Microplastics On The Menu Of Manta Rays And Whale Sharks
A new report from the Marine Megafauna Foundation highlights the impact that marine microplastics are having on large filter-feeders such as manta rays and whale sharks.
Large filter-feeders swallow thousands of cubic meters of plankton-filled water every day as they feed, and with it, the tiny particles known as microplastics that result from the breakdown of plastic products in water.
A team of marine biologists from the Marine Megafauna Foundation, Murdoch University (Australia) Udayana University (Indonesia), took samples from the waters around the island of Nusa Penida, near Bali, Komodo National Park and East Java in Indonesia. The areas were chosen for their known aggregations of mantas and whale sharks, which commonly feed in inshore surface waters where both plankton and marine debris accumulates. The researchers used a plankton net to trawl for plastics in the top 50cm of the water column, and also counted any debris visible at the surface.
Using data collected from the samples, they were able to estimate the number of microplastic particles in the water and calculate how many pieces reef manta rays and whale sharks might be ingesting as they strain nutrient-rich water through their gills.
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 added.
The collaborative study, published on 19 November 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 as many as 137 pieces per hour. More than 50 per cent of the plastic pieces counted were hard plastic fragments and the thin films of single-use bags and wrappers. 80 per cent of all the plastic recorded were smaller than 5mm - the so-called microplastics.
Plastic particles were also found in collected samples of manta ray faecal matter and vomit, a clear indication that plastics are ingested during filter-feeding. It is likely that the animals are exposed to the toxic chemicals and pollutants found in plastic products as they pass through the digestive system. These toxic substances can accumulate over decades and alter the hormones that regulate an animal’s growth and development, metabolism, and reproductive functions. Larger plastic particles can block nutrient absorption and cause damage to the digestive tract of animals.
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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 as bycatch in nets or become entangled in fishing lines.
Indonesia is currently ranked as the second-worst plastic polluter in the world and many neighbouring countries within the Coral Triangle are among the top 10. The new study found that plastic abundance was up to 44 times higher during the rainy season, with the largest seasonal effect observed in Nusa Penida.
'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,' said Dr. I. Gede Hedrawan, an Indonesian plastics researcher from Bali’s Udayana University and one of the new study's authors. '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 said.
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 by 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,' said Germanov.
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’ was published in the journal Frontiers in Marine Science on 19 November 2019 and is available here: www.frontiersin.org/articles/10.3389/fmars.2019.00679/full