Each species, population and individual manta ray is perfectly adapted to the rhythm of the ocean which shapes and encapsulates their life. Daily and seasonal movements are tuned to the ebb and flow of the oceans currents that breathe life into their world, bearing the planktonic food upon which the mantas depend.
Throughout the oceans of the world all manta rays are driven by this necessity to find food. For some, this search may be simple with ample and predictable food supplies to be found within small and localised areas throughout the year. For others this hunger may drive them thousands of kilometres on epic migrations across entire oceans. Like the great herds of wildebeest and zebra which follow the seasonal rains that pour life into the African savannah, these ocean wandering mantas depend on being able to locate regional blooms of productivity which are created by the ever changing conditions throughout the world’s oceans…
Lying close to the equator and within the doldrums, the Maldives rarely experiences any severe weather events. However, the country has two distinct Monsoons (seasons) a year which shape the prevailing weather patterns and drive the ocean currents. The Maldivian South-West Monsoon, locally known as Hulhangu, runs from May-October, while the North-East Monsoon, or Iruvai, runs from December-March each year, with the months of November and April acting as transitional periods of change in between. The S-W Monsoon typically brings with it much more rain and cloud cover, with reduced visibility and rougher seas, while the N-E Monsoon usually heralds blue skies and exceptionally clear waters for much of the time.
Rising 2,000 metres (6,600 ft) straight up from the depths of the Indian Ocean, the Maldivian Atolls straddle the Indian Ocean and act as a barrier to the monsoonal currents forcing them upwards as they flow across the Indian Ocean and through the Maldivian Archipelago. These currents create deepwater upwelling around the atolls, bringing nutrient rich water to the surface and within reach of the sun’s life giving energy, kick starting the food chain. The water is squeezed and mixed as it passes through the channels and shallow waters of the atolls lagoons; productivity blooms and life flourishes as energy flows up the food chain. By the time the nutrient laden currents have reached the leeward sides of the atolls, primary productivity is reaching its peak and the zooplankton food upon which the manta rays feed is so abundant that the Maldivian waters support the world’s largest known population of reef manta rays all year round.
After many years of photographically tracking and tagging individuals within the country’s population, scientists are now beginning to unravel the seasonal and daily movements of these animals. Not surprisingly their movements are strongly linked to the monsoons, migrating backwards and forwards across the Maldives with the changing seasons as they follow the hotspots in productivity of their planktonic food, always occurring in their greatest concentrations on the monsoonal down-current edges of the atolls. For example, during the S-W Monsoon Baa Atoll supports a huge seasonal population of over 1,000 individual manta rays, which flock to the atoll’s eastern channels to feast on the seasonal abundance of plankton at locations like Hanifaru Bay. However, during the N-E Monsoon, many of these individuals make the short ocean crossing down to Ari Atoll in the South, or up to Raa Atoll in the North, where greater abundances of food can be found in these atoll’s western channels. Research has shown that not only are these population shifts predictable, but the individuals animals themselves show an extremely high degree of fidelity to specific sites. For example, many of the manta ray individuals have been recorded at the same locations dozens of times each year, year after year, in some cases going back over the last three decades. With the changing of the Monsoon they head off to their second home, which may be on the opposite side of the same atoll, or across to another atoll altogether.
Elsewhere across the planet populations of reef manta rays also follow the daily and seasonal variations in their planktonic food supply; in Mozambique the reef mantas migrate north and south along the East coast of Africa, while in Australia’s Ningaloo & Great Barrier Reef’s the reef manta populations also range over hundreds of kilometres in search of food. In Hawaii the small population of reef mantas on the Big Island have learned to gather in a few small bays near Kona every night to feast on the plankton which is artificially concentrated in dense patches by bright underwater lights. These lights are intentionally turned on every night at sunset to attract the mantas which support a booming dive and snorkel tourism industry. As soon as the lights go off, and the plankton disperses, the mantas head back into deeper water for the rest of the night.
The oceanic mantas, as their name suggests, are more at home in the deep sea than their close relatives the reef mantas. Considered to be ocean voyagers, it is thought they follow the highway of ocean currents, travelling vast distances across the open seas in search of seasonal planktonic blooms. We know very little about the lives of these ocean giants, mainly because it’s so hard to follow and study an animal which we suspect spends much of its time so far away from land. Each year just a few dozen oceanic mantas are recorded visiting the Maldives by researchers and no individual has ever been seen more than once.
Where these giant oceanic manta rays are travelling from and going to are mysteries, but from satellite tracking data recovered from tagged oceanic mantas elsewhere around the world we know that they are capable of moving large distances in relatively short periods of time. It’s likely that the oceanic mantas seen in the Maldives are part of a population which spans thousands of kilometres across the Indian Ocean, travelling on the monsoonal and equatorial currents in search of food and mates.
Most of the best locations around the world to encounter oceanic mantas are at isolated ocean sea mounts and pinnacles. Probably the most well known of these locations are the remote Revillagigedos Islands (Socorro & San Benedicto), which lie 400 kilometres (250 miles) due south off the tip of Baja California in Mexico. Around these tiny specks of land in the middle of the Pacific Ocean truly giant oceanic mantas can be seen during the months of November to June. Some of these giants measure in excess of 6.5 metres (20 ft) from one wing-tip to the other. Scientists have been photographically identifying the mantas which visit these islands for the last 30 years, yet after all this time a massive percentage of the mantas encountered on each return expedition are still new individuals, never recorded at these islands or anywhere else in the world before. Of those few mantas which have been seen on repeat occasions, sometimes as long as a decade may have passed between sightings. These extremely low re-sighting rates suggest that the mantas which visit these islands are probably part of a much wider super population whose members voyage across large areas of the ocean, possibly following the Earth’s natural polarity to navigate between the isolated sea mounts throughout the region. Satellite tagging studies are underway to try and establish exactly where these wandering giants go and what they get up to on their journeys.
Although oceanic mantas can be seen at certain sites with a degree of predictability, this species is still much less frequently encountered by divers than their smaller reef cousins. Much of their lives still remains a mystery, even a glimpse of one of these giants is, not only an experience which many divers will never forget, it can also provide scientists with new and precious clues that may help to unravel the cloak which still casts a shadow over the lives of these enigmatic creatures.