Oceanic manta ray connections
Oceanic manta rays are endangered, with reports of up to 1,000 animals landed a year in Sri Lankan fisheries. Only 1,000 kilometres (620 miles) away, the world’s third largest oceanic manta ray population aggregates seasonally at Fuvahmulah in the Maldives. Simon wants to understand the degree of connectivity between oceanic mantas in the Maldives and those found in Sri Lankan fisheries surveys. He is documenting the population in the Maldives through photo identification and taking tissue biopsies annually to compare with those from Sri Lanka. By comparing chemical signals in the tissues, he will assess whether the rays are feeding in overlapping ranges.
Simon Hilbourne
Born and raised overseas, I spent several years growing up in Thailand, where I learned to scuba dive at a young age. I completed my Divemaster qualification days after turning 18 and then spent a gap year working as a dive instructor on the Great Barrier Reef. It was always in the back of my mind, though, that my passion and fascination was in the animals rather than the tourism side of things.
Four years of a marine biology degree in Southampton, UK, was long enough in a cold climate, so I moved back to South-East Asia...
Oceanic manta ray connections
This project aims to use tissue samples to understand whether there are any connections between the oceanic manta rays that are seen in the Maldives, and in particular around Fuvahmulah, and the manta rays that are caught and landed in Sri Lanka.
Oceanic manta rays are listed as endangered on the IUCN Red List primarily due to threats from fisheries. Fuvahmulah Atoll has been established as an important area for a huge population of this species. However, large-scale mobula fisheries in nearby countries threaten this population. A first step towards protecting the mantas is understanding the connectivity between the living population in the Maldives and mantas caught and killed in nearby countries.
The Maldives is widely regarded as one of the best locations to see reef manta rays worldwide. It has recently been confirmed that there is also a substantial population of endangered oceanic manta rays, the larger and more elusive cousin to the reef mantas, in the southernmost atolls of the country.
We have now documented more than 1,000 individual oceanic manta rays in the Maldives, making this the third-largest recorded population in the world (after the Ecuador/Peru and Revillagigedo populations in the Pacific). Sightings are highly seasonal, peaking in April and localised mainly around the island of Fuvahmulah. Resighting rates remain low (7%) among individuals, which suggests we are still only scraping the surface of this potential super-population in the Indian Ocean.
Very little is known about why these oceanic manta rays come to Fuvahmulah and where they travel outside the season. Oceanic mantas can migrate long distances and the individuals in the Maldives may leave the country´s exclusive economic zone and enter international waters, where they are vulnerable to foreign fisheries. Most worrying are the fisheries in neighbouring Sri Lanka (just 1,000 kilometres, or 620 miles, from the Maldives), where there is no national protection for this species even though the country is party to the Convention on Migratory Species (CMS). The Manta Trust’s affiliate project partners have been monitoring shark and ray landings at fish markets and estimate that several hundred – even up to 1,000 – oceanic manta rays are caught and killed each year. To inform national and international policies and management plans for the conservation of this species, we need to get a better understanding of any potential interaction between Sri Lankan fisheries and the oceanic manta rays seen in the south of the Maldives.
- To continue documenting the oceanic manta ray population in the Maldives through photo identification field seasons in Fuvahmulah. We will collect a limited number of tissue biopsy samples from oceanic manta rays each year.
- Concurrently, Blue Resources Trust will continue to collect oceanic manta samples during fisheries surveys in Sri Lanka.
- Once a representative sample size has been collected, the tissue samples will be processed and analysed for stable isotope composition. Carbon and nitrogen stable isotopes are chemical signals in the tissue that can trace what the manta rays have eaten over the past 3–6 months. By comparing signatures from both populations, we can see if they are feeding in overlapping ranges.
- The genetic analysis will involve next-generation sequencing, a method for genetic mapping that makes it possible to analyse even a small amount of DNA. Genetic sequencing will enable us to get an effective population size and look at connectivity between samples from the two locations.