Larval fish are eating plastic

Newborn / larval fish are mistaking tiny bits of trash for food. If they die, there’ll be fewer big fish—and that could rattle the food chain.

FISH NURSERIES OFF HAWAII ARE NOW A MICROPLASTIC MESS.  The naturally oily surface slicks in which many ocean fish come of age are rich in plankton and other fish food—and now also in plastics, according to researchers at the National Oceanic and Atmospheric Administration in Honolulu. They’ve been towing fine mesh nets through slicks off the Big Island and analyzing each haul. Here, a scribbled filefish, about 50 days old and two inches long, navigates a soup of plastic.

Not long ago I went snorkeling in the Pacific Ocean, a half mile off the southwest coast of Oahu. The flanks of the Hawaiian island are steep there, and the bottom quickly disappeared beneath us as we motored out to the site. Looking back, I could see the green slopes of the Waianae Range rising to 4,000 feet behind the beach. Normally the mountains shield the water here from the trade winds. But on that day a breeze created a light chop that nearly obscured what I had come to see: a thin, oily slick of surface water, rich in organic particles, in which newborn fish were feeding and struggling to survive their first precarious weeks.

My guides that day, oceanographer Jamison Gove and fish biologist Jonathan Whitney of the National Oceanic and Atmospheric Administration in Honolulu, are nearly three years into a research project that aims to make sense of this chaotic scene. The larval stage is the “black box” of fisheries science: Fertilized eggs go in, and young fish come out—but what happens inside remains sketchy. The larval fish are so small and fragile they’re exceedingly difficult to study. The overwhelming majority will never become adults. Yet fish populations around the world, and the animals that eat them, depend on just how many larval fish make it, and in what condition.

The blue glove hasn’t been in the water long enough to suffer the fate of most ocean plastic, which is to be shredded into small bits, or microplastics, by waves and sunlight. The larval fish below the thumb is a driftfish; the striped one at the base of the index finger is a mahi-mahi.

What Gove and Whitney have found lately—and what David Liittschwager’s photographs of their water samples document—is that fish and wholesome fish food are not the only things collecting in the slicks off Hawaii. Microplastics, tiny shreds of human trash, are there as well, and in such abundance that larval fish are eating them in their first days of life.

For newborn fish, to eat is to live another day; if their first meal is plastic, they’re not consuming the calories they need to sustain them until the second. “They’ve beaten a lot of odds to get this far,” Gove says. “They hatched, they found the slick, they’re feeding and growing. This is one-tenth of one percent that made it this far; they’re the lucky ones. And now plastics are coming in.”

“The most critical moment is that first feeding,” Whitney says. “If they get a piece of plastic, that could be it. A single thread in the stomach of a larval fish is potentially a killer.”

ON THE RIGHT, FISH FOOD. ON THE RIGHT, PLASTIC.A dollop of surface water from the English Channel contains a shrimplike krill, about a third of an inch long; a smaller decapod crustacean; and an orange sea star just emerging from its filmy floating larval stage. The white chip and the fraying red fiber on the right are polyethylene—but to a young fish they too may look like food. Three percent of the larval fish caught for a 2017 study by researchers at Plymouth Marine Laboratory and the University of Plymouth had eaten microplastic fibers.

Plastic waste, mostly from rivers or careless dumping on land, washes into the oceans at an average rate of about nine million tons a year, according to a 2015 study by Jenna Jambeck of the University of Georgia. The visible trash, along with heartbreaking images of its impact on everything from turtles to birds to whales, has generated a public outcry. But sunlight, wind, and waves eventually break down ocean plastic to bits that are barely visible. One of the biggest unknowns—and concerns—is the effect that these microplastics, smaller than a fifth of an inch, might be having on fish.

Fish provide critical protein to nearly three billion people and countless seabirds and other marine animals. But fish stocks worldwide have fallen by half since 1970, surveys show. Populations of the largest predatory fish, such as tuna, have fallen even more. The decline is largely because of overfishing, but pollution and waters warmed and acidified by climate change are having a growing impact.

As long ago as the early 1970s, scientists were finding plastic pellets—the material used to manufacture plastic goods—in the stomachs of fish caught off New England and Great Britain. More recent studies have documented the presence of even smaller microplastic particles in a growing array of adult fish. Larval fish have been studied much less but are likely to be more vulnerable to microplastics, as they are to everything else. “Any stressor will likely have more of an impact on early life stages than later life stages,” says Susanne Brander, a toxicologist at Oregon State University who is studying how plastics might affect the growth of fish.

A grid painted on a petri dish helps a NOAA technician sort through a sample and identify tiny organisms, such as the larval sergeant major damselfish on the left, just outside the middle row. The squares are one centimeter (.39 inch) across.

Most ocean fish are terrible parents. A few species guard their eggs on the seafloor; others protect them inside their mouths. But most fish release thousands or even millions of eggs and sperm into the wide ocean and never see their offspring. When eggs hatch a day or two later, the newborn fish are on their own.

Newborn fish look misshapen, heads oversize, tails barely formed. They have to eat like crazy to grow into their body. Whereas human babies develop in the shelter of the uterus, fish mainly develop after they emerge into an unforgiving world.

Predators or starvation will get most of them. “That’s why fish spawn so many eggs,” says Su Sponaugle, an Oregon State University marine ecologist who specializes in the early life stages of fish. “They have to hedge their bets.”

The larval phase is treacherous every step of the way—starting with the need for the larvae to find food, which they do in a surface slick. Surface slicks form mostly in coastal regions around the world, wherever currents, tides, or subsurface waves cause water to converge and concentrate the organic gunk that floats in it. Slicks can be seen by satellite as long, squiggly ribbons that run parallel to coasts.

Some larval fish swim to slicks, some drift, as do eggs not yet hatched. Predators converge on slicks too. If a baby fish manages to avoid being eaten and to find enough food, it will be about two inches long when it heads back to its permanent habitat—a reef, say. The right current will transport it there, the wrong one out to sea.

“If you miss an island, good luck with that. If there’s no reef, you cannot complete your life cycle,” Sponaugle says. Life for newborn fish was a crapshoot even before they met our plastic trash.

Off Hawaii, a single eight-minute tow of the NOAA team’s net yields a plethora of living organisms and plastic.

Pushed into a surface slick by converging currents, they’re separated in the lab by a technician with tweezers. A computer program counts the plastic pieces and measures each one; the technician uses a microscope to identify the creatures.

Leave a Reply

Your email address will not be published. Required fields are marked *