First evidence of oceanic manta rays in Fiji

Thanks to Manta Trust researcher Luke Gordon and environmental photojournalist and filmmaker Tom Vierus from the Fiji Islands, we now have the first unequivocal evidence of an oceanic manta ray (Mobula birostris) occurrence in Fijian waters.

We partly have anatomy to blame. Until revision of the genus Manta rays back in 2009 all available records in Fijian literature were recorded as Manta birostris. However, a second manta species (Manta alfredi) was resurrected based on morphological and meristic data until 2018, when both manta ray species (Manta alfredi and Manta birostris) was moved to Mobula based on phylogenetic analysis.

The first sightings take us back in time to November 2018, when two individuals were spotted in Laucala Bay, a large lagoon next to Suva, the capital of Fiji. They searched, swimming with their mouths open and pulling in zooplankton (such as copepods, mysid shrimps, crab larvae, mollusk larvae and fish eggs) with their mullet horns unrolled to guide the food to the desired location. Then three more individuals were seen in December 2018 and then… nothing. The oceanic manta rays were not rediscovered until July 2020, when the world plunged into a pandemic and two individuals brought instant joy to an otherwise gloomy month. Two more individuals were recorded in the Yasawa Island Group in western Fiji while passing and searching in a channel between Drawaqa and Naviti Island in April and September 2020. Six individuals were observed in November 2021, increasing to eight individuals in May/June this year (2022).

To some, all mantas look the same. But to the trained eye, there are many ways to tell individuals apart. Manta rays can be identified individually by the spots on the underside of the body; just as human fingerprints are unique to an individual, so are these stain patterns. Unique identification patterns were obtained for nine individuals, and all nine individuals have been seen again since the initial identification! In fact, one individual was documented in 2018, 2020, 2021 and 2022.

Interestingly, all these individuals applied in the same geographical area. But why? The authors point to a recent study that looked at nutrient measurements in Laucala Bay, which reported high chlorophyll-a (phytoplankton biomass) concentrations. These high values ​​were likely due to “accumulation of nutrients from high river discharges and anthropogenic inputs, such as the effluent released from the Kinoya wastewater treatment plant in the north of the bay combined with low water flow due to the barrier reefs that limit water exchange to and from the open ocean.” The inner bay zone – which is where the mantas were spotted – showed the highest average chlorophyll-a readings, suggesting that these mantas specifically targeted these areas to maximize their foraging success. “The current observational data and the spatial and temporal overlap of chlorophyll-a concentrations with manta occurrences suggest that Laucala Bay may be visited annually in at least November, December, June and July, presumably to feed on zooplankton blooms following high phytoplankton concentrations.” suggest the authors.

Manta rays are the largest rays in the world and live in tropical, subtropical and temperate seas. They are highly intelligent, and have the largest brain-to-body weight ratio of any living fish. One of the more threatened elasmobranchs thanks to increasing overfishing pressure and the fact that they have a relatively slow reproduction rate. The word “manta” is actually Spanish for “blanket” or “cloak” and it’s pretty easy to see why this animal got its name. And they are huge blankets of water, with fully grown oceanic manta reaching a wingspan of up to 23 feet (7 meters) and weighing a few thousand pounds.

The current sighting rate of these animals here is high, in stark contrast to other oceanic manta populations worldwide (such as the largest population in the world based on Isla de la Plata, Ecuador) where the sighting rate over the last ten years is very low. Gordon and Vierus believe this can possibly be attributed to geographical differences in habitat use. Nevertheless, they believe that Fijian waters may be a critical capture habitat for the species, not only in Fiji, but in the wider South Pacific region. “In light of the global extinction risk for M. birostris and the recent reclassification from Vulnerable to Endangered on the Red List of Threatened Species, the expansion of their known range into Fijian waters and the recurrence of individuals over successive years in the same location provide valuable information for the development of effective and data-driven conservation strategies,” state the authors in their new paper .

Gordon and Vierus hope that future research will include fine-scale and broad-scale movement tracking with genetic analysis, which will be useful in understanding the population dynamics of manta that visit Laucala Bay.

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