Microplastics on the menu for manta rays and whale sharks

Plastics pollute Indonesian feeding grounds of plankton-feeding ocean giants

Plastic pollution has a tremendous impact on marine life – and reef manta rays and whale sharks are not spared from it. These large filter-feeders swallow hundreds to thousands of cubic meters of plankton-filled water every day, and with it, tiny plastic pieces from broken down carrier bags and single-use packaging, a new study has found.

Marine biologists from the Marine Megafauna Foundation, Murdoch University (Australia) and Udayana University (Indonesia) estimated the amount of plastic particles present in the waters off Nusa Penida (Bali), Komodo National Park and East Java in Indonesia and, based on that, calculated how many pieces reef manta rays and whale sharks might be ingesting. These shark species sieve nutrient-rich water through their gills as they swim.

As manta rays and whale sharks spend a lot of time feeding in inshore surface waters where trash commonly aggregates, the researchers used a plankton net to trawl for plastics in the top 50 cm of the water column. They also counted any debris visible at the surface from the boat.

Lead author Elitza Germanov, a researcher at the Marine Megafauna Foundation and PhD candidate at Murdoch University, said: “With time, plastics break down into smaller pieces called microplastics that large marine filter feeders might accidentally scoop up because they float among their prey.”

“Manta rays and whale sharks can ingest microplastics directly from polluted water or indirectly through the contaminated plankton they feed on,” she adds.

The collaborative study, which is published today in the journal Frontiers in Marine Science, found that reef manta rays may ingest up to 63 pieces of plastic per hour of feeding in Nusa Penida and Komodo National Park. Whale sharks, which seasonally aggregate in Java, could be ingesting up to 137 pieces per hour.

Thin and bendable films from single-use bags and wrappers as well as hard fragments were the most prevalent plastics (over 50% combined). Of all plastic recorded, around 80% were small pieces of less than 5mm, so-called microplastics.

Manta ray poo and vomit also tested positive for plastics, which means that plastics are easily ingested when filter feeding and likely expose the animals to toxic chemicals and pollutants found in plastics while in their digestive system. These toxic substances can accumulate over decades and alter the hormones that regulate an animal’s metabolism, growth and development, and reproductive functions. Larger plastic particles can block nutrient absorption and cause damage to the digestive tract of animals.

Neil Loneragan, Professor of Marine Ecology and Conservation at Murdoch University said: “It is difficult to assess how much plastic manta rays and whale sharks actually ingest because conventional methods used to study animal diets, such as stomach analysis, are unsuitable for threatened species like these.”

Manta rays and whale sharks are globally threatened species facing extreme pressures from overfishing. They are often caught incidentally in nets or become entangled in fishing lines.

Indonesia is currently ranked as the second-worst plastic polluter in the world and many neighboring countries within the Coral Triangle are among the top 10. This study found that plastic abundance was up to 44 times higher during the rainy season, with the largest seasonal effect observed in Nusa Penida.

Dr. I. Gede Hedrawan, an Indonesian plastics researcher from Bali’s Udayana University and an author on this study, said: “The seasonal variability in plastic pollution shows what a difference it would make to clean up river beds before the rainy season begins.” Local authorities could also prohibit any waste disposal in areas around water sources.

“We welcome Bali’s recent ban on single-use plastic bags, straws and take away containers, although the law is yet to reach its full effect and spread to smaller businesses,” he says.

It is vital to understand the effects of microplastic pollution on ocean giants since nearly half of the mobulid rays, two thirds of filter-feeding sharks and over one quarter of baleen whales are listed by the IUCN as globally threatened species and are prioritized for conservation. Previous studies found that baleen whales may swallow microplastic particles in the thousands every day.

“We now know that, through exposure to toxic substances, plastic contamination has the potential to further reduce the population numbers of these threatened animals because they reproduce slowly and have few offspring throughout their lives,” Germanov concluded.

As plastic production is projected to increase globally, future research should focus on coastal regions where pollution overlaps with the critical feeding and breeding grounds of these ocean giants. Many areas such as the Nusa Penida Marine Protected Area and Komodo National Park are biodiversity hotspots with significant marine tourism.

The field research was supported by the Ocean Park Conservation Foundation, PADI Foundation, Foundation FortUna, Mantahari Oceancare, Arenui Boutique Liveaboard, Current Junkies Liveaboard, Happy Days yacht, Scuba Junkie Komodo, and Wunderpus Liveaboard, and was carried out under a RisTek-Dikti (Indonesian Ministry of Research) permit.

The study by Elitza Germanov et al., titled ‘Microplastics on the menu: Plastics pollute Indonesian manta ray and whale shark feeding grounds’ is published in the journal Frontiers in Marine Science on 19 November 2019 and is available here:  https://www.frontiersin.org/articles/10.3389/fmars.2019.00679/full