The Raja Ampat archipelago in West Papua is home to Indonesia’s largest population of reef manta rays (Mobula alfredi). These iconic creatures draw tourists to the area from around the world, for ecotourism and dive experiences. There are nine marine protected areas (MPAs) and, since 2014, there has been national protection afforded to both reef and ocean manta rays in the waters of Raja Ampat. However, the effects of these conservation measures on manta abundance and population dynamics has not been assessed.
Marine protected areas are known to enhance populations of corals, as well as sedentary fish and invertebrates that depend on the coral reefs. However, mantas are highly mobile, migratory animals that are more difficult to count in remote locations. But understanding their population dynamics is important in order to evaluate the effectiveness of the management strategies being used for this species that is currently categorized as “vulnerable” on the IUCN’s Red Data List. Luckily, mantas do form aggregations at certain times of year, and are philopatric – return faithfully to specific aggregation sites year after year. It is at these times that population assessments may be made.
A new study by scientists from Indonesia, New Zealand and Australia has now investigated population changes in manta rays in two of Raja Ampat’s largest MPAs, namely Dampier Strait and South East Misool. The researchers used open population mark-recapture models, based on photo-ID sighting data of M. alfredi, sourced from citizen science and from their own active surveys, to assess population changes. Photos were collected from the two MPAs between 2009 and 2019, and identification made use of individual ventral markings that are unique to each manta.
The results, published in Frontiers of Marine Science, give conservationists a “ray” of hope in the struggle to protect marine biodiversity from the ravages of constant exploitation. In Dampier Strait, the estimated population increased to 317 individuals, an annual compound gain of 3.9 percent, while South East Misool’s estimated gain to 511 individuals was 10.7 percent. Therefore, over the decade, manta populations increased significantly in both MPAs. This highlights the importance of long-term conservation and management measures, such as well-enforced MPAs and fisheries regulations, says researcher Edy Setyawan of the University of Auckland’s Institute of Marine Science.
This is the first published evidence of reef manta ray populations increasing anywhere in the world, he says. “Despite the global decline in oceanic sharks and rays because of overfishing over the past 50 years, the reef manta rays in Raja Ampat have been recovering and thriving,” Setyawan says.
According to the authors, the increased population sizes resulted from high survival rates (up to 93 percent of individuals in each group survived each year) and high rates of recruitment (typically groups got a 20 percent annual boost from new members). While conservation measures substantially reduced fishing pressures, another reason that the populations are thriving was is the occurrence of El Niño Southern Oscillation climate cycles, which boosted plankton numbers. That led to larger and more frequent aggregations of the manta rays for feeding, which in turn provided more opportunities for mating.
Mantas have a very low reproduction rate, with females only becoming sexually mature from about the age of 10. Thereafter, they produce one pup every two to six years after a gestation period of 12–13 months. This low fecundity means they are very susceptible to overfishing and population declines. Other published research has reported dramatic declines (a 98 percent decrease) in M. alfredi sightings in southern Mozambique between 2003 and 2016, where they are a target species for fishery activities. Their populations are also declining throughout their range in the Indo-Pacific, but not in the Raja Ampat waters.
Today, an estimated of 16,000 to 18,000 of the creatures may survive, with the Maldives hosting the biggest number, at least 5,000 individuals, followed by Indonesia with at least 3,500. “Unfortunately, reef manta rays are generally in decline, as in Mozambique where they have been continuously caught in targeted fisheries, or just holding steady, as in Australia and the Maldives,” says Setyawan.
This research highlights the need for more well-enforced MPAs around the world, to protect the critical habitats of the marine creatures, along with strong commitments from governments to protect the species, and fishing gear restrictions such as bans on gillnets and longlines, says Setyawan. Manta rays are known for their intelligence, graceful swimming and innate curiosity.
By Alison Bosman, Earth.com Staff Writer
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