First Evidence Of Oceanic Manta Rays In Fiji
Thanks to Manta Trust researcher Luke Gordon and environmental photojournalist and filmmaker Tom Vierus of the Fiji Islands, we now have the first unequivocal evidence of oceanic manta ray (Mobula birostris) occurrence in Fijian waters.
We partly have anatomy to blame. Until the revision of the genus Manta 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) were moved to Mobula based on phylogenetic analysis.
The first sightings have us traveling back in time to November 2018, when two individuals were sighted in Laucala Bay, a large lagoon adjacent to Suva, the capital city of Fiji. They were foraging, swimming with their mouths with open and drawing in zooplankton (such as copepods, mysid shrimp, crab larva, mollusk larvae, and fish eggs) with their cephalic horns unrolled to funnel the food into its desired location. Subsequently, three more individuals were sighted in December 2018 and then… nothing. The oceanic manta rays weren’t spotted again until July 2020, when the world plunged into a pandemic and two individuals brought momentary joy to an otherwise bleak month. Two additional individuals were recorded in the Yasawa Island Group in the west of Fiji while passing through and foraging 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 of this year (2022).
To some, all mantas look alike. But to the trained eye, there are numerous ways to tell individuals apart from one another. Manta rays can be individually identified by the spots on the underside of their bodies; just how human fingerprints are unique to an individual, so are these spot patterns. Unique identification patterns were obtained for nine individuals, and all nine individuals have been re-sighted since first identification! In fact, one individual was documented in 2018, 2020, 2021 and 2022.
Interestingly enough, all of these individuals were foraging in the same geographical area. But why? The authors point to a recent study that looked at nutrient measurements in Laucala Bay, which reporteded high chlorophyll-a concentrations (phytoplankton biomass). These high values were probably due to “the accumulation of nutrients from high riverine discharges and anthropogenic inputs, such as the effluents discharged from the Kinoya wastewater treatment plant in the north of the bay coupled with a low water outflow due to the barrier reefs restricting water exchange to and from the open ocean.” The inner bay zone – which is where the foraging mantas were spotted – displayed the highest mean chlorophyll-a measurements, suggesting that these mantas specifically targeted these areas to maximise their foraging success. “The current observational data and the spatio-temporal overlap of chlorophyll-a concentrations with manta occurrences suggests that Laucala Bay might be visited annually in at least November, December, June and July, presumably for feeding on zooplankton blooms following high phytoplankton concentrations,” the authors suggest.
Manta rays are the largest rays in the world and live in tropical, subtropical, and temperate oceans. They are highly intelligent, having the largest brain to body weight ratio of any living fish. One of the more threatened elasmobranchs thanks to mounting overfishing pressure and them having 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 the moniker. And they are huge water blankets, with fully grown oceanic manta ray can reach a wing span of up to 23 feet (7 meters) long and weigh a few thousand pounds.
The current re-sighting rate for these animals here is high, in stark contrast with other oceanic manta populations worldwide (such as the largest population in the world based out of Isla de la Plata, Ecuador) where the re-sighting percentage over the last ten years is very low. Gordon and Vierus believe this could possibly be attributed to geographical differences in habitat use. Still, they believe Fijian waters may be a critical foraging habitat for the species in not just Fiji but the wider South Pacific region. “In light of the global extinction risk of M. birostris and the recent reclassification from Vulnerable to Endangered on the Red List of Threatened Species, the expansion of their known distribution range to Fijian waters and the recurrence of individuals over consecutive years in the same location adds valuable information for the development of effective and data-driven conservation strategies,” the authors state in their new paper.
Gordon and Vierus hope that future research incorporates fine-scale and broad-scale movement tracking with genetic analysis, which would be helpful to further understand the population dynamics of manta rays that visit Laucala Bay.