Plastic Pollution In The Coral Triangle
Indonesia lies within something called the “Coral Triangle,” a marine region in the western Pacific Ocean rich in biodiversity. Among the many extraordinary species that dwell in these waters are whale sharks and manta rays. These awe-inspiring creatures attract tourists, bringing economic benefits to the country. Protecting them is thus important both for their own sake, and for the people whose livelihoods depend on their presence.
Unfortunately, Indonesia stands out not only for the richness of its ocean ecosystem, but also for being the second-largest producer of marine plastic waste in the world. Manta rays and whale sharks, globally threatened and facing the ravages of fisheries, may be vulnerable to the microplastics that inundate their feeding grounds. As filter-feeders, these creatures eat by opening their mouths to draw in seawater, from which they strain out their food. However, in polluted waters, they can end up with a mouthful of more than just plankton.
In this study, the authors set out to discover how much plastic debris these animals might consume while feeding. To do so, they examined how much plastic floats in the water of their feeding grounds in three locations, over a two-year period from January 2016 to February 2018. They also looked for plastic in their vomit and feces, collected with the help of local professional divers.
Ocean currents, weather, and seasonal changes impact levels of microplastic pollution. For example, the authors noted that rivers swollen by the monsoon rains carry waste from nearby dumping grounds into the sea. As such, the study took samples during both wet and dry seasons and avoided windy conditions. They collected samples using nets towed by swimmers and by boat. To minimize disruption to the animals, the investigators kept their boat at least 15 meters away. Similarly, swimmers kept 3 meters away from the animals while they fed.
Only 11% of trawls were plastic-free. One single sample of manta ray vomit contained as many as 66 pieces of plastic. The study found significantly more plastic debris in the water during the wet season – between 2 and almost 45 times more, depending on the location. The authors suggest that this seasonal trend might indicate that microplastics pollution originates locally, since pollution from Chinese rivers is highest during the dry season.
Data from one location suggested that during the wet season, manta rays might consume between 110 and as much as 980 grams of microplastics for every 1,000 grams of plankton. This figure seems shockingly high, but the authors observe that the ratio of plastic to plankton found in their study is lower than estimates from the Mediterranean Sea. Additionally, they suggest that, if anything, their numbers underestimate plastic levels, since the study looks only at free-floating plastics and not plastics that plankton have ingested.
Although the study provides a sense of how much plastic manta rays and whale sharks might consume, we can’t draw conclusions about how this will affect them. The presence of plastics in their vomit and feces shows that these animals can expel these substances from their bodies, which is encouraging. But it’s not clear to what extent the toxic chemicals within plastics leach into them while passing through, or how this might affect them and their offspring. The authors also note that plastic straws seem to have killed at least one whale shark.
To protect the wildlife in the Coral Triangle, the authors highlight the importance of better practices surrounding waste disposal – particularly in the run-up to the wet season, given the role that rivers play in bringing plastic pollution into the sea. They suggest that education campaigns featuring charismatic species like manta rays and whale sharks can help win public support for better regulations on plastic usage and disposal.
More broadly, the study reminds us that for all their seemingly limitless expanse, our oceans are fragile. The waste plastic we produce is choking the water and harming the magnificent creatures who call it home. The recent surge in awareness of the harms of plastics is a step in the right direction, but we’ve a long way to go.