Manta Ray Habitats: Tourism Disturbance and Microplastic Invasion
Indonesia
Manta rays are amongst the most elegant oceanic fish, often described as ‘flying’ underwater creatures. Out of all ray species, they are the largest and inhabit tropical, subtropical, and temperate oceans worldwide.
WHAT MANTA RAY SPECIES ARE THERE?
There are two distinct species: the reef manta ray (Mobula alfredi) and the giant oceanic manta ray (Mobula birostris). Reef manta rays are the smallest of the two, named after their presence in shallow waters close to coastal reefs surrounding oceanic islands and mainland’s, generally in the Indo-Pacific. Their wingspan can grow to an average of 11 feet wide, and they can weigh up to 700kg, which is small compared to the giant oceanic manta ray which can grow up to 29 feet and a weight of 2700kg. This manta ray lives further out in the ocean, hunting as deep as 560m below sea level, with others occurring deeper.
Manta rays are filter feeders, meaning their mouths remain open when they swim, allowing entry of zooplankton and krill of which they feed on. Their mouths are lined with rows of small gill rakers, called gill plates, where prey gets sifted through. The reef manta rays gather in large groups when feeding, exhibiting group behaviours to gain food: chain-feeding where individuals follow one another in a loop to create a cyclone effect, forming a spiral which traps their prey; as well as somersaulting repeatedly in one spot if it is abundant with krill.
WHERE IS MANTA POINT?
Bali, Indonesia is a hotspot for tourism, including trips to Nusa Penida, an island southeast of the mainland. Nusa Penida was declared a Marine Protected Area (MPA), one of three manta ray sanctuaries, at 200km2, in order to protect Indonesia’s manta ray tourism economy. Drawn to warm habitats, reef manta rays gather and live in large populations, enabling snorkelling and diving with them to be a unique experience, one that can occur at Manta Point on Nusa Penida, located south of the island. Surrounded by high cliffs, the waters reach depths of approximately 49 feet and are believed to be used by reef manta rays for cleaning, social activity, and courtship displays in the mating season, particularly in May.
THREATS & TOURISM
The reef and giant oceanic manta rays are classified by the IUCN Red List of Threatened Species as 'Vulnerable' and 'Endangered' to extinction, respectively. Their gill plates are used in traditional medicine and, due to high demand, they are hunted, with overfishing posing as a large threat because of their low fecundity, long gestation period, long lifespan of 40-50 years, and late sexual maturity at roughly 10 years old.
The world’s largest manta ray fishery is located in Indonesia, and in 2014, they declared manta rays a protected species and banned the trade and fishing of these rays for a more profitable purpose. Rated as one of the top countries for seeing manta rays, tourism generates huge amounts of revenue for the local economy. Thus, the focus was turned to tourists swimming with the rays. A study estimates that over one rays lifetime, a single ray can make up to $1 million USD for the industry if it is present in a tourism hotspot; in contrast to only $500 if killed. This type of tourism was valued at over $15 million USD per year, compared to manta ray fisheries of approximately $442,000 USD (in 2013).
Nusa Penida has more than 200,000 tourists every year, with Manta Point as one of the most visited locations, increasing pressure on the local manta ray population. A large number of tourists results in unlimited boats visiting the site all year round and is highest in the mating season. Noise from boat engines can cause chronic stress, and tourists can interrupt natural
behaviours which lead to fitness implications. Also, boat oil poses as a water pollution issue and further health concerns for the reef manta rays.
"Manta rays are amongst the most affected megafauna impacted by plastic pollution in Indonesia"
PLASTIC MARINE DEBRIS
Most plastic marine debris (PMD) around Indonesia is generated regionally. In 2010, an estimated 1.29 million metric tons of waste from Indonesia became PMD, making it the world’s second-largest PMD emitter. Disposing of waste in open dumping grounds near riverbeds is a common practice, leading to extreme rain in the wet season washing debris into rivers and subsequently the sea. In 2019, the abundance of plastic was 44 times greater in the wet than the dry season in Nusa Penida waters.
However, long-range drifting PMD can also contribute to plastic pollution. This is brought in by the Indonesian Throughflow southward currents, a large body of water that flows around the Indonesian Archipelago which transfers water from the Pacific to the Indian ocean. Despite providing nutrient-rich waters to the Indo-Pacific region, which results in rich marine life, it can create unpredictable water conditions, bringing PMD into manta ray habitats like Manta Point. A study on microplastics in Indonesia at Nusa Penida, East Java, and Komodo National Park waters, found most of the plastic was thin films from single-use wrappers or bags, and harder plastic fragments. Over time, plastic breaks down into microplastics, with 80% of discovered plastic at the sights being microplastics under 5mm.
Manta rays are amongst the most affected megafauna impacted by plastic pollution in Indonesia. As manta rays are filter-feeders, swallowing hundreds of thousands of cubic meters of water a day increases their risk of ingesting microplastic debris directly or indirectly by contaminated zooplankton. Studies suggest these rays cannot distinguish between the debris and plankton, arising concerns of accumulation of toxic chemicals from the plastics in their digestive tract, potentially disrupting regular gut functioning and affecting reproduction, growth, and development. In 2019, research on Nusa Penida reef manta rays showed they potentially are ingesting up to 63 pieces of plastic per hour of feeding. Yet, findings of them defecating ingested plastic or regurgitating plastic is positive.
RESOLUTIONS GOING FORWARD
Despite tourism being fantastic for the local economy, boat limits need to be put in place, regulating the number present at one time, and codes of conduct for snorkelling and diving should be introduced to reduce the impact of tourism on these rays. The seasonal closure of Manta Point during the mating season, especially in May, should be proposed to help this vulnerable species prosper and reproduce.
As for water pollution, regular surface-level clean ups should occur, aiming to remove as much PMD present, and local population waste management efforts need to be improved to keep waste disposal away from waterways, especially before the wet season.