In rare good news in the marine world, scientists have found one place where reef manta rays thrive.
Populations have increased significantly in Indonesia’s Raja Ampat archipelago over a decade, underscoring the importance of long-term conservation and management measures such as well-maintained marine protected areas (MPAs) and fishing regulations, says researcher Eddie Setiawan of the University of Auckland’s Institute of Marine Sciences. .
This is the first published evidence of a reef manta population increase anywhere in the world, he says. “Despite the global decline of oceanic sharks and manta rays due to overfishing over the past 50 years, reef manta rays in Raja Ampat are recovering and thriving,” says Setiawan.
The scientist and his colleagues studied reef manta rays (Mobula Alfred) in Raja Ampat’s two largest MPAs, Dampir Strait and Southeast Misool. They used observations of rays, each individually identified by photograph, to estimate population dynamics from 2009 to 2019. In Dampier Sound, the estimated population increased to 317, a cumulative annual increase of 3.9 percent, while Southeast Missoula’s estimated population increase to 511 was 10.7 percent. interest on the same basis.
The increase in population size resulted from high survival rates (up to 93 percent of individuals in each group survived each year) and high recruitment rates (groups typically experienced a 20 percent annual increase in new members). While conservation measures have significantly reduced fishing pressure, another cause of population growth has been the El Niño Southern Oscillation climate cycle, which has increased plankton abundance. This led to greater and more frequent gatherings of mantas for feeding, which in turn provided more opportunities for mating.
Based on a 2018 assessment, the International Union for Conservation of Nature’s Red List of Threatened Species lists manta rays as a “vulnerable” species with declining populations throughout their Indo-Pacific range .
Today, an estimated 16,000 to 18,000 of the creatures may survive, with the Maldives home to the largest number of at least 5,000, followed by Indonesia with at least 3,500. “Unfortunately, the number of reef manta rays in general is declining, as in Mozambique, where they have been continuously caught in targeted fishing, or simply preserved, as in Australia and the Maldives,” says Setiawan.
Around the world, more well-enforced MPAs are needed to protect these creatures’ critical habitats, as well as strong commitments from governments to protect the species and restrictions on fishing gear, such as bans on gillnets and longlines, he says. Manta rays are known for their intelligence, graceful swimming and traits such as somersault feeding, a technique of continuing to move while remaining in the same spot to feed on plankton or krill. Mantas must keep moving to stay alive.
The reef manta has a wingspan of up to five meters. This slow-growing species begins to mature between the ages of 9 and 13 (males) and 13 to 17 (females). A mature female gives birth to only one cub every two to six years, after 12 to 13 months of pregnancy. Late maturing and low fecundity make the species particularly vulnerable to population decline.
A study published in Frontiers of marine science, was carried out with the help of researchers from the Raja Ampat MPA Management Authority, Konservasi Indonesia, Conservation International Aotearoa and Waipapa Taumata Rau from the University of Auckland. “Since the end of the study, the manta rays seem to be doing well and their population continues to grow,” says Setiavan. “Large feeding swarms were often seen and reported. Over the past three years, we have discovered more than 300 new mantas. Not to mention the newborns and juveniles that live in Wayag Lagoon and Fam Islands nurseries that we recently discovered. “