, eating one piece of plastic could actually potentially kill them,” Whitney tells Wired.
Those fish would eat the plastic, die and fall to the bottom of the sea, and therefore, would not appear in the tow surveys.
Some species ingested more plastic than others. Economically important fish, like swordfish and mahi-mahi, for example, appeared to eat more plastic than other species. It’s hard to say what the impact of plastic consumption is just yet, but if plastic does increase mortality rates, that means fewer larval fish are surviving to adulthood.
Researchers are still working to understand how plastic affects fish health. While adult fish don’t often die from eating plastic unless it chokes them or injures their stomach, studies have found plastic can impact things like activity rates, schooling and liver function.
It’s also possible that animals that snack on larval fish are accumulating plastic bits in their tissues, a process called bioaccumulation.
“Seabirds feed on larval fish, adult fish feed on larval fish—it’s a prominent food source,” study o-author Jamison Gove, also of NOAA, tells Simon. “So that clearly has implications for how plastics can be distributed and quickly get higher up the food chain.”
The study may help answer other big questions about plastic pollution in the ocean. Melanie Bergmann, a microplastics researcher at the Alfred Wegener Institute in Germany who was not involved in the study, tells Vice’s Maddie Stone that currently scientists are unsure about what happens to 99 percent of the plastic that makes it into the ocean.
While the findings about plastic are alarming, Rebecca Helm, a marine biologist at the University of North Carolina, Asheville, not involved in the study, says the paper is significant for showing how larval fish gather in surface slicks.
“The biology outside of the plastic is in and of itself really novel and cool,” she tells Stone. “Understanding how larval fish are able to find concentrated food spots is really important.”