A new study assessing the feeding patterns and ocean processes that affect larval fish has uncovered an alarming trend. According to a paper, published today in the Proceedings of the National Academy of Sciences, larval fish are ingesting large amounts of tiny prey-sized plastics at the ocean’s surface.
The study, done by NOAA's Pacific Islands Fisheries Science Center along with an international team of researchers was one of the most ambitious to date on larval fish feeding. Done in coastal waters of Hawai'i, the study combined field-based plankton tow surveys and advanced remote sensing techniques to assess where these tiny fish spend most of their time and what they eat.
The researchers discovered that larval fish spend most of their time on smooth water features at the ocean surface called surface slicks. These slicks, which can be found worldwide, form when ocean waves converge near coastlines and become an aggregator of plankton, an important source of food for larval fish.
However, the same ocean processes that aggregate plankton also collect large concentrations of floating microplastics. Plastic densities in the surface slicks were found to be an average of 126 times more concentrated in the surface slicks than in regular surface water, found just a few hundred yards away.
Most of the plastics found in the surface slicks were less than 1 mm, which approximates the size of the prey that larval fish are known to consume. Dissection of hundreds of larval fish revealed that many fish species had indeed ingested plastic particle. "We found tiny plastic pieces in the stomachs of commercially targeted pelagic species, including swordfish and mahi-mahi, as well as in coral reef species like triggerfish," said Dr. Whitney. Plastics were also found in flying fish, which apex predators such as tunas and most Hawaiian seabirds eat.
Satellite images revealed that surface slicks comprise less than 10% of ocean surface habitats, but they are estimated to contain 42.3% of all surface-dwelling larval fish and 91.8% of all floating plastics. While researchers are still unclear just how harmful plastic ingestion is to larval fish, they have been shown to have numerous adverse effects in adult fish and it is likely that they are harmful to larval biology as well.
Image: A scribbled filefish in a sea of plastics sampled in surface slicks off Hawai'i Island. Image courtesy of David Liittschwager.