Published: 12th November 2019
Ocean's superhighways for larval fish polluted with plastic concentrates: Study
While earlier research revealed that adult fish consume plastic, it was not known until now if larval fish also do so, nor the health implications, according to the study
Larval fish species in different regions of the Earth's oceans may be threatened by plastic pollution in their nursery habitats, according to a study which suggests that the levels of this pollution is on average eight times greater than those found in the Great Pacific Garbage Patch — a swarm of floating debris in the north-central Pacific Ocean.
The researchers, including those from the Arizona State University (ASU) in the US, are the first to show that larval coral reef fish, and species neither close to the bottom nor near the shore, consume plastic as early as when they are just a few days old.
While earlier research revealed that adult fish consume plastic, it was not known until now if larval fish also do so, nor the health implications, according to the study, published in the journal PNAS. As part of the study, the researchers used a combination of advanced remote sensing techniques, field-based plankton tow surveys, and dissection tools to observe the impact of plastics on larval fish.
They conducted the study along the west coast of Hawai'i Island -- the southeasternmost island in the Hawaiian Archipelago. The researchers sampled the fishes from a prominent feature of these coastal waters called surface slicks -- naturally occurring, ribbon-like, smooth water features at the ocean surface.
"Surface slicks had never been mapped before, but we knew it would be vital to scaling up the field-based study. Our new method developed for this study can be applied anywhere in the world," said study co-author Greg Asner from ASU.
The researchers said the surface slicks act as nursery habitats for larval fish as they contain a rich and steady diet of planktonic organisms. These slicks, the study noted, contained seven times more plastics than larval fish -- with densities, on average, eight times higher than those recently found in the Great Pacific Garbage Patch.
Compared to neighbouring surface water just a couple hundred yards away, the slicks contained 126 times more concentrated quantities of plastic, the researchers said. "Slick nurseries also concentrate lots of planktonic prey, and thereby provide an oasis of food that is critical for larval fish development and survival," said study co-author Jonathan Whitney, a marine ecologist for the Joint Institute for Marine and Atmospheric Research and The National Oceanic and Atmospheric Administration (NOAA) in the US.
"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," Whitney said. The researchers also found plastic in the stomachs of flying fish — a vital food source for apex predators like tunas and Hawaiian seabirds.
They added that larval fish found in surface slicks were larger, more well-developed, and better swimmers compared to other fish. According to the study, the larval fish which actively swim can better orient with their environment.
This, the researchers said, suggested the tropical larval fish could be actively seeking surface slicks for their concentrated prey resources. "The surface slicks we mapped have turned out to be like fish superhighways that connect different bays and reefs along the west coast of Hawaii Island," said Asner.
According to the researchers, the surface slicks comprise less than 10 per cent of ocean surface habitat but may contain 42.3 per cent of all surface-dwelling larval fish and nearly 92 per cent of all floating plastics.
"This is how reef fish get around over long distances, and that has enormous implications for reef management since a change in fisheries activity in one bay can have an impact on more distant bays down the superhighway. Unfortunately, now we can also say these surface slicks are plastic superhighways," said Asner.
Study co-author Gareth Williams from Bangor University in the UK posted on Twitter that the slicks concentrated plastics and outnumbered larval fish by 7:1. "Most of these plastics were floating polymers -- polyethylene and polypropylene -- used in single-use items (bottled water, plastic bags) and fishing equipment (rope, nets)," Williams said in a Tweet.
The researchers are still uncertain whether plastic ingestion is harmful to larval fish. In adult fish, they said, plastics can cause gut blockage, malnutrition, and toxicant accumulation. The study noted that larval fish are also highly sensitive to changes in their environment and food, so prey-size plastics could impact their development and reduce their chances of survival.
"Larval fish are foundational for ecosystem function and represent the future cohorts of adult fish populations," said study co-author Jamison Gove from NOAA. "The fact that larval fish are surrounded by and ingesting non-nutritious toxin-laden plastics, at their most vulnerable life-history stage, is cause for alarm," Gove said.