Manta Rays of Hawaiʻi

By Mark Deakos, PhD

A reef manta ray filtering plankton through it’s gills as it feeds in a current line just below the surface. Photo by Mark Deakos

Many are familiar with Hawai‘i’s humpback whales, the gentle giants that visit Hawaiian waters each winter to breed and give birth. But most are less familiar with other gentle giants in Hawai‘i, who remain in our nearshore waters throughout the year — the manta ray.

Two species of manta rays exist. The less common oceanic manta ray (Mobula birostris) has a wingspan over 22 feet, and the more common reef-associated manta ray (Mobula alfredi) has a wingspan of 12 feet. Each year, thousands of visitors travel to Kona coast, on the leeward side of Hawaiʻi Island, to snorkel and dive with these graceful plankton feeders at night, using artificial lights to concentrate their food. But what about the manta rays that reside off Maui, Oʻahu or Kauaʻi?

In 2005, Dr. Mark Deakos, co-founder and chief scientist of the Hawaiʻi Association for Marine Education and Research (HAMER), began studying the reef-associating manta rays off West Maui at a known cleaning station found on the Olowalu Reef. Cleaning stations are important staging areas where turtles, sharks and other marine species visit to have cleaner fish clean wounds or remove unwanted parasites from their bodies. The fluorescent Hawaiian cleaner wrasse is the most noticeable, and it focuses generally on cleaning inside the manta ray’s gills and mouth while other cleaner fish, such as the saddle wrasse, focus more on parasites on the outside of the manta ray’s body.

Reef manta ray opening its mouth and gills to allow access for the brightly colored Hawaiian cleaner wrasse to remove parasites from these areas. Photo by Mark Deakos

Much has been learned over the past 17 years at these cleaning stations where many photographs have been taken. Individuals are distinguishable by the unique spot patterns found on the belly of each manta ray. To date, the HAMER catalog of Maui manta rays has grown to more than 600 unique individuals — the largest population known in the United States — and twice that of the Kona manta ray population.

Sadly, over the past decade, manta ray sightings at the Olowalu Reef have dropped by more than 95 percent. This is likely due to the declining health of corals suffering the devastating impacts of stormwater that carries fine sediments and pollutants down the steep mountain slopes and onto the reef, suffocating the coral and disrupting fish reproduction. This could be the reason why the once reliable manta ray cleaning stations on the Olowalu Reef have disappeared.

In 2017, Deakos, with support from Suzanne Case (executive director of The Nature Conservancy at the time) and Nainoa Thompson (president of the Polynesian Voyaging Society), successfully petitioned to have the Olowalu Reef recognized as Hawai‘i’s first Sylvia Earle Mission Blue Hope Spot. The nearly 1,000-acre Olowalu Reef has been identified by some of the top coral biologists in the world as a priority reef for marine protection.

Tala, a young female manta ray traveling near the surface, is frequenly sighted off Honolua Bay in West Maui. Photo by Mark Deakos

In response, many stakeholders are working hard to help to resolve the sediment threat through a multipronged approach within the two watersheds that flank the Olowalu Reef (Olowalu and Ukumehame). A major priority is reducing fire ignitions in the area from utility lines. Fires result in tens of thousands of acres of exposed soil and ash that are ready to be pulled downhill to the ocean during the next rain event. Other work involves installing fire breaks to reduce fire spread; eliminating deer that denude the landscape; planting native species into swales to slow the water and bind the soil; installing fencing to keep out the deer; and efforts to identify the major sources of sediment and prevent that sediment from reaching the ocean.

Meanwhile, HAMER scientists have been learning a great deal about Maui’s manta ray population. Using a paired-laser system, one can measure the size of a manta ray. After more than 10 years of measuring manta rays at Olowalu, none were smaller than seven feet and yet they are born at four feet in width. But thanks to citizen scientists who are sharing opportunistic photos of encounters in South Maui, more than 60 young manta rays have been added to the catalog. We now know they segregate geographically by age class, likely targeting different food sources.

Stormwater carrying fine sediment and pollutants enters the ocean after a heavy rain event in West Maui suffocating the corals beneath. Photo by Mark Deakos

The Olowalu Reef also hosts mating trains of manta rays that can include up to 25 males pursuing a single female. We learned that mothers, following a 12-month gestation period, give birth to a single pup every two to five years.

The photo-identification work, combined with active and passive tracking of manta rays, has shown they move between the islands of Maui Nui (Maui, Molokaʻi, Lānaʻi, and Kahoʻolawe). However, no evidence exists that they move beyond those islands.

In fact, HAMER’s recent genetic findings, which are in preparation for publication and reflect a collaboration with scientists Dr. Jonathan Whitney with NOAA and Dr. Richard Coleman with the University of Central Florida, have found the Maui manta ray population to be genetically distinct from those off Kona. The differences in the genetic signatures between the two populations suggests males may only move between the islands once per generation (about 40 years) and females, which are more likely to stay in the region they were born, would move only once per 500 years.

An adult male manta ray on the Olowalu Reef getting cleaned by several saddle wrasse. Photo by Mark Deakos

This is astonishing given that Hawaiʻi Island is less than 30 miles from Maui, suggesting that sufficient resources exist in Maui Nui that precludes the need to cross the deep ʻAlenuihāhā Channel separating the two islands. This, however, highlights the vulnerability of these isolated populations, considering their relatively small population size, low numbers of births, and late maturity. If population numbers should drop, they won’t be resupplied from neighboring populations.

Whether or not our reef manta ray populations are increasing, decreasing, or remaining stable is not yet clear. To help address this question, HAMER is working with Corey Nevels, a master’s student enrolled at the University of Hawaiʻi, who is examining multiple population parameters using mark-recapture statistical modeling. If populations from Hawaiʻi Island or Maui Nui are shown to be on a decline, this could trigger a federal designation as threatened, distinct population stocks, similar to the false killer whale populations in Hawaiʻi, mandating the need for a population recovery plan.

A female reef manta ray being pursued by multiple males in a mating train on the Olowalu Reef. Photo by Mark Deakos

In collaboration with the Manta Pacific Research Foundation out of Kona, HAMER is also using genetics to examine the first ever parentage study conducted on manta rays. This will not only help develop the family tree so we know who is related to whom, but will shed light on whether certain adult males and females are responsible for most of the offspring or if parentage is more evenly distributed across all adults.

The latest and most urgent research is identifying which critical habitats are needed for our reef manta rays to survive and thrive. Where do they spend most of their time getting cleaned, seeking mates, feeding, and giving birth? No pupping habitats have been identified to date on Hawaiʻi and on Maui. Where they feed is still an unknown. With the deployment of sophisticated satellite tags, HAMER is hoping to map out those critical habitats for both the older and younger cohorts so that we can begin to ensure those habitats are well protected.

In the past two years, citizen scientists from the islands of Oʻahu and Kauaʻi have been sharing their photos of opportunistic manta ray encounters, helping us learn about these island populations as well. This year, we had the first confirmed match connecting a manta ray from Kauaʻi with the island of Niʻihau, and the first match of a manta ray on O‘ahu’s North Shore, seen again on the south shore — evidence that the entire island of Oʻahu is part of their home range.

Mark Deakos

Through the collaborative efforts of scientists, resource managers, decision makers, and citizen scientists we are hopeful that we will find the solutions to protect and preserve Hawai‘i’s resident giants, their habitats, and the joy they bring to our lives, for many generations to come.

You can support HAMER’s research by visiting www.hamerinhawaii.org or contacting them via contact@hamerinhawaii.org.  If you have a manta ray photo you wish to contribute to the catalog, please send to reportamanta@mantatracker.org and you will be notified if it is a new individual or one previously sighted.

• Mark Deakos came to Hawaiʻi in 1996, hailing from Canada, and completed his master’s degree on humpback whales, and his Ph.D. on manta rays with the University of Hawaiʻi. He co-founded the Hawaii Association for Marine Education and Research in 2004 and has been conducting manta ray research in Hawaiʻi since 2005.

Pregnant female reef manta ray on the Olowalu Reef revealing her unique spot pattern on her underside. Photo by Mark Deakos

A reef manta ray with a wingspan of 12 feet, cruising the waters off West Maui. Photo by Mark Deakos