Larval Fish Habitat Brims with Plastic

Adult fish are an important resource commercially and ecologically. While much effort goes into tracking the health and abundance of adult fish, the future of fisheries depends on the growth and survival of younger generations of larval fish. Larval fish may be particularly vulnerable to an emerging threat in the oceans: microplastics. A first step in assessing the risk of microplastics to larval fish is to determine if larval fish and microplastics overlap in distribution. Marine scientist Jamison Gove and his team at the National Oceanic and Atmospheric Administration set out to do just that. They found that larval fish were not only swimming in a sea of plastics, but they were consuming them. Gove's team conducted a series of plankton tows—dragging a fine mesh through surface waters to collect small animals and particles—off the West coast of Hawaii to collect larval fish. When the researchers dissected the larval fish they collected, many had ingested microplastics. The microplastics were made of polyethylene—the primary compound used in most single-use plastics such as plastic bags and water bottles. But larval fish that had consumed microplastics were not uniformly distributed across the ocean's surface. More than twice as many fish in surface slicks, areas where internal waves merge and force plankton and other tiny particles to the surface, had ingested plastic. So why were so many more larval fish consuming plastic in surface slicks? One reason could be that there were simply more larval fish gathering in surface slicks. Larval fish were more abundant there, with commercially important species like swordfish and mahi found 28 times more frequently in slicks than in surrounding waters. Most of the larval fish were large enough to be considered competent swimmers, suggesting they were actively targeting surface slicks for food. The internal waves of surface slicks concentrate zooplankton, tiny animals that larval fish eat. But in a plastic-filled sea, they also concentrate microplastics. Gove's team found that microplastics were 126 times more abundant inside surface slicks, reaching densities eight times higher than those of the Great Pacific Garbage Patch, the largest area of plastic accumulation in the ocean. In surface slicks, microplastics outnumbered larval fish 7:1. Many of the microplastics found were the same size as zooplankton normally eaten by larval fish. This suggests larval fish could be consuming more plastic in surface slicks because they are mistaking microplastics for prey. The discovery of surface slicks as critical habitat for larval fish is a step towards better understanding larval fish dynamics. But the amount of plastic pollution found in these habitats is concerning, the scientists said. Little is known about how the ingestion of microplastics affects larval fish growth and survival, but the researchers suggested gut blockages, malnutrition, and reduced predator avoidance as risks. These effects could decrease larval fish survival and severely impact future fish populations. In addition, plastics are known to absorb pollutants in the environment. If larval fish ingest plastic and are consumed by other fish, it could lead to an accumulation of toxins in the fish people eat. There is still much to be learned about how larval fish and plastic interact, said Gove. He and his team plan to explore this interaction further with the use of a 20-year-old larval fish archive. They are currently developing methods to isolate the plastic fragments in each fish in the archive to learn how plastic ingestion varies between larval fish species and size, and how plastic ingestion in larval fish has changed over time. More information may be found in the team's 2019 paper, “Prey-size plastics are invading larval fish nurseries” published in Proceedings of National Academy of Sciences (PNAS).